Double Star of the Month in

• February 2026 - Double Star of the Month

  About 10 degrees east of beta Aurigae lies a coarse cluster of mostly naked eye stars all bearing the label ψ (psi). The Cambridge Double Star Atlas (2nd edition) shows that ψ², ψ⁵ and ψ⁷ are all double but they do not appear in the catalogue, presumably because they are all too wide. Nevertheless, 56 Aur = ψ⁵ Aur (06 46 44.34 +44 34 37.3), is worth looking at because it has a fine colour contrast.

  

  It was found by William Herschel in 1782 when the position angle (PA) and separation were 17 degrees and 53". and he noted colours of white and pale red. By 2022 the values of PA and separation were 44 degrees and 28" due mostly to a proper motion, largely in declination, of the A star. In 1970 I found the colours to be yellow-orange and blue with a 21-cm mirror.

  Pi Puppis (07 17 08.56 -37 05 51.0) is a wide, unequal double star found by James Dunlop at Parramatta in New South Wales. It is about 8 degrees south of the blue supergiant eta CMa (V = 2.5). The primary is a very bright (V = 2.9) K4 giant whilst there is a V = 7.9 companion at 213 degrees and 69" with this separation slowly decreasing with time.

  

  Gaia DR3 gives distances of 560 and 970 light-years respectively for A and B but the parallax measured for has a 20% error and this is because the Hipparcos mission in the early 1990s found that A itself was a double star with a V = 6.9 companion 0".7 distant. Since discovery this star has moved just 3 degrees but it does not appear in DR3.

  Pi Puppis lies in a cluster called Collinder 135 which has six members brighter than magnitude 6. It might have had a common origin with a nearby cluster UBC 7. The centres of the clusters are currently 24 pc apart but the stellar motions indicate they would have been closer in the past.

  Bob Argyle - Double Star Section Director

  If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Double Stars of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.

• January 2026 - Double Star of the Month

  The Hyades moving group contains stars which are not immediately proximate to the open cluster on the sky. The most famous is perhaps Capella, a bright spectroscopic binary with a 104 day period which is orbited by a binary pair of red dwarfs. Another member is ENG 22 (05 41 20.33 +53 28 52.7) which can be found 2.5 degrees WSW of delta Aurigae (V = 3.7).

  

  It was catalogued by Rudolf Engelmann but recent measures by the Gaia satellite have revealed the parallax and proper motion of both components. The pair consists of stars of V magnitudes 6.0 and 8.9 which lie at a mean distance of 40.0 light-years and which are moving across the sky at 0".5 per year. The separation and position angle have remained fixed at 98" and 72 degrees so this is probably a binary system. The primary is a K dwarf and the companion has spectral type M0.

  Adhara = epsilon CMa (06 58 37.55 -28 58 19.5), at V = 1.5, is the second brightest star in the constellation, but 4 million years ago, it was the brightest star in the sky reaching magnitude -4.

  

  In the nineteenth century a visual companion was seen at the Cape Observatory, some six magnitudes fainter and 7".5 distant in position angle 167 degrees. Ernst Hartung noted that the the pair can be seen in 75-mm aperture, and that the primary is brilliant white whilst the companion is deep yellow.

  The brightness of the primary is probably the reason why the error in the parallax found by Hipparcos is significantly larger than usual. Gaia has no results for this star but it is possible that the brightest stars in the sky will be investigated towards the end of the project. The satellite was switched off on March 27, 2025 and work on the Gaia DR4 catalogue is now in progress and the results are expected towards the end of next year.

  Bob Argyle - Double Star Section Director

• December 2025 - Double Star of the Month

  During the course of his long observing life, Thomas William Webb happened upon a number of wide double stars which were not included in the catalogues of the time. The Washington Double Star catalog (WDS) contains 10 pairs with the discoverer number WEB. One of these (WEB 2) lies in Camelopardalis at 03 42 42.73 +59 58 09.8.

  

  The primary star, which is a K4 giant, was catalogued by Piazzi in 1814 and is called P III 97. In fact, Piazzi did note the fainter companion to the north but did not measure it. Webb noted the companion star B on Mar 2, 1854. It is V = 8.5 whilst the primary is just above naked-eye visibility at V = 5.7. Both stars should be visible in a good pair of binoculars but a small telescope should show the distinct orange tinge of the primary and the bluish tinge of the secondary. Thomas Espin added three fainter companions. The components are currently at 36 degrees and 55". Both stars are 1790 light-years away.

  Many of the bright pairs in the Dunlop catalogues are rather wide and any relative motion is in general very small. However a small number of pairs do show significant orbital motion including p Eri (Dun 5) and Dun 23 in Puppis. The latter binary can be found at 06 04 46.76 -48 27 30.2.

  

  Since Dunlop found the pair in 1826 the companion has moved 160 degrees. A recent orbit gives a period of 553 years and the predicted position for the time of writing is 132 degrees and 2".5. With the star magnitudes of 7.3 and 7.7 this is a pair which should be well seen in 10-cm.

  Radial velocity observations using the 2.5 metre reflector at CTIO in Chile by Andrei Tokovinin showed that A is a spectroscopic binary with a period of 2.51 days and the WDS notes that B may be a very close pair although the radial velocity showed very little variation. This system is 100 light-years distant.

  Bob Argyle - Double Star Section Director

• November 2025 - Double Star of the Month

  Most double stars tend to have components of similar spectral types, or perhaps the difference might span a whole spectral class or two. In STF 188 (01 59 27.53 +62 54 20.6), however, the two main components differ by five spectral classes. The primary is a M0 star which is 2,100 light-years away whilst its B2 neighbour is over 11,000 light-years distant, so this is not a binary star but a line-of-sight coincidence.

  

  The stars lie in Cassiopeia one degree SE of the 3.4 magnitude epsilon Cas, the easternmost star in the 'W' of Cassiopeia. The catalogue visual magnitudes are 9.2 and 10.5 and they are currently separated by 32" in PA 238. Not an easy object for the small aperture but with 20-cm or so the colours of red and white should be apparent. The pair was rejected by Struve and does not appear in the volumes by Dembowski and Lewis.

  HJ 3475 (01 55 19.69 -60 18 44.6) lies in Hydrus, about 1.5 degrees NNW of the 2.8 magnitude alpha Hyi. It was swept up by John Herschel on 1836, Oct 3 and he noted it as a fine double star. Both stars have visual magnitude 7.2 and are currently 2".5 apart in PA 78 degrees. There has been substantial angular motion since Herschel found PA 30 degrees and the components appear to be closing. This appears to be a genuine binary star as Gaia DR3 gives similar parallaxes and proper motions.

  

  A further 1.5 degrees ENE is HJ 3484, which despite the considerable difference in magnitude and substantial separation, also appears to be physically connected. The stars shine at magnitudes 7.6 and 10.5 and are 52" apart in PA 62 degrees. They are 148 light-years distant.

  Bob Argyle - Double Star Section Director

• October 2025 - Double Star of the Month

  34 Piscium (00 10 02.18 +11 08 44.9) lies near the border between Pisces and Pegasus. It is 4.5 degrees south of, and slightly preceding gamma Pegasi (V = 2.8). It was found by F. G. W. Struve and included as number 5 in his Dorpat Catalogue. This is a fine, but very unequal pair, and 15-cm should give a good view of the stars.

  

  The Washington Double Star catalog gives magnitudes of 5.5 and 9.4 making them a rather tricky prospect for a small aperture and classifies the primary as B9V. The stars have remained fixed in separation since discovery and are currently 7".5 apart in position angle 159 degrees.

  Gaia DR3 gives parallaxes for the stars but the error on the primary star is almost five times that of the secondary and even allowing for the combined errors the parallaxes are different, although the proper motions are similar. Hopefully DR4 which is due out at the end of 2026 will throw more light on the situation.

  HJ 3375 (00 44 43.80 -35 00 07.3) is 2.5 degrees SSE of eta Sculptoris and about 5.5 degrees west and slightly south of the Sculptor Dwarf Galaxy. It was found by John Herschel at the Cape. Over the past 190 years there has been little angular motion and in 2015 the companion was found in position angle 171 degrees at a separation of 4".4.

  

  This is a relatively nearby binary system - the distance is 110.7 light-years and both components are moving across the sky at more than 0.5 arc-seconds per year.

  Four degrees SE of HJ 3375 is the wide pair lambda 1,2 Scl. The easterly component (lam 1) is a K giant (V = 7.1) whilst 20 arc-minutes to the west is lam 2 (B9.5V). This is a visual binary which is now slowly separating and is currently near 0".8. The components have magnitudes 6.6 and 7.0.

  Bob Argyle - Double Star Section Director

• September 2025 - Double Star of the Month

  About 1.5 degrees NW of gamma Sagittae, the bright star at the tip of the arrow of Sagitta, is STF I 48, or more recently STFA 48 (19 53 22.65 +20 20 14.1). This is a wide and bright double star which is easily seen in small apertures.

  It forms part of the additional list of wider pairs compiled by Wilhelm Struve during his Dorpat survey. It was, however, noted by William Herschel forty years earlier who dubbed it H V 106 (now H 5 106). I measured it in 2003 when the relative position was 147 degrees and 42".

  

  There is a 10.5 magnitude star some 222" distant and more recently the Hipparcos satellite resolved the B star into a pair of stars with magnitudes 7.6 and 9.4 but with a current separation of 0".2 this probably requires 40-cm and a night of good seeing. It is a binary with a period of 85 years.

  The double stars found by James Dunlop are bright, wide and sometimes colourful. DUN 236 (21 02 12,73 -43 00 07.7) lies in the constellation of Microscopium about 1.5 degrees SSW of the magnitude 5.5 star eta Microscopii which is an orange giant.

  The stars (magnitudes 6.7 and 7.0) were 67" apart when found in 1826 and since then the separation has reduced by 10" although the proper motions are almost identical. The spectral types suggest colours of pale yellow and pale orange. The parallaxes are similar but that of the brighter component has a very large error which may mean that there is another component close in.

  

  Move the telescope 4 degrees due east of DUN 236 and you will alight on MLO 6 (see this column for Sept 2012), a fine and unequally bright pair.

  Bob Argyle - Double Star Section Director

• August 2025 - Double Star of the Month

  About 3 degrees north of zeta Aql (V = 3.0) star atlases show a close pair of stars of around 6^th magnitude which are aligned almost E-W. This is STTA 177 (19 12 34,45 +16 50 47.2). The stars were 121" apart in 1874 but are closing and are now near 97", still making them a good target for binoculars.

  

  The A star is of interest as it is both a close visual binary but it is also an eclipsing binary of the Algol type with a period of 3.5 days. When S. W. Burnham was prowling the skies looking for new pairs with his 6-inch Clark refractor one of the stars he alighted on was this one. He found a pair of stars with magnitudes 7.1 and 8.0 separated by only 0".5. There has been little motion since then. I managed to measure them once in 2012 with the Cambridge 8-inch when the measure gave 0".7 but more recent measures put them at 0".6. There are two fainter and unrelated stars in the field.

  Corona Australis sits below Sagittarius and on the sky it occupies a rectangle with sides of about 15 degrees in RA and 7 degrees in Dec. About 4 degrees east of the centre of this box and 1 degree south-east of mu CrA (V = 5.2) lies HJ 5066 (18 50 58.65 -41 03 45.7). It is marked as double in the second edition of The Cambridge Double Star Atlas but it is not labelled. The primary is a magnitude 6.5 B giant and its companion of magnitude 9.2 lies 10" away in PA 85 degrees.

  

  Move another degree SSE and you will arrive at HDO 291, 6.1, 11.4, 341 degrees, 37" which is separating fairly rapidly due to the difference in proper motion between the two stars, the primary of which is a K giant. In one of his last discoveries, Robert Innes in 1926, using the then new Johanneburg 26.5-inch refractor, found B to a close equal pair (10.2, 12.2, 292 degrees, 0".6). A recent version of the Washington Double Star catalog (June 2025) records this discovery observation as the only observation.

  Bob Argyle - Double Star Section Director

• July 2025 - Double Star of the Month

  Zeta Lyrae (18 44 46.36 +37 36 18.4) is the third star in the tight, naked-eye triangle that includes Vega and the Double-Double (epsilon). It appears in many double star catalogues, including those of Herschel, the Struves and South and Herschel but according to the Washington Double Star catalog (WDS) it was found by Francesco Bianchini (1662—1729) with the date 1737 which presumably refers to a book published posthumously called Astronomicae et geographicae observationes selectas... written in Latin, but available in full on the internet.

  

  The brightest component, zeta¹ is magnitude 4.34, and is a spectroscopic binary, whilst the fainter component, zeta², is 5.62. The stars lie 157.9±0.2 light-years away according to Gaia DR3. An observation with the Cambridge 8-inch in 2010 gave 149 and 45".4 but there is very little relative motion between the two stars.

  Zeta Lyrae is resolvable in binoculars and both stars appear white. There are four faint field stars ranging from mags 13.3 to 15.7 which lie between 22" and 78", two of which were found by Burnham with the 18.5-inch at Dearborn and the faintest was detected in the Lick 36-inch.

  WNO 6 (18 28 57.36 -26 34 55.5) lies 1.5 degrees south of the 2.8 magnitude lambda Sagittarii, the orange-hued K1 giant star at the top of the 'Teapot' lid of Sagittarius. It is a fine pair for the small aperture with stars of magnitudes 6.7 and 8.0 separated by 42" with a position angle of 182 degrees. Gaia DR3 places them at significantly different distances, the A component has a distance of 639 light-years away but it has a large formal error of 24 light-years. B is 472 light-years away.

  

  It makes a tight triangle with two other stars the most north-westerly of which is BU 133, a rather difficult pair requiring at least 25-cm as the stars have passed 0".6 and are still closing, having been 1".8 apart when discovered by Burnham using his 6-inch Clark refractor. The magnitudes of 6.6 and 8.5 add to the difficulty of resolution.

  Bob Argyle - Double Star Section Director

• June 2025 - Double Star of the Month

  Two degrees of tau Her (V = 3.9) is STF 2063 (16 31 47.73 +45 35 53.8). This is a rather wide but also rather unequal pair with the two stars shining at magnitudes 5.7 and 8.7 respectively.

  There has been very little motion since William Herschel noted them on 1782 Nov 8. and the pair entered his catalogue as H 4 62 and which he called very or extremely unequal noting colours of white and 'br' although it is unclear what this actually is. Bright red? Bluish-red? Brown?

  I checked with Bruce MacEvoy's excellent complete catalogue of the Herschel double stars and he does not comment on the colours. I observed the pair with a 21-cm reflector at x96 and found colours of white and blue.

  

  The primary has a spectral type of A1V, and the Gaia DR3 catalogue reveals the stars are at the same distance (230 light-years) and have very similar proper motions. The companion lie at PA 195 degrees and the stars are currently separated by 16".3.

  A rather similar pair can be found in the southern hemisphere in the constellation of Ara. HJ 4978 (17 50 28.40 -53 36 44.7) is a little more unequal (5.7 and 9.2) and somewhat closer (12".3 and PA 268 degrees). The system, noted as nu1 Arae by John Herschel, lies 1.5 degrees due east of the globular cluster NGC 6397.

  

  The SIMBAD catalogue notes that the primary star is an Algol variable called V539 Ara, it has a period of 3.17 days and the eclipse depth is 0.53 magnitude, but SIMBAD gives no reference to the star as a wide visual double.

  Whilst in the area, check out the pair R 303 which is one degree SW of HJ 4978. The stars are magnitudes 7.8 and 9.0 with a separation of 3".6 and a PA of 108 degrees.

  Bob Argyle - Double Star Section Director

• May 2025 - Double Star of the Month

  Lambda Bootis is the prototype of a class of stars with unusual chemical composition - the abundance of metals is unusually low and they also tend to be accompanied by accretion disks or planetary systems. Two degrees NE of lambda is STF1843 (14 24 38.91 +47 49 50.0) is a wide and rather unequal pair which lie 316 light-years away.

  

  The Washington Double Star catalog (WDS) gives magnitudes of 7.7 and 9.2 and a recent measure puts the companion at 186 degrees and 19".8 with the separation widening with time. A third star of magnitude 9.3 can be seen 101" away in PA 63 degrees but the parallax given by Gaia DR3 indicates that it is more than 800 light-years away. There is, however, another member of the system - a magnitude 17.8 star found by Andrei Tokovinin. An observation which I made with a 10-inch reflector at x96 showed colours of yellow and lilac.

  In eastern Virgo lies phi Virginis (14 28 12.22 -02 13 40.6) which presents a greater challenge to the observer with a small telescope. It is the magnitude of the companion and its relatively close separation to the A star which means that a 15-cm telescope is recommended. The two stars are given catalogue magnitudes 4.0 and 10.0 and the current separation is 5".2, although when first measured in 1827 they were only 4" apart.

  

  The pair was measured with the Cambridge 8-inch in late 2015 and given that the micrometer uses a red LED to illuminate the field and hence shows the wires in outline I would have said that the B star must be brighter than 10. Both stars lie at 121 light-years distance and are moving through space with almost identical proper motions of 0".139 per year. The relatively large error on the parallax of A as measured by Gaia chimes with the comment in the WDS that A may be a spectroscopic binary.

  Bob Argyle - Double Star Section Director

• April 2025 - Double Star of the Month

  With Arcturus riding high in the Spring sky, STF 1825 (14 16 32.84 +20 07 18.7) can be easily found as it is just 1.5 degrees north and a little west of the bright, orange giant star. The components have magnitudes 6.5 and 8.4 and the current separation is 4".4 so it is a relatively easy object for the small telescope.

  

  The very slow orbital motion is reflected in the fact that my two sets of measures made in 1993 and 2019 show a difference in position angle of just 2 degrees. This is a relatively nearby system as the parallax determined by Gaia DR3 shows, the distance is 108 light-years and both stars are moving across the sky at more than 0.1 arc-second per year.

  In Centaurus, near the border with Lupus, and about 9 degrees north and slightly west of Hadar (beta Centauri, V = 0.6) is HJ 4651 (14 09 35.03 -51 30 16.8), a wide and unequal pair of unassociated stars. The magnitudes are 6.0 and 8.9 and the current separation is 65". The primary star is the eclipsing binary V869 Cen and is almost four times nearer to the Sun than its fainter companion.

  

  Of more direct interest but certainly a more difficult object is SLR 19 which is to be found 1.5 degrees north and a little preceeding HJ 4651. It is an almost equally bright pair which was first observed by R. P. Sellors in Australia using the 11.5-inch refractor at Sydney Observatory. The motion to date amounts to 45 degrees and an orbit by Izmailov gives a period of 415 years which predicts a closest separation of 0".6 in 2054. The predicted position for 2026.0 is 337 degrees and 0".9 and the magnitudes are 7.1 and 7.4.

  Bob Argyle - Double Star Section Director

• March 2025 - Double Star of the Month

  The spectral type of the primary of STF 1689 (12 55 30.32 +11 29 46.4) is given as M4III in the Washington Double Star catalog (WDS). One might expect to see a deep-orange, or red star but observations of the pair in the literature seem to indicate that the primary colour reflects an earlier spectral type.

  T. W. Webb notes yellowish, whilst Admiral Smyth recorded pale white. All three observers noted that the companion was blue or bluish. More recently, I recorded orange and blue using a 10-inch reflector at x96 in 1970. Sissy Haas gives peach-white, and a note on TheSkySearchers website records colours of orange and bluish.

  

  Gaia has observed both components and the stars are unrelated. The primary is 960 light-years away whilst B is 474 light-years distant. The components are slowly separating due to the different proper motions and in 2019 the relative position was 222 degrees and 30".3. The pair can be found 2 degrees NNW of epsilon Virginis.

  R 213 (13 07 24.30 -59 51 37.8) was discovered in 1874 by Henry Chamberlain Russell. Russell was the Government Astronomer at Sydney Observatory from 1870 until 1905. He was using a 11.5-inch refractor made by Schröder to carry out a survey of some of the John Herschel double stars which the latter discovered at the Cape. It was one of the most difficult of his discoveries being found at 207 degrees, 0."3. His description of the stars was a very beautiful double star - both alike and orange coloured.

  

  When I observed the pair in 2016 using the 67-cm refractor at Johannesburg, the PA was very similar but the distance has increased to almost 0".8. With WDS magnitudes of 6.6 and 7.0, the pair should be divided in 15-cm. R 213 lies in Centaurus close tothe border with Crux and can be swept up by moving 2.5 degrees due east from beta Crucis (Mimosa), which is DUN 125 - magnitudes 1.3, 7.2, 23 degrees, 373" - a fine binocular double.

  Bob Argyle - Double Star Section Director

• February 2025 - Double Star of the Month

  Castor (07 34 35.86 +31 53 13.8) has already been discussed in this series (Feb 2007) but since then considerable advances have been made in our knowledge of this famous sextuple system.

  

  It is eighty years since systematic radial velocity measurements of the two bright components, both of which are spectroscopic binaries, were made. Recent observations using a 1.5-metre telescope and an echelle spectrograph have refined the periods to within 4 seconds for AaAb (period = 9.2 days) and 0.1 second for BaBb (period = 2.9 days).

  Combining these data with new and direct images of the spectroscopic pairs using ground-based interferometry has produced masses for the four main stars to remarkable accuracy. They are 2.37 suns for Aa, 0.39 suns for Ab, 1.79 suns for Ba and 0.39 suns for Bb, the errors in each case being below 0.02 sun.

  The work was described by Dr. Guillermo Torres and collaborators in a 2022 paper which appeared in Astrophysical Journal. The paper also gives the elements of an orbit for AB with a period of 459 years.

  The pair was measured in May 2024 with the Cambridge 8-inch Cooke, and a mean of three nights gave 50.6 degrees and 5".76 as the stars continue to widen.

  DUN 38 (07 03 57.32 -43 36 28.9) is a physical quadruple star which is located in southern Puppis about 4 degrees east of the 3.2 magnitude star nu Puppis. Three of the four components can be easily seen in a small telescope.

  

  This is a bright and pretty pair easily seen in small apertures with the components A and B being magnitude 5.6 and 6.7 and spectral types G1V and K1V, which suggest colours of deep yellow and orange which is indeed what is observed by Ross Gould using 175-mm. He also notes that star C is bright orange.

  The current separation is 21" and the position angle is 125 degrees. Component C is 185" away in PA 335 degrees. A and B are both 55.6 light-years away, and whilst the C star is somewhat more distant it shares the large proper motion with A and B and is considered to be a physical companion. C is a rapid, close and unequally bright binary with a period of about 4.1 years.

  Bob Argyle - Double Star Section Director

• January 2025 - Double Star of the Month

  About five degrees east of the line joining the two stars which form the 'horns' of the constellation of Taurus, the Bull is a right-angled triangle of 5th magnitude stars. These are 132, 136 and 139 Tauri. If you acquire 132 Tau in the field then about 20 arc-minutes north-east is the wide pair STTA66 (05 47 56.13 +24 41 12). This is an entry in the catalogue of wide pairs which Otto Struve compiled during his survey at Pulkovo in the 1840s.

  

  The stars, which are relatively bright - 7.0 and 7.7 - and are separated by 94" in position angle 166 degrees. Rather surprisingly, they appear to be at the same distance as determined by the Gaia satellite and a search around them reveals a star of magnitude 19 also at that distance although the quoted error is substantial.

  132 Tauri itself is recorded in the Washington Double Star catalog (WDS) as a very close visual binary, although only observed on one occasion in 1979. During the Hipparcos survey another component was recorded. The two stars were given as 5.0 and 9.1 at 230 degrees and 3".8. And again the only observation appears to be the first one and such a pair should be visible in 25-cm (the WDS gives magnitudes 5.0, 9.1, 230 degrees, 3".8). It certainly it should have been detected in the recent Gaia observations and does not appear there.

  Looking through the earlier entries in this column which occupy the southern sky I find that I have been remiss in not including one of the finest of John Herschel's pairs - HJ 3945 or 145 CMa (07 16 36.84 -23 18 56) known quite widely, especially in the United States, as the 'Winter Albireo'. The stars are not quite bright as those in beta Cygni but form a spectacular pair for the small telescope.

  

  The primary is a K3 supergiant and its companion an F0 dwarf. John Herschel notes Orange and green. Fine contrast of colours. Modern observers also attest to the strong contrast of colours.

  This is certainly an optical pair - star B is 346 light years away whilst star A is 2,600. The stars can be found 3 degrees east of the magnitude 3.0 blue supergiant omicron 2 CMa. The 6th magnitude star HIP 35578 which is more than 3 degrees to the south and has almost identical proper motion to B has been formally identified as a physical companion (C). However Gaia DR3 gives its distance as 296 light-years - about 50 light-years closer than B.

  Bob Argyle - Double Star Section Director

• December 2024 - Double Star of the Month

  STF 430 Tauri (03 40 28.04 +05 07 33.2) is a coarse and unequal triple star found 1.5° WSW of 29 Tauri. The AB consist of stars of visual magnitude 6.8 and 9.8 separated by 26" at a PA of 57 degrees.

  

  The primary is a K2III and appears distinctly orange in the eyepiece. An observation made with a 10-inch mirror at x48 showed that B is blue as is the further companion, C, which is 34" away and is somewhat fainter than B.

  Although A and B have similar parallaxes they are not the same within the errors quoted in the Gaia DR3 catalogue, and actually the faintest star is much closer to us than the other stars.

  In southern Eridanus near the border with Caelum lies 41 Eridani (04 17 53.62 -33 47 54).

  

  This is a star which has an unusual chemical composition which contains significant amounts of mercury and manganese. Such stars are found to be common in spectroscopic binary systems and in 2012 a team using the VLTI in Chile resolved 41 Eri into two with a period of 5 days and a separation of a few milli-seconds of arc.

  A century before, however, Robert Innes, using a 7-inch refractor from the Cape of Good Hope, found the star to be a close double bright (I 270) with magnitudes of 3.6 and 4.

  The pair began to close in and were last seen in 1933 when the distance was 0".1. Later attempts to resolve the stars have failed - between 2017 and 2019, Dr. Andrei Tokovinin was unable to see the companion using a 4-metre telescope and it is now supposed that this is not a visual double star.

  In 1836 John Herschel found a faint companion (HJ 3636 AC) about 60" distant and gave the magnitudes as 3 and 14. Today the WDS gives magnitudes of 3.9 and 11.8 with PA and separation of 8 degrees and 49".

  Whilst in the area check out another HJ pair, no. 3642 just a few arc-minutes SSE - 6.6, 8.7, 157 degrees, 5".2.

  Bob Argyle - Double Star Section Director

• November 2024 - Double Star of the Month

  George Knott was a Victorian amateur astronomer with an interest in double stars. Using a 7.3-inch Clark refractor which once belonged to W. R. Dawes at his observatory in Cuckfield, West Sussex, he made a number of discoveries, five of which can be found in the current Washington Double Star catalog.

  The primary star of KNT 1 (01 02 18.34 +81 52 32.1) is the variable star U Cephei. It is an eclipsing binary with a period of 2.5 days and in mid-eclipse the star drops by almost three magnitudes. At maximum, it is V = 6.7 and has a faint companion of magnitude 11.8 some 14" distant in PA 63 degrees.

  

  Rather surprisingly both stars lie at the same distance (632 light-years) and have very similar proper motions. A slighter fainter, and definitely unconnected, third star can be found at PA 315 degrees (increasing) and 24" (decreasing).

  Another pair of stars with a variable primary is HJ 3476 (02 00 26.77 -08 31 25.8). This is AR Cet which is probably a semi-regular variable. John Herschel swept them up from Feldhausen and noted Large star very yellow. The primary is an M3 giant and its catalogue magnitude is given as 5.7 with the companion 61" away in PA 202 degrees. Herschel noted magnitudes 6 and 10.

  

  The position angle has reduced 20 degrees since discovery but this is due to the proper motion of the primary star which is 536 light-years away whilst its faint companion lies 2160 light-years distant.

  HJ 3476 lies about 3 degrees NE of zeta Cet which is also known as Baten Kaitos. This, too, is a very unequal and wide double with the components of 3.8 and 10.5 being divided by 106".

  Bob Argyle - Double Star Section Director

• October 2024 - Double Star of the Month

  In the north of Andromeda near the border with Cassiopeia and close to the line of zero RA lies 22 And, a star of visual magnitude 5.0. Moving about one degree west and slightly south brings us to BU 997 (00 04 57.53 +45 40 25.6), a pair of stars of magnitudes 7.6 and 9.4 and currently 3".8 apart. They should be easily seen in 15-cm although they were discovered by S. W. Burnham with the Dearborn 18.5-inch refractor. The stars show little motion, apart from a slight reduction in the pair's separation.

  

  In addition to his numbered discoveries, Burnham also noted a number of wide, faint pairs which have since been placed into the Washington Double Star catalog. One of these, BU 9001 (formerly BU 997a but renumbered to fit in with the new format of three letters and four numbers) lies about 30' due south. The stars are very unequal (6.7 and 10.6) and the companion lies at 235 degrees and 21". Larger apertures might see the 13.7 star added by Thomas Espin in 1914 at 107 degrees and 16".

  LAL 192 (23 54 21.40 -27 02 34.5) is another example of a pair being re-catalogued. It appeared in Dunlop's catalogue of 1826 as DUN 253 but was apparently changed to its current moniker at Lick Observatory when the double star catalogue existed on index cards but no-one seems to know why. It is, in any case, well worth seeking out. Easily seen in 10-cm the 6.8 and 7.4 magnitude components are presently 6".5 apart, a distance which is slowly increasing. Ross Gould using 10-cm records colours of pale yellow with a slight difference in shade.

  

  LAL 193 is in the neighbourhood. It is 1.5 degrees ENE of LAL 192 and the magnitude 6.2 SAO 192262, a distinctly orange-hued star lies between the two. LAL 193 consists of two early-F dwarfs of magnitudes 8.1 and 8.3. The current separation and position angle are 19".4 and 169 degrees and they also appear to comprise a physical system.

  Bob Argyle - Double Star Section Director

• September 2024 - Double Star of the Month

  Just two and a bit degrees north of the Saturn Nebula (NGC 7008) is the long period binary STF 2751 (21 02 09.00 +56 40 11.1). This pair of white stars should be divided easily in 10-cm and may be split in a good 7.5-cm aperture.

  

  Since 1828 when an early measure by the discoverer F G W Struve gave 343 degrees and 1".5 the stars have now moved on to 356 degrees and 1".6. Gaia DR3 indicates that the stars are at the same distance from us at least within the formal quoted errors. They lie 1,100 light-years away and are moving across the sky at close to 0".1 per year.

  Three degrees due south of the first magnitude Achernar in Piscis Austrinus is delta PsA (22 55 56.89 -32 32 22.9) which was found to be double by Herbert Howe in Cincinnati in 1876 and is number 91 in his catalogue, although the Washington Double Star catalog (WDS) also call this system BU 772 which reflects an independent, but later, discovery by S. W. Burnham.

  

  The WDS gives magnitudes of 4.3 and 9.3 whilst the Gaia DR3 catalogue records G magnitudes (similar to visual) of 3.9 and 9.8. As the current separation is 4".7 this is a pair which needs a night of steady seeing and, given its very low declination from the UK, probably 15-cm aperture. The stars appear to form a physical system and are 171 light-years away. Both beta PsA and gamma PsA are pairs with bright primaries and considerably fainter comites (see this column for Oct 2016 and Oct 2012 respectively).

  Bob Argyle - Double Star Section Director

• August 2024 - Double Star of the Month

  33 Cygni lies in northern Cygnus, near the border with both Cepheus and Draco. It forms one apex of a triangle whose sides are 1.5 degrees long with two other stars, both of which are visual doubles. The star which is directly east of 33 is STF 2687 (20 26 23.47 +56 38 19.3). This is an easy pair for the small aperture with the stars of magnitudes 6.3 and 8.1, separated by 25".

  

  The other member of the triangle is STF 2671 (H I 95). This is a brighter but closer pair than STF2687. The main components have magnitudes 6.1 and 7.5 and the current position of the fainter component is 337 degrees and 3".8, coordinates which have changed very little over 200 years. There is a 12.4 magnitude star at 53 degrees and 90" and two further 13th magnitude field stars.

  The second edition of The Cambridge Double Star Atlas shows the pair HJ 2975 in Capricornus about 5 degrees south of, and slightly following, the bright triangle of visual double stars formed by omicron (ο) Cap, rho (ρ) Cap, and pi (π) Cap (see this column for August 2015, 2016 and 2017 respectively). It is a fairly difficult pair for small telescopes as the Washington Double Star catalog (WDS) gives magnitudes of 7.5 and 11.6 and a separation of 11".

  

  The Atlas also shows that the star immediately north preceding by about 30 minutes is also double. This is a brighter and easier object (magnitudes 7.8 and 8.1) which does not warrant a label because the stars are 39" apart, but it is HJ 2973 (20 32 13.27 -22 09 17.6), a pair of yellowish F stars, one of which was found to be a close double at a lunar occultation in 1983.

  These Herschel pairs were discovered using the 20-foot reflector from Slough in 1830/1, and John Herschel allocated magnitudes of 8 and 14 to HJ 2975 and 8.9, 8.9 to HJ 2973.

  Bob Argyle - Double Star Section Director

• July 2024 - Double Star of the Month

  The constellation of Draco is draped across 11 hours of Right Ascension and perhaps the most distinctive part of it are the four stars which form the head of the beast. The faintest of these, and the most north-westerly is nu Dra (17 32 16.04 +55 10 22.5). Even a small pair of binoculars will show that it is really two equally bright, white gems, both of visual magnitude 4.9. Discovered in 1690 (by Flamsteed) this beautiful pair is worth seeking out in any aperture.

  

  The separation is slowly decreasing with time but at the time of writing is still a generous 62". Gaia EDR3 indicates that the stars are essentially at the same distance (98 light-years) and have very similar proper motions. This object forms one of the calibrating standard pairs which I use to convert the settings of the micrometer on the 8-inch Cooke telescope at Cambridge to position angles in degrees and separation in arc-seconds on the sky.

  Meanwhile, the stars of Ara sit south of Scorpius and the brighter members resemble a `butterfly' shape. Beta and gamma Ara, at the head of the butterfly, sit a degree apart and have distinctly different hues, thus creating a fine view in binoculars. Beta is red and gamma is blue whilst 4 degrees due south is 3.6 magnitude delta Ara (17 31 05.96 -60 41 01.8).

  

  John Herschel noted a distant and faint companion (V = 11) some 40" away and entered the pair as number 4951 in his catalogue. The companion should be visible in 10 or 15-cm and is now somewhat better placed having eased out to a distance of 50".4 in 2016. In 2010 a project involving deep infra-red imaging of stars which may have sub-stellar companions revealed 3 images within 12" whose K magnitudes were between 12 and 14.

  Bob Argyle - Double Star Section Director

• June 2024 - Double Star of the Month

  STF 1984 (15 51 10.09 +52 54 25) lies in south-eastern Draco almost on the border with Boötes in a rather featureless part of the sky, which may be why I have no record of observing this pair during my early visual survey.

  I did make a measure with the 8-inch Cambridge Cooke telescope giving 279 degrees, 6".4 in late 2009. An aperture of 10-cm should show the two stars which form this system - the magnitudes are 6.9 and 8.9.

  

  Although Gaia DR3 places the stars at similar distances (around 370 light-years), the formal errors on the parallaxes are significantly smaller than the parallax difference between the components, so whether the stars form a binary system is open to some doubt.

  In May 1874 S. W. Burnham was observing stars in Scorpius with his 6-inch Alvan Clark refractor and more specifically he was looking for HJ 4756 - a close pair discovered by John Herschel some 40 years earlier. He found a pair which vaguely resembled the HJ pair but its position did not correspond very well to the catalogue position so Burnham called the pair BU 228. He later realized that this was indeed the Herschel pair HJ 4756 (15 19 40.37 -24 16 11.9).

  

  The stars are magnitudes 7.9 and 8.3 and are currently slowing closing in a 500-year orbit. The current position is 229 degrees and 0".65 so a 25-cm telescope will be needed and more importantly, if observing from Europe, a night of high quality.

  Some 30 seconds of RA directly preceding is BU 227, an easier pair which drew from Burnham the comment that It is singular that he (Herschel) should have missed the other pair i.e. this one. The stars have magnitudes 7.5 and 8.6 and are currently 2 arc-seconds apart in PA 160 degrees, so should be divisible in 10—15 cm. The stars can be found 3 degrees ENE of sigma Lib.

  Bob Argyle - Double Star Section Director

• May 2024 - Double Star of the Month

  STF 1834 (14 20 17,6 +48 30 25.1) sits in the north of Boötes, about half way between theta and lambda Boo and 2.5 degrees north north-east of the latter.

  

  It is a well-known binary and the Washington Double Star catalog (WDS) notes than 250 direct measurements have been since discovery by Struve at Pulkova. From about 1830 to 1900 the two stars, magnitudes 8.1 and 8.3, closed steadily from 1".7 but then in just a few years the companion swung around its partner at which point it was out of range of even the large refractors of the time and began heading out towards the discovery position.

  The period of the pair is 413 years and at the present time the position angle and separation are 204 degrees and 1".7. The relative faintness of the stars would suggest that an aperture of 15-cm would be better to view them.

  About 1 degree south-east is STF 1843, a pair of stars of magnitude 7.7 and 9.2 currently at 186 degrees and 19".8. The stars lie at a similar distance and have common proper motion.

  Near the border of Libra with Lupus, HJ 4774 (15 28 58.69 -28 52 00.5) was discovered by John Herschel from Feldhausen during his stay at the Cape of Good Hope. The brightest star is magnitude 7.0 and has a 9.6 magnitude companion almost 10 arc-seconds distance in PA 11 degrees.

  

  In 1889 the eagle-eyed S. W. Burnham found that the primary star was double again with a 7.7 magnitude star at a distance of 0".7. Since then the separation of the close pair has closed to 0".1. A recent orbit by Dr. Andrei Tokovinin indicates that the orbit is only 0.6 degrees from being edge-on and in 2027 when periastron occurs the two stars will be only 8 milliarcseconds apart.

  Bob Argyle - Double Star Section Director

• April 2024 - Double Star of the Month

  STF 1645 (12 28 04.45 +44 47 39.5) is a fine pair in Canes Venatici found 3.5 degrees WNW of beta CVn (chara) and also 3/4 degrees north of the galaxy NGC 4449.

  

  T. W. Webb, in Celestial Objects for Common Telescopes reports an observation by Bird, a Victorian amateur astronomer, as a lovely pair as I ever saw. Webb also calls them yellow which chimes with the spectral types of F9V and KV given in the Washington Double Star catalog (WDS). In 1972 using a 10-in (25-cm) Newtonian I made the colours yellow and blue, whilst Sissy Haas using a 60-mm refractor finds both are peach-white.

  The stars have visual magnitudes of 7.5 and 8.1 and they are currently at 156 degrees and 9".8, but the stars were 16" apart when found by William Herschel in 1791. Nevertheless they both appear to be 147.7 light-years from the Earth and are moving almost directly south on the celestial sphere at the rate of 0".2 per year.

  Two degrees north of iota Librae is a coarse triangle of stars. The most north-westerly of this trio is SHJ 195 (15 14 28.13 -18 25 42.7) which again, despite the catalogue name, was discovered by William Herschel.

  

  Again this is a pair of F5 stars whose visual magnitudes are given as 7.5 and 8.1 in the WDS, which Admiral Smyth described as white and bluish. Although low in the Cambridge sky, I made a pair of measures in 2009 with the result 140.3 degrees, 45".59.

  Gaia EDR3 records parallaxes that are the same within the quoted errors, although the error of the bright star's parallax is some ten times larger than the typical value for that magnitude. The proper motions are also very similar - the stars are moving at 0".1 per year.

  Bob Argyle - Double Star Section Director

• March 2024 - Double Star of the Month

  In a sparse area of sky south of Ursa Major a faint triangle of naked-eye stars can be seen. They straddle the border between Lynx and Leo Minor. The north-eastern of these stars is 38 Lyncis (09 18 50.64 +36 48 09.3), a beautiful pair whose components are magnitudes 3.9 and 6.1 and whose position angle has reduced just 15 degrees in the 200 years since the pair was first measured. The separation is 2".6 so a moderate magnification needs to be used to separate the stars adequately.

  

  An orbit with a period of 2782.8335 years, published in 2019, needs to be regarded with some scepticism but the stars are certainly physical, as shown by the parallaxes which the Gaia satellite has provided. Speckle interferometry reveals that B is a very close pair with two observations only and a measured separation of 0".1 to 0".2, and the Washington Double Star catalog (WDS) indicates that the A component is a 2 day eclipsing binary, found by the TESS satellite.

  There are optical companions (C,D) at 79" and 173" respectively, both of magnitude 12.5, and a fainter component E (magnitude 14.7) which is 101" away, but which has the same large parallax and proper motion as the bright stars thus elevating the whole system to quintuple status.

  DUN 81 (09 54 17.66 -45 17 00.6) can be found in northern Vela. It is a bright and easy pair with components of magnitudes 5.8 and 8.2. When found in 1826 the PA was 239 degrees and separation 4".0. It is nearer to 5".6 today with no apparent change in position angle.

  

  These are distant stars but the parallax of each star suggests that they are related. The Gaia Data Release 3 (Gaia DR3) catalogue gives 1124 ± 6 and 1136 ± 16 light-years respectively for A and B, with similar proper motions. The primary is a B5 dwarf according to Simbad and whilst no spectral type can be found for B, its almost zero (B-V) colour suggests a hot, young star.

  Using a 200" search radius on the Gaia DR3 catalogue shows a number of stars around magnitude 20 at about the same distance as the bright pair although admittedly the errors in the parallaxes are large it may suggest that there is a loose cluster here.

  Bob Argyle - Double Star Section Director

• February 2024 - Double Star of the Month

  S 548 (07 27 40.54 +22 08 29.3) was noted by William Herschel in 1782 and catalogued by him as H V 66. He noted that the stars were very unequal and the larger one was pale red and the smaller one was dusky. The distance was noted as 34" 39'". Little has changed in the relative positions since then with the Washington Double Star catalog (WDS) giving 277 degrees and 35".3 for 2019.

  

  The pair can be found two degrees following delta Gem (see this column for Feb 13). The catalogue magnitudes are 7.0 and 8.9. In 1892 Thomas Espin added a faint companion of magnitude 12.4 at 24 degrees and 11".9. The three stars are unrelated. A lies 1800 light-years away, B is 940 and C is 8350 light-years distant.

  In southern Puppis right on the border with Vela is HJ 4093 (08 26 17.74 -39 03 32.3). This is a fine pair for the small aperture, it is triple in apertures of 40-cm or more, whilst the spectrograph reveals that component A is an Algol system called NO Pup.

  

  Stars A and B (magnitudes 6.5 and 7.1) are currently at PA 122 degrees and 15".0 having almost doubled their separation since discovery by John Herschel. Using the 26.5-inch refractor at Johannesburg, Willem van den Bos found that B is a close double (Ba,Bb), with stars of magnitude 7.9 and 8.1, currently 0".2 apart and a binary with a period of 103 years. In addition the WDS notes that Andrei Tokovinin finds a close component (D) only 5".4 from A but with a K magnitude of 17.3, but this star does not appear in the Multiple Star Catalogue.

  Examining Gaia DR3 with a field of radius 100" shows two stars of magnitudes 12.2 and 13.1 respectively 64" and 66" from A which share the same parallax, so perhaps this is a small cluster. There is, however, no data for the B component and the significant change in the AB distance since discovery may indicate that B is unrelated.

  Bob Argyle - Double Star Section Director

• January 2024 - Double Star of the Month

  Admiral Smyth's Bedford Catalogue contains an entry for 124 Tauri, and from his description of it as a coarse quadruple star of which components B and C are 6".2 apart in PA 318 degrees this appears to agree with Struve's catalogue entry of STF 755 (05 39 09.17 +23 17 17.7).

  

  According to the Washington Double Star catalog (WDS) A is magnitude 7.8 at 31 degrees and 149". This is HD 37387, a K2 giant star which appears close to the reflection nebula GN 05.36.2.The fourth star, D, is 50 degrees and 95" from A and has V = 11.1.

  There is no label for 124 Tauri in the the Cambridge Double Star Atlas (2nd edition) but the star appears about 30' W of a small right-angled triangle of 6th magnitude stars which in turn sits 2 degrees NNE of zeta Tauri. Smyth happened upon it in 1835 whilst looking for Comet Halley. He gives the colours as garnet (A), pale blue-white (B,C) and bluish (D).

  Some 3.5 degrees WSW of epsilon CMa is a triangle of 5th and 6th magnitude stars, the brightest of which is 10 CMa. In the same field, north following is HJ 3891 (06 45 31.20, -30 56 56.3), a double star discovered by John Herschel from the Cape of Good Hope.

  

  The primary is a B2III star of V = 5.7, and is accompanied by a 8.2 magnitude star at 223 degrees and 5".0. There has been little change since 1838. The stars have similar parallaxes and the mean distance to the system is 1968 light-years. The primary star is also called HP CMa.

  Just 90 arc-minutes to the SE is a wide pair swept up by the elder Herschel in 1782. H V 108 has stars of similar brightness (5.8 and 7.7) but they are separated by 43" in PA 66 degrees. The A component has a particularly close and faint companion just 0".6 away which was discovered from Robert Rossiter in South Africa, whilst the WDS notes that the B star has a variable radial velocity. Andrei Tokovinin regards this as a physical quadruple - the brighter stars appear in Gaia DR3 with respective distances of 634 and 623 light-years.

  Bob Argyle - Double Star Section Director

• December 2023 - Double Star of the Month

  About 4 degrees south-west of zeta Tau, is a coarse group of faint naked-eye stars, the brightest of which is 119 Tau, a red supergiant with V = 4.3. One and a half degrees WSW of 119 is 115 Tau, but between the two lie a couple of visual double stars.

  Nearest 119 Tauri is HJ 3275 (05 29 49.77 +18 24 58.3) which is a good target for a small aperture - Sissy Haas notes that they are a beautiful sight for an 8 x 50 finderscope. The stars have magnitudes 7.7 and 8.2 and they are separated by 56" in PA 21 degrees, and are physically unassociated.

  

  Both S. W. Burnham and R. G. Aitken have visited this system and added faint and close companions to both stars. In the case of A (= BU 891) it is a magnitude 12.6 star at 10".6 whilst for C (= A 2433) it is a 12.0 star at 1".4.

  A few arc-minutes preceding the A star is STT 108, a discovery by Otto Struve at Pulkovo. In this case the stars are magnitudes 6.7 and 10.4 and separated by 3".2 in PA 131 degrees so they should be visible in 15-20 cm.

  The constellation of Mensa is not well-served for visual double stars, in fact this is the first entry for this constellation since the series began in 2006! The Cambridge Double Star Atlas lists only four entries, the brightest of which is HRG 2 (05 01 09.45 -74 20 27.1).

  

  The discoverer name was entirely unknown to me. HRG is the label given to several dozen discoveries by Lawrence Hargrave from Sydney Observatory around 1880. Hargrave was born in Greenwich in 1850 and ended up as an assistant astronomical observer to H. C. Russell in Sydney. He is better known as an aviation pioneer and invented the box kite, a manned version of which he flew to a height of 16 feet. He died in Sydney in 1915.

  HRG 2 is a close pair which needs 15-cm and has remained at a separation of 0".8 since its discovery. The stars have magnitudes 7.4 and 8.0. This is a distant system - Gaia gives a parallax of 3.83 mas for the A component corresponding to a distance of 851 light-years.

  Bob Argyle - Double Star Section Director

• November 2023 - Double Star of the Month

  In the western reaches of Andromeda, about 10 degrees north of the faint naked-eye shape of Triangulum sits the glorious double star gamma Andromedae (see the entry for Dec 2006).

  Move 3.5 degrees SSE and 59 And (02 10 52.83 +39 02 22.4) will appear in the field of view. This is a fine pair for the small telecope, the magnitude 6.0 and 6.8 components being currently separated by 16".6 and position angle 36 degrees.

  

  Although it is usually known as STF 222, it was noted by William Herschel in July 1783. He noted the stars were reddish white and pale red and catalogued it as H 4 129. Herschel also noted a third star in view about 58 or 60 degrees south proceeding. In 1968 I noted a `superb' pair with colours of white and lilac. More recently Sissy Haas calls the colours of the stars pearl white and peach white.

  Gaia puts both stars at the same distance within the observational errors - 438 light-years.

  The United States Naval Observatory in Washington has a history of carrying out stellar astrometry which stretches back 150 years. During this long history they have occasionally noted new double stars which appear in the Washington Double Star catalog (WDS) under the catalogue name WNO.

  The first entry in this list, WNO 1 (00 53 12.46 -24 46 37.0), was found in 1876 and is a rather unequal pair (6.6, 8.9) which has moved very little since the first measurement. It is currently at 7 degrees and 5".4.

  

  The pair lies in Cetus about 1.5 degrees ENE of the bright spiral galaxy NGC 253. About 2.5 degrees N of WNO 1 is STN 3, another pair worth looking at. It has been closing since discovery in 1877 - 7.6, 8.4, 240 degrees, 2".

  Bob Argyle - Double Star Section Director

• October 2023 - Double Star of the Month

  About 15 degrees north of the centre of the Square of Pegasus is a relatively sparse region of stars but it does contain some interesting doubles. Near the south end of this space is STF 3050 (see this column for October 2016) whilst four degrees further north and slightly preceding is STF 3042 (23 51 52.409 +37 53 28.4) which consists of stars with magnitudes 7.6 and 7.8 and which are currently 6" apart at PA 87 degrees. I found them to be a delightful pair of yellow stars whilst the entry in Sissy Haas' volume notes them as an easy white pair.

  

  They likely form a long period binary as Gaia DR3 notes that the parallaxes differ by just 0.009 milli-arcsecond. The stars lie 226 light-years away and both appear white, but are contrasted against an orange star 3' to the south-east. Unrelated to STF 3042 this is the long period variable candidate V 397 And (V = 8.9) which was picked up by Hipparcos. The listed amplitude is just 0.03 magnitude, and the spectral type is M5.

  Move 2.5 degrees west and slightly south to find the unequal pair STT 501. The stars are 6.5 and 10.6 and have PA 162 degrees and separation 15".

  41 Aqr (22 14 18.02 -21 04 28.9) sits four degrees north of the Aquarius—Piscis Austrinus border and is just four degrees west of NGC 7293 the Helix Nebula.

  

  It is number 56 in William Herschel's third double star list and is an easy pair for the small aperture. The stars have magnitudes 5.6 and 6.7 and the primary is a KO giant; a rather ambiguous note in the Washington Double Star catalog (WDS) implies that it has a composite spectrum which is strengthened by the fact that Gaia DR3 quotes an error in the parallax about three times higher than might be expected for a star of this magnitude. The stars have similar parallaxes and proper motions and as the parallax of the companion is more precise then both stars must lie 239 light-years away.

  This is a pretty pair; Smyth recorded colours of topaz yellow and cerulean blue whilst Webb noted reddish and blue and Sissy Haas yellowish-peach and pale violet.

  The WDS also notes two more distant and unrelated field stars: an 8.9 at 45 degrees and 210" (C) with another fainter object of V = 11.6 12" away from C in PA 255 degrees.

  Bob Argyle - Double Star Section Director

• September 2023 - Double Star of the Month

  Four degrees north and slightly east of beta Pegasi is the Herschel pair H N 11 (23 07 27.73 +32 49 3.3) but now known in modern atlases and handbooks as STF 2978. It was found by Herschel on Sep 6th, 1784 and included in his third and last double star compilation. This list was not published until 1821 when it appeared in the first volume of Monthly Notices of the RAS. This is an easy and pretty pair with components having magnitudes 6.4 and 7.5.

  

  Over the last two centuries there has been but little motion in position angle and separation. A recent measure in 2018 put the stars at 145 degrees and 8".3. Gaia DR3 gives precise distances for the two stars, 450.31 and 453.31 for A and B respectively. Star A is an Algol variable with an amplitude of 0.22 magnitudes and is known as V343 Peg.

  HWE 91 in Piscis Austrinus (22 55 56.89 -32 32 2.9) was also independently discovered by two astronomers. The name represents Herbert Howe, astronomer at Cincinnati Observatory who used an 11-inch Clark refractor on a survey for new double stars. In autumn 1877, using a magnification of x230, he examined the star delta PsA and found a very faint and relatively close companion and estimated the magnitudes as 5.0 and 10.5. Two years later and unaware of Howe's finding, S. W. Burnham came across the star using his 6-inch Clark refractor but observing this time from Mount Hamilton in California. He called it BU 772 but later conceded that Howe had priority of discovery.

  

  The current position is 250 degrees and 4".9 but there is little motion in this system. The stars are equally distant from us within the errors of the parallaxes and they have similar proper motions. Gaia DR3 also indicates that the stars have G magnitudes of 3.94 and 9.80. Hwe 91 is easily found lying as it does just 3 degrees south of Fomalhaut.

  Less than one degree WSW from Hwe 91 is HJ 5367 (also gamma PsA) which presents a similar test. Here the stars are magnitudes 4.5 and 8.2 and they are currently 4".1 apart in PA 255 although this has increased from 2".5 when found by John Herschel in 1836.

  Bob Argyle - Double Star Section Director

• August 2023 - Double Star of the Month

  The constellation of Sagitta is one of the few on the sky that vaguely resemble what they are purporting to represent (Another in my opinion is Leo). The bright stars of Sagitta trace the shape of an elongated `Y' with eta and gamma in the `shaft' which then divides at delta to form the `fletching'.

  Just 45 arc-minutes NE is zeta Sge (19 48 58.65 +19 08 31.1). A small telescope will reveal the duplicity of the star - the primary is magnitude 5.0 whilst the companion is 9.0 and is currently about 8".3 distant, a system first observed by William Herschel and catalogued H 2 30. Alvan G. Clark used this star to test one of his firm's famous refractor objectives and in the course of doing so found that the primary was a very close, but equally bright pair, which subsequently turned out to have a period of 23 years.

  

  Maximum separation of 0".25 occurs in 2038, and there is some evidence that the bright star (but which component?) is also a spectroscopic binary. It is this complex structure that prevents the star appearing in the Gaia DR3 catalogue, although the magnitude 9 star is in there and is 328 light-years away.

  The double stars of James Dunlop include some of the most beautiful pairs in the southern hemisphere. No exception is DUN 227 (19 52 37.76 -54 58 15.7) which is a lovely pair for the small aperture.

  The stars are magnitudes 5.8 and 6.4 and in 2013 when I measured them they were at 148 degrees and 22".8, although there has been little change since they were first noted in 1826. There is a fine colour contrast. The primary is a K0 giant whilst the secondary has spectral class A2V. Hartung noted colours of orange-yellow and white whilst more recently Ross Gould found deep yellow and cream.

  

  Gaia DR3 places the primary at 407.50 light-years and its companion at 414.38, and when the errors on each distance are considered the two stars are formally at different distances, but if they form a a binary system then the period is likely to be very long. Its worth looking out for - find it about 5 degrees WNW of the 1.9 magnitude star alpha Pavonis.

  Bob Argyle - Double Star Section Director

• July 2023 - Double Star of the Month

  STF 2367 (18 41 16.36 +30 17 40.9) is a challenge to the observer with 30-cm available. It is always a difficult object throughout its 92 year orbital cycle, partly because the apparent orbit is inclined and very eccentric and so at times the separation of the stars dips below 0".02 and the angular motion amounts to 1 degree every 6 days. At apastron the stars reach a maximum separation of 0".45 which is where they are now, (actually apastron is reached in 2027), so this is a very good opportunity to resolve this difficult pair.

  

  The stars have magnitudes of 7.7 and 8.0 but another star of magnitude 8.8 is 14" distant and there are three much fainter components at 22" (mag. 12), 42" (15.1) and 152" (11). The system lies in Lyra in a region to the south-west of beta and gamma Lyrae but with no obvious pointers to aid location. There is a curve of stars nearby which can be found from gamma Lyrae.

  H 6 50 (18 49 40.96 -05 54 46.2) lies in Scutum close to the Wild Duck Cluster (M11) which is just 25 arc minutes to the south-east. The Cambridge Double Star Atlas (2nd ed.) also shows the pair STF 2391 (18 48 39.49 -06 00 15.5) nestling just 15 arc minutes to the west-south-west.

  

  H 6 50 is a very wide and somewhat unequal pair. The magnitudes are 6.2 and 8.2 and the stars are 112" apart in 2016 at PA 171 degress. A much fainter star, magnitude 12.5 was added by Burnham in 1879. It is due north and 25 arc seconds distant. A rather puzzling note in the Washington Double Star catalog (WDS) Notes identifies H 6 50 AB with STF 2391 but it is difficult to see how this can be. STF 2391 has, according to the WDS, magnitudes of 6.5 and 9.6 at 332 degrees and 38".

  Bob Argyle - Double Star Section Director

• June 2023 - Double Star of the Month

  HU 149 (15 24 35.30 +54 12 46.1) is from the catalogue of William J. Hussey, a well-known observer and discoverer of visual double stars who worked in the USA and who was a leading instigator of the Lamont-Hussey Observatory which was set up in Bloemfontein, South Africa and equipped with a 27-inch refractor with which R. A. Rossiter spent a lifetime on his own catalogue of discoveries.

  

  Many of the Hussey pairs tend to be close and difficult and HU 149 was only 0".2 apart when found in 1900, but it has since widened and according to the 770 year orbit of Zirm (2015), the stars should now be about 0".65 apart. A recent measurement by Andreas Alzner with a 32-cm Cassegrain confirms that the separation is close to 0".7. The pair has just passed one of the maximum separations in the apparent orbit and will now close to 0".1 by the 2180s. The stars are almost equally bright at V = 7.5 and 7.6 respectively.

  The name Jerome de Lalande appears in the Washington Double Star catalog (WDS) although just 4 pairs appear under his discovery prefix (LAL) - numbers 53, 123, 192 and 193. All are worth looking out for and this month I have chosen LAL 123 (15 33 09.52 -24 29 15.9) which sits roughly half-way between the three bright stars in the head of the Scorpion and sigma Librae.

  

  The stars are magnitudes 6.9 and 7.0 and were 9".3 apart in 2016 at PA 302 degrees. This is an easy object for 10-cm but the possessors of larger telescopes may like to examine the B star more closely. This was found to be double by T.J.J. See and has turned out to be a binary of 61 year period. The pair passes widest separation the next few years and at the moment the stars are just 0".33 apart - calling for at least 40-cm to resolve - and they have visual magnitudes of 7.0 and 8.2.

  The A component is an even closer pair and was found by speckle interferometry. CHR 232, at 0".1 and a period of 16.47 years, is beyond all but the largest apertures especially as there is almost a two magnitude difference in the brightness of the components.

  Bob Argyle - Double Star Section Director

• May 2023 - Double Star of the Month

  STF 1850 (14 28 33.239 +28 17 25.9) is a wide and fairly bright pair which can be found 3.5 degrees WNW of the spectacular pair epsilon Boötis also known as Pulcherrima - 'most beautiful' (see the column for May 2020).

  

  The stars have changed little in relative position since they were catalogued by F. G. W. Struve and they are currently separated by 25" in PA 263 degrees. Although both are given spectral class A1V by the Washington Double Star catalog (WDS), the stars are magnitudes 7.1 and 7.6. They are rather distant, and Gaia DR3 gives the mean distance of the two as 873 light-years with an uncertainty of 1% in each case.

  SHJ 263 (18 17 51.13 -18 47 54.6) sits in the yellow area marked Star Cloud M24 in The Cambridge Double Star Atlas, although it is not labelled. It is probably more accurate to define SHJ 263 as a foreground grouping of stars.

  The main components are 6.8 and 9.3, separated by 54" in PA 11 degrees. The WDS lists six other companions of magnitude 12 and 13 within 30" of A and B. The only entry in Gaia DR3 for this group is that of the primary star which is given a distance of 1038 light-years whereas the distance of the star cloud as a whole is given as 10,000 light-years in the on-line literature.

  

  Twenty minutes NE is SHJ 264 (18 18 43.26 -18 37 10.8) and the bright pair is called AC. These stars are magnitudes 6.9 and 7.6 with a separation of 17" in PA 51 degrees.

  S. W. Burnham has added a couple of more difficult tests for 25-cm aperture. A is a close pair, the companion (B) is magnitude 8.2 and only 0".5 distant from A and has moved from PA 155 degrees in 1878 to a current value of 136 degrees. Some years before this, Burnham had noted a 13.1 magnitude star (D) 6" away from C whilst Philip Fox added a 10.8 at 145" from A.

  The primary star is a B0Ib supergiant and Gaia DR3 gives distances of 1826 and 4672 light-years respectively for A and B.

  Bob Argyle - Double Star Section Director

• April 2023 - Double Star of the Month

  In the constellation Coma Berenices, in addition to the famous cluster of galaxies, there is also a grouping of naked-eye stars about 13 degrees north of the galaxy cluster. Taking the pairs 12 and 13 Comae and 14 and 16 as the vertices of an isosceles triangle, then the third vertex is occupied by STF 1633 (12 20 41.33 +27 03 16.4) a beautiful pair described by Webb as Very pretty. Solitary. Observing it with a 21-cm reflector I found the colours to be yellow and delicate light blue.

  

  Of the four bright stars in the 'triangle' only 12 appears to be in the double star catalogue. Also known as SHJ 143, this is a wide, unequal pair in 15-cm (magnitudes 4.9, 8.9, 167 degrees, 65") which should also show the more elusive 11.8 magnitude star at 57 degrees, 37" to A.

  Just over a degree south of the Sombrero galaxy (M104), and over the border into Corvus is STF 1669 (12 41 16.22 -13 00 53.6) a beautiful visual double which turns out to be at least a physical quadruple star under the gaze of the spectroscope.

  

  The AB components are both magnitude 5.9 and the current position and angle and separation is 314 degrees and 5".3. This shows a change of +15 degrees and -1".6 since being measured by Struve in 1828.

  A third star (C) of magnitude 10.3 is 46" away in position angle 228 degrees. Strangely, Gaia DR3 records similar values for the trigonometrical parallax of stars A and C (equivalent to a distance of 266 light-years) whilst the B component is significantly nearer us (235 light-years) but is certainly regarded as physical by Dr. Andrei Tokovinin is his Multiple Star Catalog (MSC). The MSC notes that A has a period of 44.5 days whilst that of B is 3.15 days.

  Bob Argyle - Double Star Section Director

• March 2023 - Double Star of the Month

  At the extreme eastern edge of Ursa Major, where it projects slightly into Leo Minor, there is a triangle of three naked-eye stars - 42 and 43 Lyn and a brighter but unlabelled star of Vmag 4.8 which turns out to be HD 82741. About a degree south of 42 Lyn is STF 1374 (09 41 21.88 +38 57 01.9) which is a long period visual binary.

  

  Since being measured by F. G. W. Struve (275 degrees, 3".3) the pair has closed slightly with increasing angle and is now to be found at 312 degrees, 2".8. This motion is apparently sufficient to allow the calculation of an orbit with a period of 1815 years.

  This is a relatively unequal pair with magnitudes of 7.3 and 8.7 given in the Washington Double Star (WDS) catalogue. In the neighbourhood is STF1369 (7.0, 8.0, 150 degrees, 25".1) which is just north of HD 82741. Another star of magnitude 8.4 lies 116" distant and the A component is the close pair COU 2084 with a period of 50 years and a current separation of 0".1.

  SEE 115 (09 37 12.65 -53 40 06.6) lies in southern Vela near the border with Carina. It sits neatly at the centroid of the isosceles triangle formed by φ, L and κ Vel.

  

  It was discovered by T. J. J. See, but in the 125 years which has elapsed since then the position angle has only increased by 8 degrees whist the separation remains at 0".7. This is a good test for a 20-cm aperture as the stars are magnitudes 6.1 and 6.3.

  Although it has an entry in Gaia DR3, there is no information on either parallax or proper motion even though a relatively wide pair such as this might be expected to have been resolved satisfactorily by this stage of the satellite mission.

  Just half a degree WNW is the open cluster NGC 2925 some 10'x10' in size and discovered by Sir John Herschel.

  Bob Argyle - Double Star Section Director

• February 2023 - Double Star of the Month

  One and a half degrees just east of the fine but unequal binary pair delta Gem (see this column for February 2013) is the pair STF 1108 (07 32 50.63 +22 53 15). The star is displayed on page 8 of the second edition of MacEvoy and Tirion's fine Double Star Atlas (CUP) but without a label. Having checked back over my file of measures I note that this pair does not appear in it, nor does it appear in my earlier visual survey carried out with a 21-cm in the late 1960s.

  

  The Washington Double Star catalogue gives the visual magnitudes as 6.6 and 8.2 whilst Gaia DR3 notes G mags of 6.3 and 8.8. There has been little motion since the pair was found by F. G. W. Struve. DR3 notes that the parallaxes are the same within the errors (and indicate a distance to the pair of 643 light-years) although these are some ten times larger than might be expected for stars of this brightness.

  Half a degree south is the unequal, wide pair 63 Gem and about a degree to the south-west is STF 1081 (7.7, 8.5, 204 degrees, 1".9, slowly increasing)

  2 Pup = STF 1138 (07 45 29.14 -14 41 25.7) lies in a string of naked eye stars which stretch about 30 minutes of RA along the line of south declination 15 degrees. Other objects in this area include M46 and M47 and the binary STF 1104 (see this column for February 2020). STF 1138 is about a degree due east of M46 and is a beautiful, easy pair for the small aperture.

  

  During a visual survey carried out in the late 1960s I found colours of yellow and lilac whereas Admiral Smyth records hues of silvery white and pale white for `2 Argo Navis'. The stars of magnitudes 6.0 and 6.7 were separated by 16".7 at PA 340 degrees when I measured them in 2015, having closed from 17".4 when measured by William Herschel in 1782.

  A third star of magnitude 10.6 lies 100" away in PA 229 and is some five times further away than the stars in the pair. They both have very precisely determined parallaxes thanks to Gaia DR3 which puts them 279 light-years away with an uncertainty of about 1 light-year.

  Bob Argyle - Double Star Section Director

• January 2023 - Double Star of the Month

  STF 845 lies in Auriga (06 11 36.59 +48 42 39.6) and is also known as 41 Aurigae and it is also H 3 82, observed by William Herschel in November 1782 and recorded by him as a pretty double star. It is a beautiful pair for the small telescope. The writer first saw it in 1969 using the 12-inch reflector of a friend. At x208 the colours were noted as yellow and lilac. The Washington Double Star (WDS) catalogue gives the spectral types as A1V and A6V.

  

  James Dunlop found the 30th entry in his catalogue in 1826. It is located in Pictor at 06 29 40.03 -50 14 20.7 some 2.5 degrees NNE of Canopus. It offers a fine sight to the small telescope user.

  The stars are magnitudes 6.0 and 8.0 and at present they are separated by 11".5, being 14" apart when observed by Dunlop. The position angle has hardly changed between these two epochs and is currently 312 degrees.

  

  In 1871, Russell, observing with an 11-inch refractor at Sydney Observatory, found the primary star to be a close double. R 65 has a period of 111 years according to Docobo and Ling in 2021. The stars are almost equally bright but according to the orbit, which is extremely eccentric, the separation never exceeds 0".7 and, at times, drops to 0".012 which was the case in mid-2021. By 2027 the stars will be at least 0".4 apart.

  In the 1890s Harvard Observatory was site testing in the Peruvian Andes and was using a 13-inch refractor. This telescope discovered several hundred new double stars amongst which was the fainter companion of DUN 30. This has also turned out to be a binary pair of period 101 years, giving a predicted position of 224 degrees, 0".44 in early 2023. The stars have visual magnitudes 8.0 and 8.7.

  Bob Argyle - Double Star Section Director

• December 2022 - Double Star of the Month

  47 Tauri = BU 547 (04 13 56.39 +09 15 50) could be be described as a typical Burnham discovery. The stars are both unequal and close, a type of visual binary that Burnham seemed particularly adept at finding.

  The V magnitudes are 5.1 and 7.3 and at present they are 1".3 apart in position angle 339 degrees. They have moved slowly apart since discovery and seem likely to continue this trend for some years to come.

  

  Hipparcos determined a parallax of 9.83 mas for the whole system whilst Gaia DR3 contains only the results for the primary star. This comes in at 7.96 mas.

  I was able to measure the pair on 2 nights in 1993 using the 8-inch Cooke refractor at Cambridge Observatories, but it was not an easy object. 47 Tau is about 8 degrees south-west of Aldebaran and sits in a low power field with magnitude 4.3 mu Tau. A good 6-inch should show the stars which have been allocated a preliminary orbit of 479 years.

  The Washington Double Star (WDS) catalogue also lists an unassociated field star of magnitude 13 which is at 228 degrees and 28".

  SKF 949 (03 53 33.33 -46 53 37.2) came to the attention of Brian Skiff at Lowell Observatory whilst examining stars in Nancy Houk's spectral type reclassification project in order to add to his extensive collection of spectral types.

  

  Examination of Vizier showed a common proper motion companion, V = 8.5 and spectral type G, some 76" distant from the K2III primary which has V = 6.1. and which is almost due south of its companion. Gaia DR3 confirms that the stars have similar parallaxes, indicating that they are 359 light-years distant.

  This is an easy pair for the small telescope and may even be visible in a pair of suitable binoculars. The system is in a unremarkable area of Horologium, about 6 degrees south-west of alpha.

  Bob Argyle - Double Star Section Director

• November 2022 - Double Star of the Month

  In the corresponding column for this month in 2021, I mentioned a number of pairs close to the bright, close binary phi Andromedae. As promised this piece will include the pairs MAD 1 (01 00 35.58 +47 19 14.6) and STT 21 (01 03 01.54 +47 22 34.1).

  

  STT 21 is about 1.5 degrees west of phi And and is now considerably easier to observe than in the 1840s when the separation was 0".6 The current orbit of 450 year period was published by W. D. Heintz in 1966 and predicts a position of 176 degrees and 1".2 for late 2022. The apparent orbit is one of high eccentricity and inclination and the apparent motion appears to be almost linear. The magnitudes are 6.8 and 8.1 and the pair should be resolved in 15-cm aperture.

  Johann Madler (1794 - 1874) succeeded F. G. W. Struve as Director of Dorpat Observatory where the main instrument was Fraunhofer's 9.3-inch refractor with which Struve had carried out his great survey of double stars. MAD 1 can be found half a degree west of STT 21. It is a long period binary with main components of magnitudes 7.7 and 9.1. When first observed the separation was 1" but the stars have been closing slowly and at the present time are 0".74 apart in PA 356 degrees, if the 2127 year orbit currently in the catalogue is accurate.

  HJ 5437 (00 00 34.35 -53 05 51.8) sits in southern Phoenix 20 arc-minutes to the south-east of the red giant pi Phe (V = 5.1).

  

  It is not labelled in the second edition of the Cambridge Double Star Atlas, and was found by John Herschel in 1836 at 340 degrees and 2".5 since then has been slowily closing. In 2019 the stars were at 340 degrees and 1".4 and as the magnitudes are respectively 6.9 and 9.9 this is a rather difficult pair which probably needs at least 20-cm.

  Although the observed motion only amounts to 44 degrees, a premature orbit with a period of 904 years is listed in the online USNO 6th orbit catalogue. Incidentally the USNO double star website is back on-line and can be found at crf.usno.navy.mil. The WDS catalogue is available again but the latest update appears to have been about 2 months ago.

  Bob Argyle - Double Star Section Director

• October 2022 - Double Star of the Month

  Just to the north-east of the Blue Snowball Nebula, NGC 7662, in Andromeda is an asterism of naked-eye stars in the shape of a `Y'. The star at the position where the figure forks is kappa And and just half a degree to the west is STT 500 (23 37 32.53 +44 25 44.5).

  

  This is a long-period binary, a preliminary orbit computed in 1981 gives a period of 351 years, and an aperture of 25-cm is probably needed to divide the stars. The V magnitudes are 6.1 and 7.4 and when the binary nature of the system was discovered in 1843 by Otto Struve at Pulkovo, the stars were 0".3 apart and at a position angle of 273 degrees. They are currently separated by 0".4 and they have moved around to a position angle of 21 degrees.

  Gaia DR3 has an entry for the brighter star but no data on parallax and proper motion. The Hipparcos satellite gave a distance of 807 light-years but with a formal error of 104 light-years. At a distance of 118" and a position angle of 334 degrees is a more distant star of V = 11.0.

  DUN 249 (23 23 54.52 -53 48 31.5) is located in the extreme SE corner of Grus and about 2.5 degrees due east of kappa Gruis. Like many other pairs in the Dunlop catalogue this is a wide and bright pair, easily seen in the smallest of apertures. I measured it using the 26.5-inch refractor at Johannesburg in 2016 when the position angle and separation were 211 degrees and 26".4 respectively.

  

  Gaia DR3 gives distances of 408 and 494 light-years for the stars which, nevertheless, have very similar proper motions. The errors on the parallaxes nowhere near overlap so it seems that these stars are not connected physically.

  Head 3 degrees north and there is a line of DUN pairs stretching along the 51st parallel of south declination including DUN 246, DUN 248 and DUN 250.

  Bob Argyle - Double Star Section Director

• September 2022 - Double Star of the Month

  I noted, rather surprisingly, that this column had yet to deal with gamma Delphini (20 46 39.50 +16 07 27.5) so this is a good opportunity to do so. One of the finest pairs in the sky because of its glorious colours, gamma has been known since the time of James Bradley (1755).

  

  The stars are magnitudes 4.4 and 5.0 and they are currently 8".8 apart in position angle 264 degrees. Although there has been little motion between the two since Bradley's time, an orbit with a period of 3249 years has been calculated which predicts a closest approach of 0".8 in around 2300. The apparent orbit as plotted in the USNO 6th orbital catalogue shows little, if any, sign of orbital curvature but the stars have the same parallax, if only just.

  The results of radial velocity observations of both stars with the intention of finding exoplanets have not confirmed any.

  Smyth observed the pair on many occasions and found colours of yellow and light emerald, whilst I made them golden yellow and blue-green when I first observed them during a youthful survey of double stars in 1968. Thirty arc-minutes south preceding gamma is the pair STF 2725, also a long-period binary. It has stars of magnitude 7.5 and 8.2 at 12 degrees and 6".2.

  Right on the border between Sagittarius and Microscopium, R 321 (20 26 52.95 -37 24 10.5) is a fine binary which is well worth sweeping for.

  

  Found by H. C. Russell at Sydney Observatory, this pair has a period of 186.4 years and the motion of the stars is in decreasing position angle (retrograde). In 1880, when first observed, the stars according to Russell were 1".1 apart , then by 1950 they had closed up to 0".26 before later opening out. At the present time they are 1".6 apart, with widest separation occurring around 2029. The stars have visual magnitudes of 6.2 and 7.9 and are orange in hue - each is an early-K dwarf. They should be comfortably divided with 10 or 12-cm.

  Whilst in the area, move about 3 degrees SSW, and you will find DUN 230 (20 17 49.68 -40 11 05.1) which is an easy object for the smallest aperture. The two components are both of late-F spectral type and whose visual magnitudes are 7.4 and 7.7; the stars are 9".7 apart in PA 117 degrees, and lie 257 light-years away.

  Bob Argyle - Double Star Section Director

• August 2022 - Double Star of the Month

  In the column for August 2019, I described 16 Cygni, a bright and easy double foundabout 6 degrees north of delta Cygni. Starting with 16 Cygni, and moving west about 5 degrees a coarse triple of 6th magnitude stars will appear in the finder field. The easternmost of these is STF 2486 (19 12 05.03 +49 51 20.7) which consists of stars with V = 6.5 and 6.7 which are currently 7".1 apart in position angle 202 degrees.

  

  This is a relatively nearby (82.5 light-years) binary pair whose period has been found to be 1459 years but clearly this is a preliminary value, since the position angle has decreased but 21 degrees since 1819. The proper motion of the stars is more than 0".6 per year, so they are rapidly leaving behind star D (V = 11.1) at 196", but rapidly approaching star C which is V = 13.2 at 27".

  Sir William Herschel's third and last catalogue of double star discoveries includes many pairs which are low in the sky from the UK, and two of them were found close to omicron Sagittarii.

  

  The first of these, H N 126 (19 04 20.28 -21 31 53.7), is the more difficult of the two. It is about 15 arc-minutes NW of omicron and consists of stars with magnitudes 7.9 and 8.1 which are currently 1".3 apart in PA 183 degrees. This binary has almost completed 270 degrees of its apparent orbit since discovery, has an orbit of about 502 years, and lies 168 light-years distant.

  About 1.25 degrees due south of omicron is H N 129 (19 04 14.20 -22 53 47.5), significantly easier at 309 degrees and 8".3, but with two components which are considerably unequal (V = 6.9 and 9.2).

  Bob Argyle - Double Star Section Director

• July 2022 - Double Star of the Month

  Delta Serpentis (15 34 48.14 +10 32 20.0) is a bright, easy, and attractive pair in small apertures. It lies in a rather undistinguished part of the sky below Corona Borealis and a quick way to find it is to locate alpha CrB and then move 16 degrees south in declination.

  

  Since it was found by Herschel in 1782, when he noted that it was a fine double star with colours of white and greyish, it had been slowly widening until about 2020 since when it has begun to close.

  Looking at the apparent orbit, there is no question that this is a binary system and yet Gaia EDR3 puts the stars at 302±14 and 172±1 light-years respectively for the primary and secondary components. Neither of these stars appears in the Hipparcos catalogue and the large error for the primary star suggests that it may be a system of higher multiplicity. The plot of the apparent orbit in the Washington Double Star (WDS) Sixth Orbit Catalogue shows rather large errors on the earlier observations of this pair which are difficult to reconcile with the ease of measurement that such a system should present, again possibly evidence for an oscillation in the movement of one of the stars.

  I have made eight sets of mean measures since 1993 and the separation has ranged between 3.94 to 4.43 arc-seconds. The orbit gives 171 degrees 4".0 for mid-2022.

  H.F. Donner was a student at the University of Michigan when he was sent out to South Africa to assist Robert Rossiter with the survey for southern double stars at the Lamont-Hussey Observatory in Bloemfontein using a 27-inch refractor. He stayed for 6 years and accumulated a total of 1030 new pairs, many of which are faint and close. Xi Ophiuchi (17 21 00.37 -21 06 46.5) is number 832 in his catalogue of discoveries.

  

  The primary magnitude is V=4.4 and companion V=8.9. This is a difficult pair for the small aperture and probably needs at least 15-cm to see distinctly. The current separation is only 1".7 with the corresponding position angle 20 degrees. A orbit was calculated with a period of 421.5 years which is clearly rather premature since the motion since discovery amounts to only 40 degrees.

  There is a very faint star at 261 degrees, 11" which is K=13.7 and was discovered during an imaging survey for exoplanets with the 200-inch reflector, but it remains unconfirmed and there is no subsequent astrometry.

  Bob Argyle - Double Star Section Director

• June 2022 - Double Star of the Month

  Gamma Herculis (16 21 55.24 +19 09 10.9) appears in William Herschel's first double star catalogue as H V 19. Gamma is easily found as it sits just 3 degrees SW of the most south-westerly star, beta, in the Keystone of Hercules.

  

  It is a late-A giant star of magnitude 3.8 and in 1780 Herschel found a magnitude 10 companion at 251 degrees and 41".8. By 2013 the position angle had reduced to 226 degrees and the separation was 43".3. This change is due entirely to the difference in proper motions. The Gaia satellite finds the primary star is 193 light-years away whilst the companion is more than 1,770 light-years distant. An additional optical companion, of V = 13.3, can be found at 298 degrees and 82" from B.

  The Washington Double Star Catalog (WDS) states that A is a spectroscopic binary, but it does not appear in the Ninth Catalogue of Spectroscopic Binaries (SB9), compiled by the late Dr. Dmitri Pourbaix. Additionally an infra-red survey has revealed an object 8" from A which has a K magnitude of 8, and thus a V magnitude of perhaps 10-11. Whether it is connected to A is not known.

  Eta Lupi (16 00 07.33 -38 23 48.1) is a hot, early B-type star of magnitude 3.4 which lies in Lupus, 16 degrees due south of delta Sco, the middle of the three bright stars in the Scorpion's head. Eta is part of the Upper Centaurus - Lupus association and thus has a similar distance (460 light-years) and proper motion to a group of stars in the neighbourhood.

  

  The WDS shows three companions, two of which have similar parallaxes to eta, and all of which appear white in the eyepiece. The B component is magnitude 7.5 at a distance of 14" and position angle 19 degrees, a relative position which has changed little since the pair were first noted by Rumker from Parramatta. C is magnitude 9.4 at 115" and PA 248 degrees. This star was also noted by Rumker who appears to have estimated a distance of 1 arc-minute but this may be in error (possibly a misprint for 2 arc-minutes?) as the star moves through space together with the brighter components. The star listed in the WDS as ANT 2 AD (the designation refers to Rainer Anton who measured the previously uncatalogued component D in 2007) does not appear to belong to the association and is almost four times more distant. It can be found at 293 degrees and 136".

  Searching the Gaia EDR3 catalogue within a radius of 200 arc-seconds from eta shows a magnitude 14 star which also moves with, and is equally distant from us, as A, B and C, and which can be found at 290 degrees and 118".

  Bob Argyle - Double Star Section Director

• May 2022 - Double Star of the Month

  In the second edition of the Cambridge Double Star Atlas, SHJ 191 (14 59 34.58 +53 51 36.7) is the most northerly labelled double star in the constellation of Boötes and lies just over a degree south of the border with Ursa Major.

  

  It was measured by Sir James South on 1823, April 27 and again on May 3 of that year using his 5-foot equatorial and he found the mean separation to be 40".85 (the text in the catalogue gives 48".85) and the mean position angle 343° 10'. These values are little changed today and whilst the proper motions determined by Gaia in the EDR3 catalogue are similar, the parallaxes appear to be significantly different, with 9.590 ± 0.122 mas for the A component and 8.875 ± 0.015 mas for B. The large error for A may indicate additional structure.

  The Washington Double Star Catalog (WDS) gives magnitudes of 6.9 and 7.6 for A and B and both stars are F1 dwarfs. An interesting feature in the finder chart plotted using SIMBAD is the presence of a nearby pair of faint red stars and which are described at greater length in the upcoming Double Star Section Circular (DSSC) Number 30.

  One of S. W. Burnham's more attractive pairs for the small telescope is mu Lib - also known as BU 106 (14 49 19.09 -14 08 56.3). It is easy to find sitting just 2 degrees north and slightly west of Zubenelgenubi, or alpha Cap, which is, in turn, a fine very wide pair, the stars being 376" apart.

  

  Mu Lib was discovered by Burnham with his 6-inch Clark in 1873 when the stars were separated by 1".3, and since then they have slowly separated (currently 1".9) with a change in position angle of just 27 degrees, even so an orbit exists for the system with a period of 614 years. A and B have V magnitudes 5.6 and 6.2 and whilst the A component does not appear in EDR3 the B star has a measured distance of 245 light-years. A magnitude 12.6 star lying at PA 232 degrees and 27" from A has the same parallax and would appear to be physically connected to the binary pair.

  Bob Argyle - Double Star Section Director

• April 2022 - Double Star of the Month

  The two pairs in this month's column have a common feature. Each is accompanied at a distance by a ninth magnitude companion.

  STF 1781 (13 46 06.75 +05 05 56.1) is a visual binary with a period of 179 years which is 206 light-years away, according to Gaia EDR3. It can be found in Virgo, near the border with Boötes, about 2 degrees NE of 84 Vir, itself a double star (5.6, 8.3, 227 degrees, 2".7).

  

  Although the components of STF1781 are rather faint (V magnitudes are 7.9 and 8.1) the current position, 200 degrees and 1".1, indicates that the stars should be resolvable in 15-cm aperture. They are currently closing and by 2050 they will be separated by 1".2. I have not observed this system - in the late 1970s the stars were less than 0".3 apart and have been widening ever since. More recently Dr. Andrei Tokovinin found a common proper motion companion of magnitude 9.8 at PA 128 degrees and distance 1594".

  The primary of JC 16 (11 29 38.35 -24 27 50.6) is just above naked-eye visibility at V = 5.8. Eight arc-seconds away in PA 82 degrees is the B component of V = 8.6. The catalogue number tells us it was discovered by Captain Jacob from India in 1847. He also added a third star of V = 8.9 at 120".

  

  The Washington Double Star Catalog (WDS) notes that of 2018 this had increased to 166", but it is difficult to reconcile this large change with the very similar proper motions that all three stars have. In fact Gaia EDR3 gives a significantly different parallax for B than it does for A and C.

  The coarse triple is a fine sight in small apertures.

  Bob Argyle - Double Star Section Director

• March 2022 - Double Star of the Month

  15 Boo (14 14 50.85 +10 06 02.2) is a naked-eye star about 9 degrees due south of Arcturus. Discovered as a close double by Gerard Kuiper in 1936, it has shown little motion since that time. I measured it on 3 nights in 2001 using the 8-inch Cooke refractor at Cambridge and got a relative position of 119 degrees, 1".1. The Washington Double Star Catalog (WDS) indicates that in 2015 the position was 111 degrees and 0".8.

  

  The difficulty of resolving this pair is exacervated by the difference in magnitude between the components: 5.4 and 8.4. In 2020, Dr. Marco Scardia using the 1-metre Epsilon telescope at the Plateau de Calern near Nice, found that B was itself a close visual pair: 8.4, 10.0, 175 degrees, 0".2.

  In an area just north of a line drawn between theta and iota Centauri there is a rich profusion of visual double stars. One of the more interesting is HWE 28 (13 53 32.75 -35 39 51.2) a binary whose components shine at magnitudes 6.3 and 6.4. The period is 363 years and at the present time the stars are 1" apart in PA 317 degrees. An 8.7 magnitude star lies 68" away in PA 7 degrees and two much fainter stars can be seen - a 12.3 at 150 degrees 28" and a 14.8 at 133 degrees 38".

  

  Just 1 degree due west is HWE 94 (13 48 55.07 -35 42 15.1) where the components are 6.6 and 10.2 and the separation 12" in PA 0 degrees. The HWE stars were found by Herbert A Howe, who started his career at Cincinnati Observatory, and went on to become Director of the Chamberlin Observatory which was attached to the University of Denver, Colorado. It boasted a 20-inch Clark refractor.

  Bob Argyle - Double Star Section Director

• February 2022 - Double Star of the Month

  To the naked eye, Canis Minor is essentially two stars - Procyon and beta (or Gomeisa). In the field with Procyon is STF 1126 which has already been described in this column (Feb 2014). Starting at Gomeisa, move 1.5 degrees due south and slightly east and you will alight on eta CMi (07 28 02.1 +06 56 32), a magnitude 5.3 star. It is also known as BU 21, the low number indicating it was in the first list of discoveries, made with the famous 6-inch Clark refractor, which S W Burnham published in 1870.

  

  The Washington Double Star Catalog (WDS) gives the magnitude of B as 11.1 and the latest measures places it 4".3 from the bright primary. Gaia EDR3 tells us that the stars are at the same distance from us, within the errors of measurement i.e. 319 light-years, and that the proper motions are almost identical.

  Two and a half degrees south of eta CMa (Aludra) is an equilateral triangle of 5^th magnitude stars, the north-easterly of which is a fine triple for medium apertures and a difficult quadruple for 30-cm upwards.

  

  The region was noted by James Dunlop at Paramatta when he saw a very wide and unequal pair of stars which he catalogued as DUN 47 (07 24 43.9 -31 48 32), and which is now given as AC in the WDS. These are 5.4 and 7.6 at 344 degrees and 97". A third star (B) was added by Bernhard Dawson observing with a 15-inch refractor at La Plata in Argentina. He noted a magnitude 9.7 just 2".2 from the primary and some years later Willem van den Bos added D, a magnitude 10.8 at 0".9 from C, a star which has not been measured since 1965.

  Stars A and B have parallaxes close to 5.2 milli-arcseconds (630 light-years), whilst C and D are considerably more distant (mean parallax of 2.4 mas = 1360 light-years) and thus placing them at about the same distance as the nearby cluster Collinder 140. (This is a revised version - I'm grateful to John Greaves for pointing out an error in the original text).

  Bob Argyle - Double Star Section Director

• January 2022 - Double Star of the Month

  phi Tauri (04 20 21.2 +27 21 03) can be found about 10 degrees north of the Hyades. Found by William Herschel, it is an attractive double star for small apertures, offering as it does, a beautiful colour contrast between the two components. The primary is a K1 giant which W. H. Smyth found to be pale red but I noted as golden yellow with a 21-cm mirror in 1968. The companion is blue and the stars are magnitudes 5.0 and 7.4.

  

  I found the position angle (PA) and separation to be 258 degrees and 48".4 in 2015 and whilst this is significantly different to the first measures of this pair, the differences are due entirely to differences in proper motion due to the differing distances to each star; 284 light-years for the primary, and 346 light-years for the secondary.

  Just 2 degrees south, and slightly east of phi is chi (04 22 34.9 +25 37 45), another probably optical pair, but with magnitudes of 5.4 and 8.5, it is also well-seen in small telescopes. The stars are separated by 19" in position angle 25 degrees.

  The 4.4 magnitude tau (or 30) Canis Majoris (07 18 42.49 -24 57 15.8) is at the centre of the open cluster NGC 2362. Like many stars in this region it is young, hot and very luminous and has a spectral type of 09II.

  

  It attracts the interest of the double star observer because it appears in the catalogue of John Herschel as HJ 3948. The nomenclature applies to companions of 10.2, 11.2 and 8.2 at distances of 8".3, 14".2 and 83".5 respectively and it is of interest to the binocular observer as well as the small aperture telescope.

  Tau itself, has 5 components all within 1 arc-second. Quite recently, Dr. Andrei Tokovinin found a magnitude 9.7 star nestling 0".9 away whilst in 1951, the great interferometric observer William Finsen noted another star 0".2 distant. Tau is also a tight triple star with a fainter eclipsing binary, orbiting tau itself as a spectroscopic binary with a period of 154 days.

  Bob Argyle - Double Star Section Director

• December 2021 - Double Star of the Month

  STT 65 (03 50 18.9 +25 34 47) is close visual pair about 2 degrees north-east of the Pleiades but which does not appear to be part of the cluster. Although the compilers of Gaia EDR3 decline to pronounce on the value of its parallax, the Hipparcos mission found it to be less than half the distance to the Pleiades at 184 light-years.

  

  The pair was discovered by Otto Struve using the 15-inch refractor at Pulkova in the 1840s and it has been followed regularly ever since the orbital period was found to be relatively short (it is currently thought to be 61 years). The orbit is highly inclined and quite eccentric which means that the apparent distance between the stars ranges from 0".03 (in 1996) to almost 0".7 in 2033. At the end of 2021 the stars will be 0".59 apart and thus within range of a good 20-cm aperture, although there is a significant difference in magnitudes between A and B - 5.7 and 6.5 respectively.

  Located in the narrow part of Eridanus which squeezes between Fornax and Horologium, is the K giant star y Eri (magnitude 4.6). Half a degree east is the fine pair DUN 15 (03 39 45.49 -40 21 08).

  

  Since John Herschel's measure in 1836, the distance has increased from 4".0 to 7".5 at the current time, whilst the position angle has reduced from 335 degrees to 327 degrees. The stars are both of spectral class A and the Washington Double Star Catalog (WDS) gives the V magnitudes as 6.9 and 7.7. This does seem to be a physical pair as the Gaia EDR3 catalogue lists a parallax for each star which is the same (mean value 4.70 mas = 694 light-years) within the errors.

  Moving 1 degree ESE from DUN 15 will bring the observer upon HJ 3589, an unequal pair of magnitures 6.5 and 9.3 with position angle 350 and a current separation of 5".0 but which appears to be slowly decreasing.

  Bob Argyle - Double Star Section Director

• November 2021 - Double Star of the Month

  On the border of Andromeda and Cassiopeia, about 15 degrees north-west of gamma And, is the naked-eye star phi And (01 09 30.12 +47 14 30.6). This has long been known as a close and difficult visual binary, having been discovered by Otto Struve at Pulkova in the 1840s.

  

  The orbital period is long, a recent estimate put it at 554 years, but at present it is slightly wider than 0".5 and is thus within range of 25-cm, although 20-cm might well show an elongation. The stars are not equally bright, the Washington Double Star Catalog (WDS) gives values of 4.6 and 5.6 for the magnitudes.

  Within 2 degrees of phi there are three other double stars marked in the Cambridge Double Star Atlas, 2nd edition. Two of these (MAD 1 and STT 21) will be discussed in this column next year.

  The other is BU 397 which lies 40' south-west of phi. The stars are magnitudes 7.5 and 10.3 at PA 142 degrees and separation 8".7. This pair was actually discovered by John Herschel, but S. W. Burnham noted that Herschel had made an error of 1 degree in declination. Burnham also added a fainter and more distant star - 12.9 at 72 degrees and 19".3.

  66 Cet (02 12 47.54 -02 23 37.1) is a beautiful wide pair to be found 1.5 degrees WNW of Mira Ceti, the famous long period variable star. Mira reached maximum brightness in August 2021, so judging by previous light curves it is expected to be about V = 7.5 by mid-November.

  

  66 Ceti has stars of spectral types F8V and G1V and visual magnitudes 5.7 and 7.7. The position angle of 235 degrees and separation of 16".8 has remained unchanged since 1783 when the pair was noted by William Herschel. This is almost certainly a physical pair, since in addition to very similar parallaxes the two stars have the same considerable proper motion.

  In addition, the A component, which was known to be a spectroscopic binary, has recently been resolved by Dr. Andrei Tokovinin at Cerro Tololo using the 4 metre SOAR telescope, The period is 94 days and the angular separation is under 0".02.

  A more distant star of V = 11.5 is optical. I found the primary stars to be yellowish and lilac with a 21-cm reflector.

  Bob Argyle - Double Star Section Director

• October 2021 - Double Star of the Month

  The fine pair STF 3053 (00 02 36.1 +66 05 56) is located on the border between Cassiopeia and Cepheus and is about 1 degree south of the large emission nebula Cederblad 214. It is marked on the Cambridge Double Star Atlas 2nd Edition (CDSA2) but not labelled.

  

  This is an easy object for the small aperture with the components of magnitudes 6.0 and 7.2 currently separated by 15".2 in PA 70 degrees, although the separation has decreased from 18" over 200 years. I have not measured this pair, possibly because the high declination makes access using a German-mounted refractor awkward. There is a third star, V = 11.0, at 99" and 291 degrees.

  Gaia EDR3 has pinned down the distance to all three stars, in the case of the main pair to better than 1% - the mean value is 803 light-years. The faint companion is 33 light-years further out, but with a significantly greater error.

  One of the double stars which William Herschel included in his last list of discoveries is H N 112, better known as STF 3008 (23 23 45.3 -08 27 36) which lies in Aquarius and is again included in CDSA2 but again is unlabelled. It can be found as the faint point about one degree north following the three stars marked psi, and also, incidentally, one degree due west of the spiral galaxy NGC 7606.

  

  This is certainly an optical pair but is noticeable for the measurable change in aspect over a number of years. I measured this pair with the Cambridge 8-inch three times with the following results: 2000 - 153 degrees, 5".97, 2005 - 151 degrees, 6".27 and 2014 - 149 degrees, 6".67. The stars are magnitudes 7.2 and 7.7 and easily seen in smaller apertures.

  Bob Argyle - Double Star Section Director

• September 2021 - Double Star of the Month

  STF 2883 (22 10 38.8 +70 07 57.2) is located in the pentangle of Cepheus, about 3 degrees following beta Cephei, which is itself a fine pair (see this column for Sept 2012).

  

  This pair was somehow missed in my early survey carried out in the late 1960s, probably because of its high declination. It also eluded the elder Herschel and only made it into his final double star catalogue of 1822 as H N 121. It was subsequently observed by South and John Herschel and they noted the stars were white and blue with the colours very decided. I measured it with the Cambridge 8-inch in 2003 (253, 14".4) and at the time I noted another fainter and wider pair about 15 arc minutes south-west (7.9, 8.1, 207 degrees, 66").

  STF 2883 is almost certainly a long period binary. The distances as they appear in Gaia EDR3 are almost identical and the proper motions are similarly equal. The stars lie 106.29 light-years away. This is a nice pair for the small telescope with the stars shining at magnitudes 5.6 and 8.6.

  HIP 110109 (22 18 15.6 -53 37 38) is a Sun-like star of magnitude 5.4 located in the south-western corner of Grus and which can be found six degrees due north of alpha Tucanae.

  

  During the period in the late nineteenth century when observers from Harvard were site-testing at Arequipa in Peru, they discovered a number of new double stars and this star appeared in that list (HDO) as number 298. When first observed the magnitude 9.7 companion was 2".5 distant in position angle 10 degrees but since then the stars have closed and the last measurement in 2015 found the companion at 10 degrees and 1".8.

  Hipparcos found both stars to have the same distance (45 light-years) within the admittedly large errors on each parallax.

  Bob Argyle - Double Star Section Director

• August 2021 - Double Star of the Month

  I have been following STT358 in Hercules (18 35 33.22 +16 58 32.5) for almost 50 years. In 1970, with the 28-inch refractor at Herstmonceux, a measure gave the values of 168 degrees, 1".7. In 2018 using the Cambridge 8-inch Cooke, the separation had remained virtually unchanged and the position angle reduced to 149 degrees. This fine binary has a period of 380 years according to Wulff Heintz in 1995 and it is currently at 142 degrees and 1".5. The pair will remain within range of 15-cm or so for decades to come, only reducing to 1".3 by 2100 before widening again.

  

  Gaia EDR3 shows that both stars have the same trigonometrical parallax to within the stated errors (±0.1 light-year in each case) and the mean distance of the system is 111.2 light-years. The pair can be found 3 degrees WSW of 111 Her. In 2007 I. N. Reid found a magnitude 12.6 star 349 degrees and 35" from A which is 0.3 light-year further out. This turns out to be an M3 dwarf.

  One of the brightest triples in the sky is β Sgr which sits in the extreme south-west corner of the constellation and is unfortunately not visible from the UK. The two brightest components, called β¹ (19 22 38.30 -44 27 32, V = 4.0) and β² (19 23 13.14 -44 47 59.2, V = 4.3) are separated by 21' on the sky and thus easily visible to the naked-eye.

  

  β¹ has a magnitude 7.1 companion at a distance of 28" and position angle 76 degrees, and was first noted as double by James Dunlop in 1826. Ernst Hartung thought these stars physically connected and indeed Gaia EDR3 seems to indicate that they lie at a similar distance, but the parallax for A has a very large quoted error, which may be partly due to the star's brightness. Hartung also noted the colours of the components of β¹ as bright pale yellow and ashy-white, while Ross Gould found them to be white and yellowish, more in line with the spectral types of B9V and A5V.

  Draw a line from β¹ through β², extend it by about 1.5 degrees and you will come upon I 116, a fine triple star which will be well seen with 20-cm. The close pair has magnitudes of 8.6 and 9.4 and they are currently at 24 degrees and 2".7, whilst 16" from A is another 8.6 magnitude. star at PA 190 degrees. The discoverer, Robert Innes, re-visited the system some years later and added a magnitude 13 star only 2".4 from C. This star was last measured in 1960.

  Bob Argyle - Double Star Section Director

• July 2021 - Double Star of the Month

  On summer evenings the beautiful binary star 70 Ophuichi (see this column for July 2008) is well placed for observation. It is part of a triangle of naked-eye stars which also include 68 Oph (= H VI 2), a wide and unequal Herschel pair) and 67 Oph (= BU 1125), a very unequal and close Burnham pair which is now closing. Draw a line from 70 through 68 and extend it by the same distance and you will alight on STF 2244 (17 57 04.32 +00 04 00.0), also a close binary, but more tractable than BU 1125 for the observer with moderate aperture.

  

  The period is 475 years and in mid-2021 the separation is 0".69. The stars are magnitudes 6.6 and 6.9 so 20-cm should be sufficient to see them separated. By 2027 they will start to slowly close again so this is a good opportunity for observation. Note that if you are using the second edition of the Cambridge Double Star Atlas, the star appears as STF 2224. Gaia EDR3 offers no up-to-date information on parallax, but Hipparcos found a distance of 386 light-years but with an uncertainty of 33 light-years.

  The components of kappa CrA (alias DUN 222 - 18 33 23.13 -38 43 33.6), on the other hand, probably do form a physical system. The stars, of magnitudes 5.6 and 6.2, are currently 20" apart and lie almost in a north-south direction. When James Dunlop discovered this pair they were closer to 30".

  

  The proper motions from Gaia EDR3 show that the stars are moving in similar directions through space, the radial velocities are very close, and the parallax measurements from the satellite tell us that both stars are about 717 light-years away. Both components are white, being late B in spectral type. The system lies close to the 'Teapot' of Sagittarius and can be found 5 degrees south and slightly east of epsilon Sgr (V = 1.9).

  Incidentally, epsilon Sgr has a companion of magnitude 14.3 - discovered by T. J. J. See in 1896 which has been measured only four times since 1896. The current distance is 39".

  Bob Argyle - Double Star Section Director

• June 2021 - Double Star of the Month

  About three degrees north of zeta UMi (magnitude 4.3) is a group of naked eye stars, the most southerly of which is the binary pi¹ UMi = STF 1989.

  

  The brightest star in the group is pi² (magnitudes 6.6, 7.3) which is also a pretty, wide pair STF 1972 (15 29 11.19 +80 26 55.0). (This pair is listed in the notes of the second edition of the Cambridge Double Star Atlas (CDSA2) but not identified with its Struve number on page 1 of the Atlas). CDSA2 notes doubles 6' south-following and 10' north. William Herschel found the pair in 1782 when the separation was 26", whilst a recent measure in 2018 gives 32".

  This is actually a physical quadruple system as both bright stars are 71.1 light-years away and each is also a spectroscopic binary. A recent deep imaging investigation, looking for exoplanets has also noted two twentieth magnitude objects within a few arc-seconds of B but the single epoch of observations does not indicate whether they are co-moving or connected to B. The relatively large proper motions of the bright stars has increased the distance to a 11.4 magnitude field star by 55" since 1911.

  The southerly objects chose for this month's column are just south of the celestial equator, in the constellation Virgo. In 1781 William Herschel noted a wide unequal pair which appears as H 6 51 (14 57 29.32 -00 11 05.74) in his second list of double stars, published in 1784. He records the position as in Monte Maeneli Heveliana. In the Hevelius star atlas, Boötes appears to be standing on ground called Mons Maeneli or Mount Maenelus which was a constellation created by Hevelius in 1687.

  

  The Washington Double Star Catalog (WDS) catalogue gives V magnitudes of 5.6 and 10.4 with position angle 224 degrees and separation 86". The primary has a strong orange hue, a consequence of its spectral type of K1III. The SIMBAD catalogue calls the bright star 1 Ser and rather surprisingly Gaia EDR3 shows that both stars have the same parallax within the quoted errors, showing that they are 322 light-years away.

  Nearby (just over one degree due east) is the much more difficult BU 348 (2 Ser - 15 01 48.92 -00 08 24.9) which requires 30-cm to resolve clearly, although S. W. Burnham discovered it with his 6-inch Clark refractor. Here the stars are magnitudes 6.1 and 7.5 and the current separation is only 0".5. If this is not challenge enough, try and see the 14.5 magnitude star C, 37" distant, which was found by Burnham in 1899. The close pair has a parallax of 3.252 mas corresponding to 1002 light-years.

  Bob Argyle - Double Star Section Director

• May 2021 - Double Star of the Month

  1 Boötis (13 40 40.50 +19 57 20.4) is a star just about visible to the naked eye on a good night. It can be found about 4 degrees WNW of eta Boötis. It is not an easy double star for the small aperture and I have not actually observed it, even thought the primary is a naked-eye object.

  

  The Washington Double Star Catalog (WDS) gives the magnitudes as 5.8 and 9.6 with the current position angle and separation being 133 degrees and 4.4 arc-seconds. A 15-cm aperture should give a good view of the stars, the primary being white - its spectral class is A1V. There is a faint field star, magnnitude 12.2, 88 arc-seconds away whilst the 7.4 magnitude star at 208 arc-seconds has a similar parallax and proper motion as the stars of the binary. According to Gaia EDR3 the three lie at a distance of 310 light-years.

  For the first time in this series, (as far as I know, and deliberately at least!) a binary star is going to be included a second time. This is because it is an exceptional object, about which much has been learned recently so it seems a good time to take another look at alpha Centauri (14 39 36.50 -60 50 02.3).

  

  In May 2007, alpha Cen was at PA 235 degrees and 8".4. This month it can be found at 354 degrees and 6".35, although it is now widening, but only as far as 10".4 in 2020, and then it closes to 1".7 in 2038. Extensive direct imaging has been done of both the stars in the bright binary and the physically connected Proxima Centauri.

  To date there definitely seems to be one planet orbiting Proxima and recent observations suggest there is a second one. Both bright components have been suspected of having planetary companions and a recent paper purports to show a candidate close to the A component.

  The latest data from the Gaia mission - data release EDR3 - includes a measurement of Proxima Centauri. The satellite finds the star to be 4.2465 light-years away with a quoted error of 0.0003 light-years. The main stars are far too bright to have been considered for data reduction at present though it is hoped that this might be done towards the end of the present mission.

  Alpha Centauri is always a 'goto' object for the observer fortunate enough to be south of latitude +20 degrees or so. I have seen the stars in the 67-cm refractor at Johannesburg, and they appear like car headlights - dazzlingly bright.

  Bob Argyle - Double Star Section Director

• April 2021 - Double Star of the Month

  35 Sex (10 43 20.91 +04 44 51.6) lies south of the main body of the constellation of Leo and about 5 degrees SSE of the magnitude 3.9 star rho Leonis. It is a fine pair with the principal stars shining at magnitudes 6.2 and 7.1 and the current separation of 6".8 puts the pair within range of small apertures.

  

  The angular motion in 2 centuries has amounted to about 2 degrees with the stars closing in very slowly. There seems to be no doubt that this is a binary pair - the Gaia EDR3 parallax data show that they are at 687 and 718 light-years respectively each with a formal error of about 22 light-years. Each star appears to be an early K giant which would imply that they should appear orange or deep yellow. Webb found colours of yellow and blue and later yellow and ruddy and noted that there was Much difference as to colours.

  Just over 5.5 arc-minutes away from AB in PA 210 degrees is a star of magnitude 8.1, which is the long period binary A 2769 - 8.4, 9.4, 207 degrees, 0".5.

  Nestling within the bounds of the five main bright stars of Crux, CPO12 (12 28 16.88 -61 45 55.6) can be found 1.5 degrees N. of alpha Crucis (Acrux) and 20 arc-minutes east of the open cluster NGC 4349. The stars are V magnitudes 7.3 and 8.2 and with a current separation of 2".1 they can be easily seen in 75 to 100-mm aperture.

  

  Since the pair was discovered in 1880 the position angle has reduced from 271 degrees to 183 degrees and a preliminary orbit by Dr. Andrei Tokovinin suggests that the period is about 690 years. In 1939, Robert Rossiter, using the 27-inch refractor at Bloemfontein in South Africa, noted that the B star was a close double. The Washington Double Star Catalog (WDS) gives a magnitude of 8.8 for each component. This is also a binary system but the stars are about ten times closer than A-B. The period is 28 years and the stars are currently 0".18 apart. By 2030 they will have reached their maximum separation of 0".24. There is another, much fainter companion - a star of magnitude 13.7 at 331 degrees and 7".9.

  Acrux, of course, is a magnificent pair - see the column for April 2007 for more details.

  Bob Argyle - Double Star Section Director

• March 2021 - Double Star of the Month

  STF 1426 (10 20 32.32 +06 25 47.6) is a long period binary located about six degrees south-south-east of Regulus. More specifically it is about 45' preceding the 6.1 magnitude star 43 Leo.

  

  It is also a triple with the C component (magnitude 9.4) having been first found by Herschel in 1782. The stars are magnitudes 8.0 and 8.3 and the current position is 317 degrees and 0".9, so 15-cm is needed to see them clearly divided. The stars have moved prograde by 60 degrees and widened from 0".6 since discovery. The C star is a relatively easy object for the small aperture and is currently at 9 degrees and 8", and Asaph Hall added a magnitude 12.6 with the Washington 26-inch which is currently at 41 degrees, 39".

  The catalogue orbit for AB gives a period of 2209 years and predicts that the stars will now slowly close reaching 0".6 around 2175. One degree south-east is the wide and unequal optical pair SHJ 115, the primary of which has the significant proper motion of 0".25 per year and a distance of 190.6 light-years.

  The pair chosen for the southern part of this column offers a real challenge to observers. BU 208 (08 39 07.90 -22 39 42.8) was one of the pairs which S. W. Burnham found with his famed 6-inch Clark refractor.

  

  When discovered the pair was 1".3 apart but later obserations saw the stars close to 0".25 and the period was recently found to be 123 years. The stars have magnitudes 5.4 and 6.8 and in early 2021 they can be found at 77 degrees and 0".34, but they are closing so any plan to resolve them would benefit from an early attempt!

  Gaia EDR3 shows a parallax for the system as a whole which converts to a distance of 64 light-years, and its considerable annual proper motion of half an arc-second per year is continuing to widen the distance to a field star of magnitude 11.4 currently 121" away.

  In addition to reports of the spectroscopic binary nature of the A star, the Washington Double Star Catalog (WDS) also contains a note on another component which has an H magnitude, of 13.7 at a distance of 7".5. It was noted whilst using the Subaru telescope on Mauna Kea to look for debris disks and planetary bodies. The authors noted it was near the edge of the detector and they do not give much weight to it.

  Bob Argyle - Double Star Section Director

• February 2021 - Double Star of the Month

  In December 1968 I was observing with friends in the back garden of a house in Newcastle-upon-Tyne using a 12-inch reflector. We looked at a number of double stars that night, including phi² Cancri = STF 1223 (08 26 47.08 +26 56 07.8) and 24 Cancri = STF 1224 (08 26 39.82 +24 32 03.7). These two pairs form a kind of wide double-double and can be found in northern Cancer.

  

  Starting with the beautiful pair iota Cancri, move about 4 degrees SE to find phi². This is a pair of magnitude 6.9 and 7.5 stars separated by 5".7 and currently at PA 53 degrees. With the 12-inch I noted that the stars appeared white and lilac at a power of x208. Gaia indicates that these stars are at the same distance from us (347 light-years).

  STF 1224 is the brighter of the two pairs with the components having V = 5.2 and 6.2. The separation is 5".2 and the position angle 219 degrees. I recorded both stars as being blue-white. Robert Aitken found that the B component was a very close binary of short period. The BC pair revolves in just 21.8 years and the separation stays close to 0".15 throughout the cycle. The Gaia EDR3 catalogue gives a parallax for the A component of 14.429 mas giving a distance of 226 light-years.

  The magnitude 2.5 star pi Puppis (07 17 08.56 -37 05 50.9) lies in a rich area of the Miky Way which is part of the Vela-Puppis star forming region. It is surrounded by a number of naked-eye stars and the open cluster Collinder 135.

  

  Pi, which is distinctly red, is a close, very unequal double star which was discovered by Hipparcos in 1991 but whose nature does not appear to have been confirmed since. Along with a magnitude 7.9 star some 67" distant in PA 213 degrees it also forms the pair DUN 43.

  To the north of pi are the bright stars upsilon 1 (V = 4.7) and upsilon 2 Puppis (V = 5.1) which are 4 arc minutes apart. Both these bright stars are variables. Upsilon 1 is also known as NV Puppis whilst upsilon 2 is NW Puppis. A V = 8.8 star lies at 119" and 215 degrees from B whilst William Jacob, observing from India, discovered a fainter companion (V = 9.1) to C at 3".1 and 209 degrees; this pair is now known as JC 10. There is another open cluster nearby called UBC7 and its possibly binary relation with Collinder 135 has been discussed.

  Bob Argyle - Double Star Section Director

• January 2021 - Double Star of the Month

  

  32 Orionis (05 30 47.06 +05 56 53.3) is easily found. It follows, and is slightly south of, Bellatrix (gamma Ori) by about 2 degrees. It was picked up by William Herschel on Jan 20, 1782 and he noted that the stars were Considerably unequal and The distance or black division between the two stars with 278 is about ¼ diameter of L(arge star)…. Herschel noted that the position angle was 232 degrees 10'. During the following century, measurements of 32 Ori showed the stars closing down to a distance of 0".3 before slowly increasing again to the current distance of 1".3.

  Thomas Lewis in his 1906 volume on the Struve stars thought the motion was explained by the proper motions of the two stars whilst van den Bosin 1962 also thought that the stars were not associated. The USNO Orbit catalogue gives a period of 614 years and predicts that the stars will widen until about 2100 before closing again. Gaia EDR3, unfortunately, does not help since it contains only an observation of the brighter component. The stars have V magnitudes of 4.4 and 5.8 and should be nicely seen in 10-cm aperture.

  Six degrees due south of 32 Ori, and just a little below the celestial equator is Mintaka, or delta Orionis (05 32 00.40 -00 17 56.7, V = 2.4), the most westerly of the three Belt stars.

  For the small telescope, the magnitude 6.8 companion (actually component C) located 52" away in PA0 degrees is an easy object to see. This star is itself a spectroscopic binary but it seems to be unassociated with its much brighter neighbour. Gaia EDR3 gives the distance of C as 1245 light-years and whilst the position of the A component is in the catalogue, there is no information on either parallax or proper motion. Hipparcos in 1997, however, found a distance of around 690 light-years, so it seems almost certain the stars are unassociated.

  The bright star is a close triple system. In 1978, Wulff Heintz using the 24-inch Sproul refractor, found a close visual companion at a distance of 0".2 which has a visual magnitude of 3.8; a preliminary orbit gives a period of 346 years. These two stars are very bright and hot, late O-type giant stars. In addition A is also an Algol-type eclipsing system. In 1877, Sherburne Burnham found a magnitude 14 companion (B) at 229 degrees and 33".

  Bob Argyle - Double Star Section Director

• December 2020 - Double Star of the Month

  About 5 degrees due south of alpha Persei (Mirfak) is the rather faint pair STF 391 (03 29 13.74 +45 02 57.5). The Washington Double Star (WDS) catalogue gives the magnitudes as 7.6 and 8.3 and the stars are currently 3".9, a value that has remained unchanged for 200 years or so.

  

  The primary is a B1 dwarf and its companion a giant star of spectral type G8. The colours of the stars might therefore be expected to be white and yellow. Sissy Haas in her book 'Double stars for small telescopes' gives the colours as white and red, as observed in her 60-mm refractor. On the other hand, when I observed them with a 21-cm reflector some years ago, I noted that there was a superb contrast of yellow and blue. I have not yet observed the stars in the Cambridge 8-inch refractor but will try to do so at an early opportunity.

  The Gaia satellite provides a distance of about 3,100 light-years for each component with an error of about 150 light-years in each case. Whilst in the area check out S 430 (03 38 18.53 +44 48 06.5), approximately 2 degrees east and slightly south, a fine pair - 7.2, 7.5, 96 degrees, 41".

  In the far south-eastern and rather sparse corner of Hydrus near the border with Octans is HJ 3568 (03 07 32.13 -78 59 21.3). This is one of John Herschel's Cape discoveries. The primary is a naked-eye star (+5.7) whilst some 15" distant is a magnitude 7.7 companion. In addition, A is also a spectroscopic binary and a delta Scuti variable, known as BN Hyi.

  

  The stars are equally distant (263 light-years) according to Gaia DR2 and both are moving through space in the same direction and at similar velocity but there has been little change in the relative motion of the stars since the 1830s giving no clue as to the orbital period. Ernst Hartung notes that this is a fine, unequal pair yellow and bluish-white.

  Bob Argyle - Double Star Section Director

• November 2020 - Double Star of the Month

  Some 90 arc-minutes to the east of alpha Cas is the open cluster IC 1593 which is inside the nebulosity NGC 287. The cluster is dominated by a group of 9th and 10th magnitude stars which attracted the attention of S. W. Burnham soon after he obtained his 6-inch Clark refractor.

  

  He found that the brightest star in the cluster (V = 8.6) is a close double. There is a magnitude 9.3 star at 1.5 arc-seconds distance, whilst star C is magnitude 8.9 is 4 arc-seconds away with another 9.7 (D) at 9 arc-seconds distance. It is not clear where this group ends and the cluster begins.

  The WDS lists 16 components altogether with most of the stars lying between magnitudes 12 and 16. The accepted distance to the cluster is 2.94 ± 0.15 kiloparsecs whereas the parallax of star A in Gaia DR2 corresponds to a distance of 2.80 ± 0.38 kpc. The bright multiple is known as BU 1 (00 52 49.22 +56 37 39.5) although this was not actually the earliest Burnham discovery.

  The southern part of this column in its second appearance in 2006 considered the glorious pair theta Eri or Acamar. Starting at this star and moving three degrees west you will alight upon the closer pair HJ 3527 (02 43 20.36 -40 31 38.8), also in Eridanus (as can be seen from the relevant map in the Cambridge Double Star Atlas, but the WDS catalogue mistakenly has it in Fornax - my thanks to James Whinfrey for pointing this out).

  

  One of John Herschel's discoveries from Feldhausen, this is a beautiful pair, the primary of which is a late B dwarf. The magnitudes are 7.0 and 7.2 and the current separation of 2".3 appears to be increasing. Whilst observing this star in 2013 with the 67-cm refractor in Johannesburg, the writer found a faint and distant star of magnitude 11.6 at 133", unassociated with the bright pair.

  Bob Argyle - Double Star Section Director

• October 2020 - Double Star of the Month

  In this column exactly 10 years ago, the fine binary star 72 Peg was described. It remains a severe test for 20-cm aperture but as the stars are almost equally bright at V=6, so separating the components, which are now 0".59 apart, is made somewhat easier.

  About 2 degrees to the north-east is another Burnham discovery, BU 858 (23 41 17.6 +32 33 40), somewhat wider than BU 720 (BU 858 is 0".8 at present) but with components whose V magnitudes are 7.8 and 8.8 this is an equally challenging pair.

  

  Since discovery in 1881 the companion has moved almost 60 degrees in position angle, but there is no orbit as yet. A magnitude 12.9 star at 23" is being left behind by AB and was also found by Burnham (BU 389) but earlier in his career and with the famous 6-inch refractor. Gaia DR2 does not give data on A and B but quotes a parallax for both stars of 6.66 mas, equivalent to 490 light-years. The faint star is much more distant and clearly optical.

  The second edition of the Cambridge Double Star Atlas notes that tau Aqr is preceded, about 45 minutes to the south-west, by a somewhat fainter but still naked-eye star called 69 Aqr (22 47 42.7 -14 03 23). The SIMBAD catalogue also lists alternative names for tau and 69 as tau 2 and tau 1. Tau is unmistakeable - it a magnitude 4.0 star with a distinctly reddish hue - an MO giant in fact.

  

  The closer pair, of which 69 Aqr is the brighter component, are separated by about 20" but this distance has been closing since 1781 when William Herschel found the stars 35".8 apart. Measures I made in 1994 and 2005 show the distance decreasing over that time interval. This is purely down to difference in proper motions as the stars are at significantly different distances. The pair is also known as STF 2943. The WDS notes that A is also a spectroscopic binary and a putative faint and distant companion found in 1918 does not exist. Herschel noted colours of reddish-white and dusky. A is a hot blue dwarf of spectral type B9 and the companion has a somewhat later spectral type, but Simbad does not elaborate.

  Bob Argyle - Double Star Section Director

• September 2020 - Double Star of the Month

  STF2872 (22 08 36.04 +59 17 22.2) is located in Cepheus about 1¼ degrees north and slightly west of zeta Cep (a distinctly reddish star of magnitude 3.4). To the small telescope it appears as a wide pair. The catalogue magnitudes are 7.1 and 8 and the current separation almost 22 arc seconds.

  

  When F. G. W. Struve examined the system during his great survey at Dorpat he noted that the companion was a close pair with a separation of around 0".5. Since then the position angle has decreased about 40 degrees and the separation has slowly increased giving a position angle of 296 degrees and separation 0".80 in autumn 2020. I was able to measure this pair in 2016 with the Cambridge 20-cm refractor and it may be divisible in 15-cm. The 840 year orbit shows the stars slowly closing, reaching 0".2 in 100 years time, but for the foreseeable future a moderate aperture will suffice. Gaia DR2 does not show BC as two stars, rather surprising as many pairs wider than 0".5 appear as two entries in that catalogue.

  

  Surprisingly missing from Hartung's Astronomical Objects for Southern Telescopes, 57 Aql (19 54 37.65 -08 13 38.3) is one of the finest pairs in Aquila. The stars are of magnitude 5.7 and 6.4 and both are late B dwarf stars.

  It might be expected that both stars appear white but there have been wide variations in reported colours. Webb in 1851 thought they were distinctly contrasted pale yellow and pale blue whilst he notes in 1855 cols. entirely diff. Smyth though both were pale blue whilst Struve reported both stars as white. I have often wondered if the colour of a star can appear to change if it is a spectroscopic binary, especially if the components have different spectral types and the orbits are almost edge-on to the plane of sight. The WDS notes tell us that both components are, in fact, SBs, whilst the paper by Chini et al. in 2012 notes both stars have constant radial velocities.

  There has been no change in relative position in the last 200 years with the two stars fixed at 171 degrees and 35".5. William Herschel noted 29".5 in 1781 which may suggest a misreading of his micrometer screw. Gaia DR2 places them 440 light years away; within the search radius of 150 arc-seconds, I also noted a magnitude 20 star with nominal parallax slightly greater than 57 Aql A and B but with an error of 30%.

  Bob Argyle - Double Star Section Director

• August 2020 - Double Star of the Month

  

  In preparing this column I checked my list of measures for 23 Aquilae = STF 2492 (19 18 32.50 +01 05 06.5) and found to my surprise that I had never observed it with the 8-inch refractor.

  William Herschel first noted it in 1781 when he recorded Excessively unequal; the small star is just visible with 227... Since then the stars appear to be widening and Gaia DR2 astrometry puts them at 2 degs and 3".27 for 2015.5. The Gaia results show that they are not strictly at the same distance - the difference between the parallaxes for each component is about ten times as large as the quoted error in the distance of B, although the error on the bright star is large and may be expected to be improved upon when the Gaia Early DR3 catalogue is issued. This was supposed to happen in the next month or two, but it now seems likely it will be put back due to COVID-19.

  

  Also in Aquila is STF 2597 (19 55 19.50 -06 44 05.22), another system which has not graced my eyepiece. One reason for this is that in the early 1970s it was only 0".04 apart and has been slowly widening since then.

  The USNO 6th catalogue of orbits shows a highly elongated apparent orbit and the eccentricity of the true orbit is actually 0.94, one of the highest known. The stars are also quite unequal (6.9 and 8.0) which adds to the difficulty of visual resolution.

  The great Italian observer Ercole Dembowski suspected an elongation of the brighter star in PA 140 degrees in 1864 but this has not been confirmed. More than 50 observations over the last 40 years with speckle cameras on large telescopes have not shown a third component.

  The 425 year orbit for this system puts the components at 100 degrees and 0".75 for 2020.5. STF 2597 can be found just 1.5 degrees north of the pretty pair 57 Aquilae, more on which will appear in next month's column.

  Bob Argyle - Double Star Section Director

• July 2020 - Double Star of the Month

  

  Mira was not the first Long Period Variable to be shown to be a visual binary. T. H. E. C. Espin found X Oph (18 38 21.13 +08 50 02.6) in 1886 and in 1900 William Hussey, observing at Lick Observatory, noted that the star was a close pair and catalogued it as HU 198. On several nights with the 36-inch refractor he saw two equally bright stars at a separation of 0".2. Later spectroscopic observation showed that the star spectrum was composite with a K giant combined with an M6 giant. George van Biesbroeck, when observing X Oph visually noted its very deep orange colour.

  Since then the stars have separated somewhat and in 2018 the companion was found to be at 126 degrees and 0".5. This fact, and also that X Oph has only a 3.3 magnitude amplitude, and the companion is significantly brighter than Mira B makes this star easier to resolve than Mira, although at least 30-cm would be needed, preferably.

  An ephemeris for X Oph gives a maximum brightness on Feb 5, 2020 and Jan 8, 2021 so during July 2020 the variable should be near minimum and therefore helps visual resolution. It is also 10 degrees higher in the sky than Mira. X Oph can be found 4 degrees due north of the open cluster IC 4756 or 18 degrees due west of Altair. According to David Boyd of the BAA Variable Star Section, the primary ranges from V = 6.5 to 9.8 whilst the B star is magnitude 9.0. Gaia DR2 gives a parallax for the variable of 4.66 ± 0.30 mas or 700 ± 45 light-years but there is no entry for the companion.

  

  SEE 316 (17 00 26.96 -48 38 52.2) is in northern Ara and lies 2.5 degrees due east of DUN 211 which was described in May's column.

  A discovery of T. J. J. See, this fine pair can be well resolved in 15-cm. The stars are magnitudes 6.3 and 7.7 according to the Washington Double Star Catalog (WDS) and when the writer last measured them in 2016 the position angle and separation were 173 degrees and 1 arc-second. This represents a small increase in angle and a doubling of separation since the first observation in 1897.

  Hipparcos gives a distance of about 350 light-years but with a significant error. Surprisingly, the stars do not appear in the Gaia DR2 catalogue even though significantly closer pairs are included. E. J. Hartung noted that the field is rich in faint stars.

  Bob Argyle - Double Star Section Director

• June 2020 - Double Star of the Month

  In this column for July 2013, I included the difficult pair 26 Dra. One of Burnham's discoveries at the time of writing it was closing and by mid-2020 it will be widening (186 degrees, 0".5) but will still probably require 30-cm and a good night as the stars are almost 3 magnitudes apart.

  Attention this month turns to STF2218 (17 40 18.07 +63 40 31.4) which is 2 degrees N of 26 Dra and slightly E. Discovered at Dorpat by F. G. W. Struve this pair has been closing slowly over two centuries but is still within range of 10-cm. The components are magnitudes 7.1 and 8.4 and at 2020.5 they can be found at 307 degrees and 1".4. The orbit is preliminary as the angular motion amounts to just 50 degrees and predicts a period of 2130 years.

  R. T. A. Innes used a 7-inch refractor at the Cape Observatory around 1900 to survey the sky for new double stars and to take up work again which he was doing as an amateur astronomer in Australia about 5 years before.

  One of his discoveries was I 333 (15 39 55.12 -78 01 38.1), a relatively bright and easy pair with magnitudes 6.9 and 7.5 and 0".8 apart when he happened upon them. He was not the discoverer though. This was Solon Bailey who found the pair from Arequipa in Peru in July 1897 but the observation did not get published until 1908 so Innes got the credit.

  When Willem van den Bos made a pair of mean measures from Johannesburg around 1930 the stars were just 0".3 apart and clearly in rapid motion. The WDS Observations Catalogue then reports no further observations until 1990, a stretch of almost 60 years. By then they stars were almost back to where they were discovered and it seems that here is a binary with a period of about 150 years.

  I 333 was brought to my attention by Andrew James who had noted the probable binary nature of the pair and at my request Rainer Anton secured a measure in Namibia in mid-2019. His result was 323 degrees 0".85.

  Like iota Octantis in April this pair is close to the South Pole but should repay observation with 15 or 20-cm.

  Bob Argyle - Double Star Section Director

• May 2020 - Double Star of the Month

  Epsilon Boötis (14 44 55.44 +27 04 29.9) is one of the best-loved and most well-known of visual double stars - and one of the brightest - its components are 1.9 and 4.8 in the Gaia G band - the equivalent of Johnson V.

  Its spectacular colours prompted F. G. W. Struve to call it Pulcherrima (most beautiful). It was found by William Herschel and was catalogued by him as H I 1. W. H. Smyth calls it pale orange and sea green but leaves any discussion of it out of his book Sidereal Chromatics. Apertures of 15-cm will show it well, especially if a higher magnification is used.

  Gaia DR2 includes both components but does not give a parallax for the bright star. The quoted error in the parallax for B is significantly larger than normal, which may be due to the proximity of A or the fact that B is a spectroscopic binary, although it does not appear in the Ninth Spectroscopic Binary catalogue. DR2 makes the distance of B to be 219 ± 6 light-years. The pair appear to be physical with the position angle having increased about 45 degrees to 347 degrees in 2018. The distance at that epoch is 2".8 which has changed little since 1777. Assuming the orbit is circular then the period will be about 2,000 years.

  The galactic equator passes through the north-east corner of Ara and close to the very young cluster NGC 6193, which in turn is just east of the HII region NGC 6188. About 1 degree ENE of NGC 6193 is the wide binocular triple star DUN 211 (16 47 28,13 -48 19 10.1).

  The brightest pairing (AB) is stars of magnitude 6.5 and 8.1 in the G band of Gaia. The primary is clearly a red star and is a bright M giant whereas the companion 106" distant in PA 125 degrees is early F. The third obvious component can be found at 144 degrees and 130" and has G = 8.1. However, 10-cm aperture should be sufficient to show the close companion to C found by John Herschel at the Cape (HJ 4885). D is 3".8 away in PA 239 degrees, and both C and D have similar parallaxes and proper motions. Gaia DR2 puts them at around 440 light-years. Star B is unconnected and star A is much more distant than its companions.

  Bob Argyle - Double Star Section Director

• April 2020 - Double Star of the Month

  Six degrees NE of delta Leonis is 54 Leo (10 55 36.80 +24 44 59.0). It is an attractive and brightbut unequal pair (V magnitudes 4.5 and 6.3) which is well seen in small apertures.

  It is H 3 30 in William Herschel's catalogue and the great observer noted the colours as white and ash-colour or greyish-white. Struve catalogues it as STF 1487 and Admiral Smyth found them white and grey whilst Webb noted greenish-white and blue and in 1972, I recorded white and blue using a 25-cm reflector.

  Gaia DR2 finds that the parallaxes for A and B are respectively 9.83 and 10.17 milliarcseconds corresponding to distances of 332 and 321 light-years, although the error on the A component parallax is 12 light-years, indicating the possibility of another star in the system. In 2018, a measure made with the Cambridge 8-inch refractor put the stars at 114.3 degrees and 6".5.

  The most southerly double star to appear in this series so far is iota Octantis (12 54 58.80 -85 07 24.1).

  It came to light as a double star in April 1935 when Robert Rossiter was surveying the sky with the 27.5-inch refractor at the Lamont-Hussey Observatory in Bloemfontein, South Africa. He found a pair of stars with magnitudes 5.5 and 6.3 at 0".67 and numbered the pair RST 2819.

  Unless the stars have widened significantly, it is difficult to understand why they were not found before by southern observers. Perhaps the closeness of the stars to the southern celestial pole where traditional refractors are hard to maneouvre played a part.

  Since discovery the stars have moved just 13 degrees and shown little motion in separation. The last observation in 2006 put them at 240 degrees and 0".7, and they were elongated by Ross Gould with 17.5-cm; he noted the primary was orange. In 1941 Willem van den Bos added a third star of mag. 10.9, currently at 53 degrees and 62".

  Bob Argyle - Double Star Section Director

• March 2020 - Double Star of the Month

  STF 1728 = 42 Com = alpha Com (23 09 59.29 +17 31 46.0) is probably the shortest period visual binary star which is resolvable in 20-cm but such is the nature of the apparent orbit that it can only be seen briefly.

  The apparent motion in the 26 year orbit is in a straight line because the orbit is edge-on to the line of sight and in this case it seems that the stars do undergo mutual eclipses although the last such event in late 2014, was missed because the orbit used had been biassed just enough by three observations (out of 600) that the time of eclipse occurred several weeks before the date on which they were widely expected to reach conjunction. For full details see Astronomy Now for December 2014.

  In Spring of 2020 the components are at 0".47 but they are now closing at the rate of 0".08 per year. I measured them for the first and only time in April 2018 when the separation found was 0".59. The stars are magnitude 4.9 and 5.5 and 42 Com can be found 6.5 degrees north and 2 degrees east of epsilon Vir.

  James Dunlop found a number of his bright wide discoveries in the rich star fields of the southern summer sky. DUN 78 (09 30 46.09 -31 53 21.2) consists of the stars zeta1 and zeta2 Antliae which, taken together, are just visible to the naked-eye.

  This pair of A1 dwarf stars have magnitudes 6.2 and 6.8 and form a beautiful sight for the small aperture. They clearly form a long-period binary system as the Gaia DR2 results shows that their distances are respectively 350.0 and 347.0 light-years with formal errors of 2.2 light-years on each value.

  Hipparcos found that the A component was a close unequal double (0".4 and closing) whilst B is accompanied by a faint M dwarf also at 0".4 which was found in the K band. It seems likely that this is a physical quadruple.

  Despite the spectral types Ross Gould using 175-mm found that both components were light yellow.

  Bob Argyle - Double Star Section Director

• February 2020 - Double Star of the Month

  Starting at the fine pair 38 Gem (see this column for Feb. 2016) and moving 2 degrees due East, brings the observer on the pair STF 1007 (07 00 37.52 +12 43 24.2) and, a further 20 arc-mins East, upon HJ 3288.

  The brighter and wider of the two is STF 1007 which was left out of Lewis' treatise on the Dorpat pairs because it was too wide (the writer found 28 degrees, 67".4 in 2014). In fact, Burnham noted two fainter and closer companions on March 16, 1873 with his 6-inch refractor, neither of which could be seen in the 8-inch Thorrowgood with the micrometer field illumination on. C is 11.4 at 300 degrees, 15" and D is 10.0 at 244 degrees, 22" whilst Burnham called them magnitudes 14 and 12 respectively. The Washington Double Star Catalog (WDS) notes that D was found to be a close lunar occultation double.

  HJ 3288 is a pair with magnitudes 7.3 and 8.7 and the writer found 217 degrees, 38" in 2013.

  Originally found as a close bright pair by F. G. W. Struve, STF 1104 (07 29 21.91 - 14 49 53.40) turns out to be a physical quintuple system. The AB pair has magnitudes of 6.4 and 7.6 and at discovery was found at 292 degrees, 2".4. At 2017 it was 38 degrees, 1".8 and a preliminary orbit was computed by A. A. Tokovinin in 2014 who found a period of 729 years. This predicts a minimum separation of 1".7 around 2045 so the pair is always within range of 10-cm.

  In the 1880s two further stars were noted - an 11.8 at 20" (C) and a 13.2 at 72" (D). Since then D has been rapidly left behind by the considerable proper motion of AB, which is 0".3 per year. C, however, is keeping pace and is clearly physical. Dr. Tokovinin also found that C was a close pair of dwarf stars separated by 0".1 and also noted that a star 1072" away which was noted by Luyten and labelled LP 722-24, is also moving through space with a similar proper motion and distance.

  The group is 120 light years from us.

  Bob Argyle - Double Star Section Director

• January 2020 - Double Star of the Month

  118 Tau (= STF 716) is well-placed for observing in mid-evening (RA 05 29 16.49 +25 09 01.1) and can be located almost half-way between the stars forming the points of the 'horns' of Taurus, (zeta and beta Tauri).

  It is a distinctly neat pair for the small aperture - 10 or 15 cm will show it very well. First noticed by Herschel (H 2 75) this pair of B and A type dwarfs appeared white and pale blue or white and bluish to the early observers.

  With little change of separation over 200 years (it is currently 4".7) the position angle has increased by 27 degrees to its current value of 210 degrees and Gaia DR2 shows that the parallaxes of the components are similar but not identical, being 8.96 mas ± 0.10 mas for the magnitude 5.8 A, and 8.42 ± 0.12 mas for B (magnitude 6.7).

  In late 2002 Roberts and colleagues found faint companions to both A (at a distance of 1".7) and B at a distance of 1" but no confirmatory measures have yet been made. There is a 11.9 magnitude field star 140" away in PA 99 degrees.

  The constellation of Lepus is unfortunately low in the sky for UK observers but repays some attention as there are attractive double stars to be found, especially if the seeing is good.

  38 Leporis (RA 05 20 26.91 -21 14 23.1) is 2 degrees WSW of beta Leporis and was discovered by John Herschel (HJ 3750) from the Cape in November 1835 with the comment A most beautiful double star.

  The magnitudes are 4.7 and 8.5, whilst the stars appearing to be slowly widening. In 2015 they were at 279 degrees and 4".0. Observing from Victoria in Australia, Ernst Hartung found them pale yellow and white, as did Sissy Haas adding easily seen with 75-mm.

  If the seeing is good then try beta Lep itself. This Burnham pair is magnitude 2.9 and 7.5 at 8 degrees, 2".7 substantially different from the discovery PA 268 degrees indicating significant orbital motion. Gaia DR2 appears to show a third component with G mag 8.68 (similar to V) at a distance of 1".3 which may be new.

  Bob Argyle - Double Star Section Director

• December 2019 - Double Star of the Month

  14 Aurigae (05 15 24.39 +32 41 15.3) sits in a small cluster of naked-eye stars 5 degrees north and a little preceding beta Tauri. It was first found by William Herschel on 24 September 1780, who called it H IV 19. He gave the colours as reddish-white and dusky but by the time that F. G. W. Struve had observed the system the colours had become greenish and blue.

  Struve also added a fainter companion B of magnitude 10.9 at 11 degrees and 9".8 whilst the Herschel companion was reclassified as C. There is an additional 10.8 magnitude star at 180". The writer measured AC in early 2016 at 225 degrees and 14".5.

  Hubble Space Telescope (HST) observations of star C have shown the existence of a white dwarf companion 2" away but 7 magnitudes fainter than C in the V band. There are no confirming observations to attest that the white dwarf is a physical member of the system, but if it is then 14 Aur is quintuple as both A and C are spectroscopic binaries and C is attached to A.

  A low power field also includes 16 Aurigae, a considerably more difficult pair found by Otto Struve in 1848. The Washington Double Star Catalog (WDS) gives magnitudes of 4.8 and 10.6 whilst the position of B has changed little relative to A. In 2009 it was at 55 degrees and 4".1. This implies a physical relation as the primary star, a K3 giant has a proper motion well in excess of 0".1 per year. Gaia DR2 shows the B star but is coy about it's parallax and proper motion.

  BU 311 (04 26 56.93 -24 04 52.8) lies in an extensive sparse area of Eridanus about 10 degrees west of Lepus. It was found by S. W. Burnham with his 6-inch refractor on 24 October 1874, who estimated the distance at 1". Burnham noted that there was uncertain change by the time he compiled his General Catalogue (1906).

  Since then the companion has passed through periastron and is now on the opposite side of the apparent orbit at half the discovery distance. The period is 596 years and the stars will remain close for some years to come. The ephemeris for 2020.0 gives 162 degrees and 0".4. The magnitudes are 6.7 and 7.1, so about 30-cm will be needed to divide these stars.

  Bob Argyle - Double Star Section Director

• November 2019 - Double Star of the Month

  STF 79 (01 00 03.56 +44 42 47.7) This is a beautiful, easy pair in Andromeda about 4.5 degrees north-east of M31. It was missed by William Herschel on his first two surveys for new double stars but swept up eventually in 1786 and is catalogued as H N 45.

  Sissy Haas notes that the stars are pearly white and pale blue-violet. When I last observed it in 1968 I recorded it as 'bluish-white and bluish(?)' in a 21-cm reflector at x96. Strangely it has not been measured with the Cambridge telescope at all, although an easy object and with the stars of magnitudes 6.0 and 6.8, and the current position is 195 degrees and 7".9.

  An image of the pair appears on the Asociacion Astronomica de Hubble website. The observer JCS noted that the stars appeared to be a delicate shade of sky-blue and there did not appear to be any contrast between them.

  Both stars are spectroscopic binaries and probably form a quadruple system. DR2 places them 494 light years away.

  H 2 58 (01 59 00.72 -22 55 11.2) is in Cetus, about 0.5 degree south-east of 56 Cet. It is one of William Herschel's discoveries. He noted that the stars were considerably unequal but the WDS gives 7.3 and 7.6 and Gaia gives a magnitude difference in the G band of just 0.17. Herschel also gave both colours as dusky white and the spectral types are A7 and G0 according to the WDS.

  Found at 315 degrees and 5".0 in 1782, the stars had widened to 8".3 in 2015 with the PA decreasing to 302 degrees. More recently, the primary was found to be a W UMa ellipsoidal eclipsing binary system which is now known as AA Cet. The period of variation is 0.536 days and the magnitude range is 6.2 to 6.7. Both stars are given as F2 in the Catalogue of Eclipsing Variables by Avvakumova (2013).

  Bob Argyle - Double Star Section Director

• October 2019 - Double Star of the Month

  STTA 1 in Cepheus (00 14 02.61 +76 01 37.2) can be found 2.5 degrees south-east of gamma Cephei. It is the first entry in the appendix catalogue of 256 pairs with separations between 32" and 2' which Otto Struve compiled when he surveyed the heavens for new close pairs with the large refractor at Poulkova.

  Gaia DR2 confirms that the difference in the stars' distances is more than 300 light-years. The primary is an M4 giant whilst the secondary is spectral type G5.

  Just over one degree north is the binary STF 13.

  In late 2019 the stars can be found at 48 degrees and 1". The writer always found it somewhat difficult to see clearly due to the difference in the magnitudes, but the Washington Double Star Catalog (WDS) gives 7.0 and 7.1, whilst in the G-band, similar to V, DR2 gives a deltaM of about 0.4.

  About 4.5 degrees north and east of the first magnitude star Fomalhaut is epsilon PsA, a star of magnitude 4.2. Head due south by about 1.5 degrees and you will alight on the wide pair H VI 119, (22 39 44.12 -28 19 32.0) an object found by the elder Herschel in 1783.

  The current position is 159 degrees and 86" and the magnitudes are 6.4 and 7.5. Even with small apertures, however, it should be apparent that B itself is a double star, a fact that Herschel discovered when he revisited the system around 1800 and called it H N 117. The close pair was rediscovered by John Herschel in 1834, giving it the number HJ 5356, but this has now been discontinued.

  Ross Gould, who has observed it with 175-mm, notes that the primary is deep yellow whilst both stars in the close double companion are pale yellow. The 7.5 and 8.6 magnitude stars are separated by 3" in PA 70 degrees with both these values increasing slowly since discovery. This is a physical triple - DR2 shows almost identical parallaxes and proper motions.

  Bob Argyle - Double Star Section Director

• September 2019 - Double Star of the Month

  STF 2744 is located in northern Aquarius (21 03 03.09 +01 31 55.9) near the border with Equuleus.

  It seems to have missed the attention of William Herschel which is surprising considering that the current orbit of 1532 years gives a separation of fully 2" for 1780 and the stars are of magnitude 6.3 and 7.0. That fact and the general run of observations plotted in the United States Naval Observatory (USNO) 6th Orbit Catalogue tend to suggest an orbit of smaller angular size and orbital period. In any case observations of position angle are almost ten degrees away from the predicted position.

  In 2014 I found it at 113 degrees and 1".4 and motion is slow so it should still be within the resolution range of 10-cm. Greater aperture would be needed to spot two faint field stars - one of magnitude 12.9 at 99 degrees and 98" and another of 14.3 at 300 degrees and 74". The A component does not appear in the Gaia DR2 catalogue, whilst the distance of B is given as 233 ± 5 light-years.

  Lambda Sco, also known as Shaula, is the brilliant white star in the tail of the Scorpion (17 33 36.52 -37 06 13.8).

  It first appeared in a double star catalogue when James Dunlop noted a magnitude 9.2 star which is currently at 330 degrees, 94" (2016). Dunlop recorded the distance as 60" but this is clearly an error. In 1897 T.J.J. See found a magnitude 14.9 star 42" distant from the magnitude 1.6 primary which has, unsurprisingly, only one observation in the Washington Double Star Catalog (WDS) since discovery.

  Slipher found the bright star to be a spectroscopic binary (SB) in 1903, which has subsequently been shown to have a 6 day period, whilst the SUSI interferometer array in Australia showed that this system rotates around another star in a period of 2.9 years. The brighter member of the SB is an early B star whilst its companion is either a massive white dwarf or a TT Tau star. The SUSI companion is another early B dwarf. Like lambda, the Dunlop companion is a brilliant white star.

  Bob Argyle - Double Star Section Director

• August 2019 - Double Star of the Month

  16 Cyg (19 41 49.1 +50 31 32) is a beautiful, wide double star easily found by heading due north from delta Cygni, itself a bright although unequal and much closer pair (see the column for Aug 2011).

  16 Cyg is a much measured pair and the Washington Double Star (WDS) observations catalogue has almost 600 entries.

  It is clear from the astrometry of both stars given by the Gaia DR2 mission that they at the same distance from Earth (A is 68.8 light years away whilst B is 69.2). It seems certain that they form a binary system of long period where here long is taken to mean anywhere from 18 centuries to 485 centuries. This is the range of possible solutions from three different research groups.

  In 1998 a star of visual magnitude 13, thought to be a M dwarf was found just 3 arc seconds from A and it is clear that it shares the large space motion of the bright stars and is therefore physically associated with A. In 1996 a planet associated with A was independently found at Lick and McDonald observatories which has a period of 2.19 years.

  16 Cygni is a binocular pair but best seen in telescopic apertures. The components are spectral types G1 and G3, so slightly larger and more luminous than the Sun, and they shine with magnitudes 6.0 and 6.2. Smyth calls them pale fawn colour, whilst Webb just notes that they are yellow.

  AC11 (18 24 57.2 -01 34 46) is in Serpens, about 2 degrees north-west of the 3.3 mag eta Serpentis. It was found by Alvan Clark on 30th July 1854, and reported by W. R. Dawes to the Monthly Notices of the RAS. "A very difficult object", he reported, "though decidedly elongated with a 7.5-inch aperture".

  The stars are of magnitudes 6.7 and 7.2 and at present this 248 year binary is just closing from maximum separation. The orbital position in late 2019 is 354 degrees and 0".8 and it is well seen in the Cambridge 20-cm refractor. It remains above 0".6 for another 20 years or so and then dives down to about 0".03 towards the end of this century.

  Continue another 2 degrees NW to find 59 Ser, a pretty pair separated by just under 4".

  Bob Argyle - Double Star Section Director

• July 2019 - Double Star of the Month

  STF 2398 (18 42 46.69 +59 37 49.4) is a pair of M dwarfs whose large proper motion has been known for well over a century. It was inevitable that the stars would be shown to be close to the Sun, and such indeed has proved the case.

  Gaia DR2 puts the primary star at 11.487 light-years whilst B comes out at 11.490 light-years, with formal errors of 0.002 and 0.004 light-years respectively. The large proper motion of 2.3 arc-seconds per annum means that two fainter and unassociated field stars (C = mag. 12.2 at 158 degs, 215", 2000) and D (13.5 at 110 degs, 100", 2008) are being rapidly left behind.

  The current orbital period is 408 years and in mid-2020 the companion can be found at 182 degrees, 10".9. This pair needs 20-cm to see well, but the writer has yet to measure it with the Cambridge 8-inch as the red field illumination tends to swamp the stars; 30-cm might show the colours of orange-red. it can be found 1 degree west and slightly north of omicron Dra, itself a colourful pair worth seeking out (4.8, 8.2, 317 degs, 38") which Burnham calls orange and blue and Haas finds yellow peach and clear grey.

  Just 3 degrees north of Antares and a little proceeding is the naked-eye star rho Ophiuchi (16 25 35.03 -23 26 47.0). This beautiful pair, whose components are magnitudes 5.1 and 5.7, is currently 3".0 apart in PA 334 degrees and has been slowly closing since discovery by Herschel with the position angle decreasing by 30 degrees over the same interval.

  Whether it is binary is not yet fully established, as the measures of distance by Gaia as given in the DR2 catalogue show that the parallaxes just overlap within the quoted errors but the proper motion of B is significantly larger than that of A.

  The stars are 467 light-years away putting it about 40 light-years beyond the rho Ophiuchi dark cloud which lies 1 degree to the south.

  Rho has a number of faint companions - C is 7.3 at 0 degrees, 149" and D is 6.8 at 252 degrees, 156"; both distances are slowly reducing. S. W. Burnham divided D into stars of 6.8 and 8.4 which are currently 0".28 apart in a binary orbit which takes 675 years.

  Bob Argyle - Double Star Section Director

• June 2019 - Double Star of the Month

  This month's choices are both fine sights in small telescopes but also of great interest to enthusiasts of stellar multiplicity.

  Mu Boötis (15 24 29.54 +37 22 37.1) was noted as a very wide pair by William Herschel in 1780 when he gave a distance of 2' and 8" ("exact est.") for AB. When he revisited the star a year later he noted the secondary was itself a close double (BC) which became H I 17 in his first catalogue. The modern orbit for this pair which suggests a period of 265 years which gives a separation close to 2".1 for 1781 so its odd that Herschel did not see the two stars in 1780.

  The distance of AB given by Herschel must be a transcription error as the three stars have common proper motion and there has been little relative change over the last 240 years. During the middle of the C19 the separation closed to less than 0".5 since when it has been widening. For the summer of 2019 the stars can be found at 3 degrees and 2".2, an easy split for 10-cm, as the stars are mags 7.1 and 7.6.

  In 1988 using the CFHT on Hawaii, the CHARA team led by H. McAlister found that A was also a close pair whose separation varied between 0".06 and 0".12 in a period of 3.75 years. Although the four stars may seem to be physically connected the astronomer Olga Kiyaeva speculates that because of elemental abundance differences what we are seeing is the close passage of two unassociated pairs.

  Gaia DR2 puts A at 116.1 light years but with an uncertainty of 2.4 light years, no doubt due to the interferometric companion, whilst BC are at 120.0 light years with an uncertainty around 0.3 light years.

  One of the few constellations which has not yet been visited in this series is Apus, which lies between Pavo and Musca and whose southern border impinges on the northern edge of Octans at the South Celestial Pole.

  Perhaps the best pair is I 236 (14 53 13.57 -73 11 24.3) in which the stars are visual magnitude 5.9 and 7.6 and they are currently separated by about 2".2 in PA 123 degrees. In fact there have been no measures since 1996, but there is intriguing evidence that this may be a binary pair with a highly inclined orbit.

  Innes, in his Southern Double Star Catalogue of 1927 notes that the first observation was made by Pickering in 1891 in which he estimates a separation of 0".6. Innes independently noted the pair during his early double star searches but Pickering's report to Astronomische Nachrichten at the time mentions only that the primary star had two companions within 30 arc-seconds, and this prompted Innes to claim the close pair for himself.

  Just 7 degrees due north is the binary pair HJ 4707. Orbiting in 346 years these stars are currently 1".3 apart in PA 266 degrees, and the magnitudes are 7.5 and 8.0. Very fine was John Herschel's comment when he found the pair in April 1835.

  Bob Argyle - Double Star Section Director

• May 2019 - Double Star of the Month

  BU 800 (13 16 51.05 +17 01 01.9) was re-discovered in 1881 using the 15.5-inch refractor at Washburn Observatory by Burnham who recorded a separation of 1".27 in position angle (PA) 121.5 degrees, with estimated magnitudes of 7.1 and 10.2. In fact Herschel had found it almost a century earlier and catalogued the system as H 2 46.

  Burnham noted This is a very interesting physical system. By 1898 the separation had doubled with little change in angle leading Burnham to suspect that it was an orbital pair with the plane in the line of sight. By 2015 the stars were 7".7 apart and, according to the 770 year orbit of Hale (1994), they should start to close around 2040.

  Hale's orbit suggests that the inclination of the plane to the line of sight is 93 degrees; almost edge-on. Gaia DR2 confirms that they are close together in space at distances of 35.83 light-years (A) and 35.80 light-years (B), respectively. That evidence plus the relatively large and similar proper motions confirms that this is a long period system.

  For the small telescope user this will be a challenge. The modern magnitudes are 6.7 and 9.5 but 12.5-cm suffices to see the stars which are orange and red in colour according to Hartung. Haas notes that it is 40 arcmins south of the globular NGC 5053. Recent observations with the CHARA array using baselines of 331 metres show no close companions to A.

  Zeta Centauri, a V=2.6 magnitude B star, forms an equatorial triangle with alpha Cen and beta Cru. A wide-field view of zeta will also include two double stars, one 0.5 degrees SW, HJ 4619 (13 52 02.91 -47 51 56.6) and another 1 degree SW, CPO 61 (13 51 32.35 -48 17 35.7).

  Both are easy for the small telescope. HJ 4619 was noted by John Herschel on July 2nd, 1834 on Sweep 434 without comment apart from a note on the position angle and separation. It transpires that these stars are unrelated - Gaia DR2 notes that the V=6.9 magnitude primary is 748 light-years away whilst its V=8.4 companion is only 208 light-years away. The error on the DR2 parallax for this star suggests a higher multiplicity. I measured them at 198.3 degrees, 23".24 in 2016.

  CPO 61, on the other hand, (7.4, 7.4, 130.6 degrees, 30".59, 2016) is almost certainly a binary pair. The slightly brighter primary is slightly closer (222.0 light-years) compared to the B star at 223.4 light-years.

  Bob Argyle - Double Star Section Director

• April 2019 - Double Star of the Month

  This month's pairs consist of a very easy pair in the southern hemisphere, and a much more difficult one in the north.

  When I used the 28-inch refractor at Herstmonceux in 1970, I observed STF 1606, an orbital pair in Canes Venatici (12 10 47.34 +39 53 29.5), which was found at a separation of 0".41. The pair then closed to 0".29 in 1990 and is now opening.

  For 2019.0 the position angle and separation will be 141 degrees and 0".6, so it should be just resolvable in 20-cm and I will look forward to seeing this pair as double for the first time in 49 years.

  It sits in a little group of three Struve pairs which also includes STF 1622 (see the column for April 2012) and STF 1624. The group is four degrees preceding and slightly south of beta CVn. STF 1606 is also practically coincident with NGC 4145, abarred spiral galaxy of V=11.3.

  In 2011, Shaya and Olling published a paper in Astronomical Journal in which they identified over 800 very wide pairs which they concluded were physically connected. From that list, number 588 (SHY 588) (12 02 39.44 -10 42 48.9) is a pair of stars with V magnitudes of 7.5 and 8.6. The current separation is 331" at PA 115 degrees.

  Gaia DR2 indicates that the brighter star has a distance of 177.6 light-years whilst the fainter is 186.8 light-years distant. The proper motions are similar but the difference in distance is supiciously large for them to constitute a binary, although Shaya and Olling used the Hipparcos data which actually suggests that the two stars are further apart in distance than does DR2. The telling factor may be the quoted error on the parallax of A which is seven times that on the parallax of B and suggests higher multiplicity.

  For the binocular user this is an easy pair and the field is enlivened by a V=8.5 star some 19 degrees and 262" distant from A. DR2 indicates that this is more than three times more distant than the SHY pair and therefore unrelated.

  Bob Argyle - Double Star Section Director

• March 2019 - Double Star of the Month

  During his work at Pulkovo using the 15-inch refractor to survey for new pairs, Otto Struve came across 256 wide pairs (with separations between 32" and 2', and by no means all new discoveries) which he collected an published in an Appendix catalogue. Many are rather faint and uninspiring but several are worth seeking out. One such is STTA 123 (13 27 04.7 +64 44 07.6) in Draco, found about 4 degrees preceding Thuban (alpha Dra).

  The stars are given as yellowish and blue together with the description striking object in the Dover edition of Celestial Objects for Common Telescopes, Volume 2. Although Sissy Haas calls both stars solid blue, Simbad gives the spectral types of both stars as F0.

  Even though they are separated by 69" at position angle (PA) 145 degrees, the stars are identically distant within the errors in the parallax as determined recently by Gaia DR2 and the mean distance is 225.87 light-years with an error of 0.01 light-year. The WDS lists an additional faint companion of magnitude 12 at 95 degrees and 39".

  Three and a half degrees east of the 1.8 magnitude gamma Velorum, and the same distance south of the Vela Supernova Remnant is A Velorum, although on the Cambridge Double Star Atlas (2nd edition) it appears only as HJ 4104 (08 29 04.76 -47 55 44.2).

  This is a bright triple, the closer pair (AB) are magnitudes 5.5 and 7.2 and they were separated by 3".5 at PA 244 when I measured them in 2008; both quantities are slowly increasing with time. At 19" and 39 degrees (2008) is a magnitude 9.2 star.

  In 1951, W. S. Finsen, using his eyepiece interferometer on the 26.5-inch refractor at Johannesburg found that the primary was double at a distance of 0".1. Recent measures have shown that this a binary of high inclination and the projected period is 340 years. If the orbit is correct the apparent separation reaches only 0".25 before falling back again.

  Whilst the easily resolvable components appear to be early B stars, Ernst Hartung found the AB pair to be pale yellow. More recently, and also from Australia, Ross Gould, using 175-mm, notes only that the primary is pale yellow but confirms that the triple is embedded in an interesting field.

  All three components appear to be equally distant - 1600 light years away, according to Gaia DR2.

  Bob Argyle - Double Star Section Director

• February 2019 - Double Star of the Month

  Epsilon Hya (08 46 46.51 +06 25 07.7) is one of the most observed double stars in the catalogue. According to the Washington Double Star Catalog (WDS) it has been measured 432 times since it was discovered by Wilhelm Struve in 1825. It seems to have escaped the attention of William Herschel although it would have been within the capability of his telescope.

  Since 1825 the companion, known as C, has moved 120 degrees in position angle with little change in separation. A measurement at Cambridge in 2017 showed it at 309 degrees and 2".93. It should be resolvable in 10-cm; the stars have visual magnitudes of 3.8 and 7.8.

  In 1860 Otto Struve, using the 15-inch refractor at Pulkovo suspected that the primary star was elongated, an impression he received again in 1864. In April 1888 Giovanni Schiaparelli, observing with the 15-inch refractor at Milan, noted a clear elongation and subsequent follow-up observations allowed him to say that the primary star was a close binary of short period.

  Eight revolutions have been traced out since discovery and the period of AB is close to 15.05 years. The stars are never wider than 0".27 and at the start of 2019 they are 0".22 apart and closing.

  A more distant star D (V = 12.5) at 210 degrees, 18" is also physical, and C is a spectroscopic binary of period 9.9 days meaning this is a quintuple system.

  The Struve pair is a fine sight on a good night - the stars are given as yellow and purple by Smyth but I saw them more as yellow and light blue.

  In the visually barren but telescopically interesting area between Sirius and Procyon there are a number of fine double stars and clusters.

  About 5 degrees west and a little north of 5 Pup (see the column for February 2018) is STF 1097 (07 27 56.66 -11 33 24.7), an easy 6.3 and 8.2 magnitude pair with colours of yellow and bluish.

  I came across it in Spring of last year and obtained 311 degrees and 20".8. I did not see the close companion to A that Dembowski had suspected in 1865, and Burnham confirmed nine years later with his 6-inch Clark. It should be visible in 20-cm although the low altitude of the star would have been a factor.

  BU 332, as it is known, is currently at 0".7 and may be closing; the stars are magnitudes 6.2 and 7.4. There are two faint comites. D is 9.7 at 157 degrees 23" (distance decreasing) and E is 12.4 at 43 degrees and 32".

  Espin noted that A varied between 6 and 6.8 with a period of 14 days, whilst Otero, more recently, suggests that it is the Burnham component which is likely varying by around 0.6 magnitude to produce the small observed variation in AB (0.13 magnitude) found by Hipparcos.

  Bob Argyle - Double Star Section Director

• January 2019 - Double Star of the Month

  Epsilon (or 8) Mon (06 23 46.10 +04 35 34.2) sits about 7.5 degrees ESE of Betelgeuse. It was found to be double by William Herschel on February 15th, 1781 and he briefly dismissed it with a note saying Double, distance about 12".

  This is, in fact, an attractive pair for the small telescope. Admiral Smyth found colours of golden yellow and lilac for A and B whilst John Nanson's observed hues of yellow and pale yellow. It earns the accolade of 'showcase pair' in Sissy Haas' book Double Stars for Small Telescopes.

  The stars have shown little motion since discovery indicating a long orbital period, the physical connection being demonstrated by astrometry of both components by Gaia. The DR2 catalogue puts the A star 134.3 light years away with its companion at 130.4 light years, but the formal error on the parallax of the bright component is 8 times that of star B. A is known to be a spectroscopic binary with a period of about a year which may explain the difficulty which Gaia has had in pinning down its distance.

  In 2014 I found B at 29 degrees and 12".4.

  The small constellation of Caelum sits to the south of Lepus and precedes Columba. The second brightest star is gamma (V = 4.7) which was found to be double (JC 9) by Captain W. S. Jacob in India in 1847 using a 6-inch Lerebours refractor. It is located (05 04 24.40 -35 28 58.7) in a rather sparse area of sky and can be found by moving 13 degrees due south of epsilon Lep.

  The primary is a K giant whilst B is spectral class G8. E. J. Hartung found the stars orange and white and noted that 7.5-cm shows them clearly, whilst more recently Ross Gould suggested that they were a test for 10-cm.

  In 1985, a paper in the International Bulletin of Variable Stars series suggested that B was variable. The authors found a range in delta magnitude of 3.5 to 5 (presumably in the V band), whilst their own data from direct blue-sensitive plates indicated that the B star was between 1.4 and more than 2 magnitudes fainter than that of A, but there are no confirming observations.

  The pair forms a binary system as Gaia DR2 shows the stars to be 186.3 light years away with similar (and substantial) proper motions.

  For 30-cm users, move 13 arc-minutes south to acquire HDS 658 - 6.4, 9.7, 196 degrees, 1" - a pair discovered by Hipparcos.

  Bob Argyle - Double Star Section Director

• December 2018 - Double Star of the Month

  Both pairs in this month's columns are long period binaries accompanied by distant and faint, but co-moving companions.

  STF 326 (02 55 39.06 +26 52 23.6) was unknown to me until recently. Observations of it on the web indicate that the stars are yellow-orange and reddish. It is 1.5 degrees ESE of 41 Arietis, itself a wide pair found by Herschel (3.6, 8.8, 237 degrees, 123"), but also a more complex system according to the Washington Double Star catalogue (WDS).

  Despite having moved just 5 degrees in position angle (PA) since 1831, STF 326 was allocated a hyperbolic orbit in the 1960s - suggesting that the stars make one close approach and then fly off into different directions in space. The existing astrometry hardly supports this theory but the stars have certainly closed since discovery and are now at 221 degrees and 5".5 with the K2 primary at magnitude 7.7 and the M0V secondary at V = 10. They are thus rather faint but the fine colours make this a system worth looking out for.

  There is a background star (C) at 171 degrees and 41" (magnitude 11.9), but the 13.9 magnitude comes at 266 degrees and 44" is LDS 883 D. It is at the same distance as the AB pair, and moving with the same substantial proper motion- 0".3 per year.

  Gaia DR2 puts them all at 73.5 light years.

  BU 1004 is in the constellation of Eridanus (04 02 03.44 -34 28 55.7) and located about 3.5 degrees west of 41 Eri. It was found by Burnham in 1881 and with magnitudes of 7.3 and 7.9 it must have been an easy object in the Mount Hamilton 12-inch.

  Since then the position angle has reduced by 100 degrees and the separation has changed from 1".7 to the current value of 1".2 at PA 50 degrees, making it a rather tricky object from the UK due to the very low altitude.

  J. Docobo finds an orbital period of 410 years and predicts that the stars are now close to minimum separation and may reach a maximum of 1".8 by around 2280.

  In the last century W. J. Luyten found a faint star, probably a white dwarf, moving through space with a similar proper motion to AB. LDS 3551 B is visual magnitude 18 and lies 64" distant in PA 313 degrees.

  DR2 pins all three stars down to 151 light years give or take 0.1 or 0.2 light years so this is a physical triple star.

  Bob Argyle - Double Star Section Director

• November 2018 - Double Star of the Month

  As a by-product of his survey at Dorpat for new double stars F. G. W. Struve came across hundreds of pairs which were very wide, and obviously of not much significance. He consequently placed them in two appendix catalogues - now denoted by STFA and STFB in modern WDS parlance.

  His son, Otto also compiled a catalogue of wide pairs which he happened across during his searches and which appear in the Washington Double Star Catalog (WDS) as STTA. Most of these pairs can be seen in binoculars but some of them are quite attractive and worth seeking out.

  One such pair is STTA19, also known as S 398 (01 28 22.92 +07 57 40.9). It is 3.5 degrees East of the attractive bright pair zeta Psc. The WDS gives magnitudes of 6.3 and 8.0 and the K1 giant primary appears orange, although T. W. Webb calls it rosy, whilst the companion is 'bluish'. W.H. Smyth found yellow and pale blue.

  Modern observations have shown that there is more to this system than meets the eye. Gaia DR2 indicates that the stars are at the same distance (391 light years) from us and moving through space with the same considerable proper motion of more than 0".1 per year. In addition the B star is a close pair which has moved about 30 degrees in Position Angle (PA) since discovery in 1999. The separation is currently about 0".4 whilst the components are mags 8.1 and 11.9.

  Some of the more interesting and difficult visual binaries were found by the Clark brothers, Alvan and Alvan G., during the course of testing some of their objectives on stars.

  In 1853 Alvan was assessing the performance of a 7.5-inch objective when he alighted on 95 Cet = AC2 (03 18 22.43 -00 55 49.0) and noted it had a faint and close companion. When William Rutter Dawes heard about this his interest was aroused. The following year Dawes met Clark during the latter's visit to England and bought the objective and telescope.

  Dawes soon looked at 95 Cet and was able to measure the new companion which was at 73 degrees and 0".7. What made it hard to measure was the significant difference in magnitude.

  After Dawes' observations, made on three nights, there were no further measures for 30 years according to Burnham. The American master relates how he spent many nights with various apertures only to find no trace of the companion and he succeeded only once, in 1888, in seeing the B star.

  By 1900 even Aitken with the Lick 36-inch could not see the companion. It has long been suspected that the B star is variable, which would explain some of the negative results.

  Modern observations show that the visual magnitude difference is between 2 and 3, but currently the stars are close to maximum separation (2019.0, 260 degs, 1".18) and this represents a good opportunity to divide the pair; it probably requires 20-cm and a night of fine seeing.

  Bob Argyle - Double Star Section Director

• October 2018 - Double Star of the Month

  In Cassiopeia, about 3 degrees west of the magnitude 2.2 star beta Cas (the westernmost of the five in the well-known 'W') is tau Cas. Move a further 3 degrees west and you will alight on SHJ 355 (23 30 01.92 +58 32 56.1).

  There are nine components in the Washington Double Star Catalog (WDS), most of which are visible in a 6-inch, and a drawing by John Nanson on the Star Splitters website using that aperture, shows the halo of faint stars around the brightest member of the group which is magnitude 4.9. This B3 star is a well-known eclipsing binary of the Algol type (AR Cas) with a primary dip of 0.14 V magnitudes and a period of 6.06 days.

  The small aperture will have no problem in picking out the C component at 269 degrees and 75". Larger apertures may see that both A and C are close, unequal doubles. AB is one of Otto Struve's discoveries (STT 496) and B is some 4.4 magnitudes fainter than A yet now only 0".8 distant. At least 30-cm will probably be needed for this. 20-cm may suffice to show the companion to C discovered by W. R. Dawes in 1841. This pair (DA 2, CD) are magnitudes 7.2 and 9.0, at 213 degrees and 1".3.

  John Herschel discovered that delta Sculptoris (23 48 55.48 -28 07 48.1) was double before his journey to South Africa.

  The primary is magnitude 4.6 and lies about 12 degrees east of Fomalhaut. He estimated the distance to the magnitude 9.4 companion as 80", but Burnham in his 1906 catalogue suggests that this was a little large.

  It appears in that volume because in 1881 Burnham added a close and faint companion to delta (BU 1013) using the 36-inch refractor at Lick. This star now known as B is only 3".4 away and is magnitude 11.6.

  Since 1881 there has been but 11 degrees of direct motion between the two stars which are clearly physical, because delta is moving though space at more than 0".1 per year. In fact the distant C, (297 degrees, 74") also possesses the same transverse motion as AB. Gaia DR2 tells us that delta is 139.2 light years away, with an uncertainty of less than 0.1 light year, whilst C is 144.25 +/- 0.01 light years distant.

  Bob Argyle - Double Star Section Director

• September 2018 - Double Star of the Month

  STF 2735 (20 55 40.64 +04 31 57.70) is a pretty pair discovered by William Herschel in 1782. It is 1 degree WNW of 1 Equulei which is a bright triple, although the primary pair is now beyond all but the ground-based arrays.

  F. G. W. Struve noted the stars were yellow and ash, whilst Smyth in the Bedford Catalogue noted orange tint and purple. More recently John Nanson on the Star-Splitters blog, using a 5-inch f/15 refractor at x191 thought the primary was white with a weak but noticeable gold-yellow tinge.

  The stars form a very long period binary and have moved only 8 degrees in almost 200 years. Gaia DR2 puts them at a distance of 351 light-years. Webb noted that the stars were magnitudes 6.2 and 7.5 but the Washington Double Star Catalog (WDS) reduces the difference in magnitude to 1.0. John Nanson suspected that they were more unequal than that and indeed Gaia DR2 gives G magnitudes (similar to V) of 6.1 and 7.4. Using the Cambridge 8-inch refractor the writer found 283.6 degrees and 2".21 in 2011.

  Three degrees west of the nearby dwarf Epsilon Indi is a coarse binocular triple which seems to have evaded Dunlop and been first pointed out by W. S. Jacob and which sits in the WDS catalogue as JC 25 (21 43 59.16 -57 19 30.4).

  The two brightest stars sit 152" apart in PA 4 degrees and have both very similar and quite significant annual proper motions (115 mas in RA, -53 mas in Dec) and parallaxes (22.54 mas and 22.49 mas, respectively for A and B) such that they are almost certainly physical. The third star, C, is 187" away in PA 214, is magnitude 7.5 and is unconnected. Gaia DR2 puts it 600 light-years away.

  Bob Argyle - Double Star Section Director

• August 2018 - Double Star of the Month

  Embedded in the Milky Way in Cygnus, about 2 degrees East and slightly south of 22 Cyg, is HJ 1470 (20 03 39.5 +38 19 38.3) a deep-red star which lies at a distance of 1630 light years (with an error of about 30 light years) according to the latest results from the Gaia mission (DR2). Simbad gives the spectral type as M0III and the star is about 225 times as bright as the Sun.

  John Herschel noted a distant companion of magnitude 9.3. The Cloudy Nights website contains drawings of HJ 1470 and three nearby pairs which together form an arc of stars about 22' across and known as Chaple's Arc or the Fairy Ring. The other pairs are considerably less impressive.

  An observer in the US using an 8-inch at x53 noted that the primary was strong yellow-orange/reddish and greyish-blue. At the beginning of 2005, I measured the pair with the Cambridge 8-inch. The result was 340 degrees and 28".6.

  Browsing though Sissy Haas' excellent descriptive guide to visual double stars, I came across the pairs S 715 (19 17 39.96 -15 58 01.7) and S 716 (19 18 05.55 -15 57 13.4) which can be found in Sagittarius.

  The brighter pair is S 715 where the two components have magnitudes of 7.1 and 7.9 and they are currently at 17 degrees and 8".4. Just 6 arc minutes preceding and 1 arc minute north is S 716 with magnitudes 8.4 and 8.6 at 194 degrees and 5".0.

  Gaia has observed both pairs; each appears physically connected but the components of S715 are 480 light-years away whilst the stars in S 716 are both 1030 light-years distant.

  S 716 is also known as Stone 46. Ormond Stone (1847-1933) was Director of Cincinnati Observatory where he found a number of pairs using the 11-inch refractor, in this case about 40 years after South first noted it.

  I measured S 715 in 2016 with the Johannesburg telescope, but S 716 was not noticed, although it should have been clear in the 6-inch finder.

  Bob Argyle - Double Star Section Director

• July 2018 - Double Star of the Month

  On a straight line between alpha Oph and 93 Her, but two-thirds of the way towards 93, and therefore just in Hercules, is STT 338 (17 51 58.46 +15 19 34.9) a neat close pair which was discovered by Otto Struve at Pulkova.

  At the time of discovery the stars of magnitudes 7.2 and 7.4 were separated by only 0".6 in position angle (PA) 223 degrees. Since then orbital motion has taken them almost 60 degrees retrograde in angle and the separation has increased to 0".8.

  Sissy Haas, recalling T. W. Webb, describes them as them gold and green white, but in fact the latter term was abbreviated in Webb to mean Greenwich. A recent orbit by Dr. Jean-Louis Prieur and colleagues assigns a period of 1276 years.

  In a small rectangular area of about 20 x 9 degrees, just below the Teapot of Sagittarius, is the constellation of Corona Australis. It has a number of attractive double stars two of which HJ 5014 (August 2009) and gamma CrA (August 2010) have already been described in this column.

  Kappa CrA (18 33 23.13 -38 43 33.6) is a fine pair which was noted by James Dunlop and is number 222 in his catalogue. The stars are magnitudes 5.9 and 6.2 and the current PA and separation are 358 degrees and 21".5.

  Dunlop's 1826 separation of 30" must be an error, as the stars appear to have common proper motion and Gaia DR2 also indicates that they are both around 695 light-years away. Not connected however, are two fainter and more distant stars, a 13.1 magnitude at 202 degrees and 33", and an 11.6 magnitude at 247 degrees separated by 96".

  About 2 degrees following is lambda CrA (COO 227), a pair of stars of magnitudes 5.1 and 10.0 at PA 213 degrees and separated by 30". It is, nevertheless, a physical pair and DR2 gives distances of 205 and 200 light-years respectively, with similar proper motions. A third star (mag. 9.9) is at 51 degrees and 43".

  Bob Argyle - Double Star Section Director

• June 2018 - Double Star of the Month

  Easily found four degrees due west of alpha CrB, STF 1932 (15 18 20.19 +26 50 24.7) is a visual binary whose period is 203 years, so the current aspect of the stars closely resembles that at the time of discovery.

  The separation around the cycle ranges from 0".6 to 1".6 and at the moment the stars are as widely separated as they get (2018.5, 266 degrees, 1".62). The magnitudes are nearly equal (7.3 and 7.4) and both stars are F-class giving a yellowish aspect to the observed colours.

  Whilst in the area look at eta CrB (STF 1937), four degrees NNE which is now a test for 25-cm. The position angle is currently increasing 20 degrees per year and by the middle of 2018 will be at 246 degrees and 0".42.

  Beta Scorpii (16 05 26.23 -19 48 19.4) skirts the southern horizon during the short northern summer nights. It was first seen as double by Benedetto Castelli in 1627 and was later catalogued by William Herschel as H 3 7. With the two bright components of magnitude 2.6 and 4.5 separated by 13".7 and 20 degrees the pair is not difficult even low down.

  A century after Herschel, S. W. Burnham noticed a close and very faint companion to A about an arc second away and of magnitude 10. More modern measures show that this system has closed in considerably and that the estimated period is 610 years.

  Slipher found that A was a spectroscopic binary and a lunar occultation observation of A in 1976 indicated another component at a distance of 0".1, but no further observations of this pair have been forthcoming. McAlister found that C was also a close binary with a period of 39 years and a separation of about 0".1. Take into account the distant magnitude 7.5 at 519" and 30 degrees from A and this is a physical sextuple star.

  Bob Argyle - Double Star Section Director

• May 2018 - Double Star of the Month

  Not far from Arcturus in the Spring sky is pi Boötis (14 40 43.56 +16 24 05.9) a beautiful pair of white stars found by Christian Mayer and later called H III 8 by William Herschel and STF1864 by F. G. W. Struve. The stars are magnitudes 4.9 and 5.8 and have shown little motion since discovery. Smyth, Webb, Sissy Haas and me all find the both stars are white and the spectral type are B9 and A6.

  The primary star, at least, is over 300 light years away but the quoted error on the Hipparcos parallax is significantly large and is probably not affected by the presence of the visual secondary. In 1984 the primary was found to be a spectroscopic binary and the Washington Double Star (WDS) Catalog notes that the secondary is also a spectroscopic binary. In 2015 I found the stars at 113 degrees and 5".4 apart.

  The upcoming Gaia catalogue may well settle the issue of whether the visual pair is physical, and if it is then we have here a quadruple system. A third star of mag 10.6, first noted in 1881 is 163 degrees and 127" distant.

  k (not kappa) Lupi (15 25 20.21 -38 44 01.0) is a magnitude 4.6 star located in central Lupus about 2 degrees north of delta.

  It was observed by James Dunlop who noted a couple of distant 9th magnitude companions. Dunlop's original paper reads for entry number 183: A star of the 6th mag with two stars of the 10th and the measured separations are 12 and 15". It's possible that Dunlop meant 120 and 150" as the latest WDS positions (for 2016 and 1999, respectively) are 203 degrees and 93" for AB and 134 degrees and 149" for AD.

  In 1896 Robert Innes, observing from Cape Town using a 7-inch refractor, found that the B component was an almost equal double (I 87) at a distance of 1".4 since which time the position angle has reduced by 40 degrees to 207 degrees and the separation is now just below 1". The relative faintness of the two stars means that this is now a stiff test for 25-cm aperture. Innes also added a magnitude 11.5 at 17 degrees and 42".

  Bob Argyle - Double Star Section Director

• April 2018 - Double Star of the Month

  Just 6 degrees north of Denebola (beta Leo) sits 93 Leo (11 47 59.23 +20 13 08.2) a rather undistinguished white star of magnitude 4.6.

  It caught the attention of William Herschel in 1782 who called it H VI 80. Some 40 years later it came to the attention of F. G. W. Struve during the Dorpat survey but the companion star at magnitude 9.0 and distance 77" did not impress him enough to put in the main catalogue, so it was relegated to one of the two Appendix Catalogues of essentially wide pairs.

  In 1900, the primary was found to be a spectroscopic binary by Campbell and Wright at Lick Observatory where four spectra of the star showed a range of 38 kms per second.

  More recently 93 Leo has been shown to be an RS CVn star, a variable type which involves chromospheric activity on one component of a pair of giant stars, in this case the spectral types are F8III and A6III.

  The period was shown to be almost 72 days and then the Mark III interferometer (which was based at Mount Wilson Observatory but closed in 1992) resolved the two stars and defined a visual orbit of great precision even though the separation varied from only 5 to 8 milliseconds of arc.

  Hipparcos showed the primary star to be 232 light years away but the companion has recently been observed by Gaia which shows that it is at the same distance and moving through space with the same proper motion as A making this a physical triple system.

  Smyth neglects to include in his catalogue. Webb notes a curiously similar pair in the field. This is SHJ 130 (7.5, 10.0, 30 degrees, 71") and is about 13 arc-minutes south-west of 93 Leo.

  STF 1604 (12 09 28.54 -11 51 25.4) lies in a barren area of sky on a line between beta Virginis and gamma Corvi and about 6 degrees along the line from the latter star.

  There are three components for the small telescope, although the B star is rather faint and is of late spectral type.

  AB is magnitude 6.9 and 10 at 89 degrees, separated by 9" (2016). Both components have a significant and similar proper motion and have been approaching the third unconnected star C (magnitude 8.1) since 1831 when the distance AC was 58 arc-seconds. Closest approach was 9".6 in 1983, and I measured the distance at 10".8 in 2017.

  The stars were in Virgo when Webb wrote his book but have now sneaked over the border into northern Corvus.

  Bob Argyle - Double Star Section Director

• March 2018 - Double Star of the Month

  Tucked away is an obscure part of Ursa Major is the red dwarf binary STF 1321 (09 14 22.79 +52 41 11.8). Containing stars of magnitudes 7.8 and 7.9 it sits about half a degree west of the centroid of a triangle of stars formed by 15, 18 and θ UMa.

  Although discovered by Struve almost 200 years ago the orbital motion has amounted to only 50 degrees or so and the projected period is 975 years. Both stars are a distinct yellow colour and currently separated by 16".8 in position angle 99 degrees.

  This is one of the nearest stellar systems to the Sun. Hipparcos measured both stars and came up with a distance of 19 ± 0.6 light years for the A component but most recently an interim measurement from Gaia gives a distance for the B star of 20.52 ± 0.05 light years.

  The Hipparcos parallax error is exceptionally large and implies there may be underlying structure. Both stars have been suspected of being spectroscopic binaries but this was disproved, at least at the 0.1 km per second level by Morbey and Griffin. Further searches for faint companions have so far revealed nothing. It will be interesting to see what Gaia finds for the A star - the fact that the parallax has not yet been published may be revealing in itself.

  The pair move across the sky at more than 2.5 arc-seconds per year and is fast approaching two faint companions found by Ball (mag. 11.9) and Espin (mag. 14.5).

  BSO 18 in Vela (08 42 25.41 -53 06 50.5) contains two stars bright enough to be in the HR catalogue, the primary HR 3467 of magnitude 4.8 and HR 3466 of magnitude 5.6, 76 arc-seconds distance in position angle 311 degrees, so this is a fine pair of white stars for the binocular user.

  It is easy to find as it is 25 arc-minutes south following the bright star o Velorum (mag. 3.6) which is itself embedded in the galactic cluster IC 2391, so the whole area is a spectacular telescopic view.

  B in turn has a magnitude 9.9 star (D) at 266 degrees and 60 arc-seconds. The bright components of BSO 18 share the proper motion of the cluster and are both at a distance of about 500 light years.

  In 1929 W. H. van den Bos found a close companion to the B component some 2.5 magnitudes fainter and 0.5 arc-seconds away. Since then the distance has increased only slightly and the position angle has increased by 40 degrees to 153 degrees. This pair (B 1625 BC) was last measured in 1991 and poses a challenge for 30cm.

  Bob Argyle - Double Star Section Director

• February 2018 - Double Star of the Month

  Lambda Geminorum (07 18 05.61+16 32 25.7) sits in the ecliptic zone which means it can be occasionally occulted by the Moon. High speed photoelectric measurements of the star's brightness during occultation show that there is another star close in. The star is also known as a spectroscopic binary which may be the same object.

  For the visual observer the challenge is to see the faint companion discovered by F. G. W. Struve. The WDS give its magnitude as 10.7 and I can honestly claim never to have seen it. John Nanson, however, finds it slightly easier than delta and kappa Gem, which I don't, so clearly I will have to take another look this Spring.

  Lambda is only 101 light years distant and the position angle and separation, currently 36 degrees and 9".3 have changed little since the 1830s. As lambda has a significant proper motion then it seems that the faint star is travelling with it through space.

  The duplicity of 5 Pup (07 47 56.71 -12 11 33.8) is also down to Struve, and it is known as STF 1146. During the 19th century the components, of magnitudes 5.7 and 7.3, changed very slowly relative to each other but by 2016 the pair were about 1 arc second apart.

  This is a highly-inclined long-period system, like STF 1527, and the current orbit predicts a period of 1331 years and a close approach of 0".7 by 2044. At present it is well separated in 15-20cm but the difference in magnitude and low altitude in the UK sky makes the task of resolving it a little trickier. I have only been able to make two measures in the last five years.

  The surrounding area of sky is very rich. Move 3 degrees south and then swing west by 4 or 5 degrees and you will encounter more bright Struve pairs as well as M46 and M47.

  Bob Argyle - Double Star Section Director

• January 2018 - Double Star of the Month

  7 Cam (04 57 17.2 +53 45 07.5) is a fairly unprepossessing system in the F G. W. Struve Dorpat catalogue and appears as number 610 in that list.

  The stars are magnitudes 4.9 and 11.3 with the current position 242° and 26 arc-seconds, and there is no evidence that the stars are in anyway connected. It is 7 degrees north of Capella and about 4 degrees preceding, and is part of a group of 4th and 5th mag stars which also contain the splendid pairs 1 and 2 Cam.

  In 1864, Baron Ercole Dembowski discovered that A was an unequal, close pair, the new component being magnitude 7.9 at 307° and 1".2. Since then the stars have closed in, but are now slowly widening, although by 2020 the separation will still only be 0".62 in PA 196°.

  A recent orbit with a period of 2733 years has since been replaced in 2014 by a linear ephemeris by Drummond, although this has now, in turn, been updated by Hartkopf in 2017.

  The two recent linear solutions give 315° 0".88 and 196° 0".62 respectively for epoch 2020 but it is difficult to understand why there is such confusion about the quadrant in which the companion is located as there is such a large difference in magnitude. Suffice to say it will take sustantial aperture to see Dembowksi's companion but visual observations will help to confirm which of the two predictions is right.

  With Canis Major skimming along the southern horizon at present there is a brief window of opportunity to delve into its treasures.

  Mu (μ) CMa (06 56 06.59 -14 02 34) is a pretty pair but rather close and unequal and tends to be more difficult than it really is due to its low altitude from the UK.

  The stars are magnitudes 5.3 and 7.1 with a current separation of 3".1, showing a very slow closing since discovery almost 200 years ago. Struve gave colours of orange and reddish although the Washington Double Star Catalog (WDS) gives spectral types of G5III and A2.

  By the way, don't forget to take a look at Sirius, just 3.5 degrees south-west of μ CMa - the companion is now fully 10 arc-seconds distant.

  Bob Argyle - Double Star Section Director

• December 2017 - Double Star of the Month

  The two pairs selected for December are both wide, unequal double stars.

  WEB 2 (03 42 42.73 +59 58 09.8) appears in the first edition of Celestial Objects for Common Telescopes. In it, Webb calls the primary star P III 97 and in examining Piazzi's Palermo Catalogue you will find that Piazzi did indeed record the primary star and also the secondary. He adds a note saying another telescopic (star) follows to the north, so this is, in fact, a discovery by Piazzi.

  Webb was almost certainly drawn towards this star because of his abiding interest in red stars. His notes say orange with scarlet blaze and bl(ue). for the colours of the components which is a good enough reason to observe this pair.

  The magnitudes are 5.7 and 8.9 and the current position of B is 33° and 54 arcseconds but there is no evidence that the stars are are in any other way connected. Espin added two fainter, closer stars in PA 33° and 98° and there is an 11.8 magnitude at 168 arcseconds along PA 162°.

  WEB 2 is close to STF 385, STF 389, STF 396 and STF 400, 15 degrees due north of Mirfak (alpha Per).

  H 3 80 (02 26 00.38 -15 20 28.0) is in Cetus in a fairly sparse area of sky about 12° south of Mira, 1.5 degrees preceding σ Ceti towards τ Ceti.

  Herschel came across this pair on October 13th, 1782. He measured the distance as 11.2 arcseconds and the PA at 292°. There has been little change in either coordinate since then.

  The stars are magnitudes 5.9 and 9.1 and the primary is a A star although William Herschel called it reddish white, whilst the companion appeared bluish-red to him.

  The primary star (AB Ceti) is both a spectroscopic binary and a member of the Alpha2 CVn class of variables. These are main-sequence stars which display strong magnetic fields and whose spectra show abnormally strong lines of Si, Sr, Cr, and rare earths. They exhibit magnetic field and brightness changes and the amplitudes of the brightness changes are usually no greater than 0.1 magnitude in V.

  Bob Argyle - Double Star Section Director

• November 2017 - Double Star of the Month

  STF 163 (01 51 16.93 +64 51 17.9) is a colourful pair which can be easily found just over a degree north of epsilon Cas, the easternmost star of the 'W' of Cassiopeia.

  The colours astonished F. G. W. Struve when he observed them. He recorded aurea (cuprea) golden (coppery) and caerulea (light or sky blue). The stars are magnitudes 6.8 and 9.1 and are currently 34".5 apart and PA 38 degrees.

  There has been little movement over the past two centuries or so. Both stars are at great distances from us but seem to be unrelated. A is a mid-K supergiant and the Hipparcos parallax gives a distance of 2200 light years with an uncertainty of 770 light years. More recently the Gaia mission has measured B and gets 800 ± 130 light years.

  A number of observers on the Cloudy Nights website find that companion to be pale blue or gray whilst most see orange in the primary.

  One degree further east is a triangle of three 6th magnitude stars, the westerly of which, according to Mullaney and Tirion is HJ 1000, but which is, in fact, HJ1100 5.3 +11, 309°, 43" (distance increasing).

  Reticulum is a kite-shaped grouping of stars about 1 degree north-west of the Large Magellanic Cloud. The brightest star is alpha (V = 3.4) which forms a wide naked-eye pair with HR 1340, 17' to the north, itself a telescopic double (HJ 3641, 5.6, 11.0, 215°, 13".3 - an optical system).

  Moving about 1 degree south of alpha and slightly east brings you to theta Ret (04 17 40.27 -63 15 19.7). This bright pair is number 3 in the small catalogue of stars compiled by Rumker at Parramatta.

  John Herschel estimated the stars to be magnitude 6 and 9 and gave distances of 5".53 and 6".85. The WDS catalogue gives 6.0 and 7.7 with a separation of 3".9.

  There has been virtually no angular motion since discovery and as the stars seem to be slowly widening again from a minimum of 3".7 in 1907 it might be assumed that this is a very long period binary whose apparent orbit is highly inclined to the line of sight.

  Notwithstanding the B9 spectral type of the primary, Hartung notes the stars are pale and deep yellow, and reports that they lie in an attractive field.

  Bob Argyle - Double Star Section Director

• October 2017 - Double Star of the Month

  William Sadler Franks was an astronomer who assisted Isaac Roberts at his observatory in Crowborough from 1892 to 1906 and then went on the act as astronomer for Mr. F. J. Hanbury in East Grinstead from 1908.

  One of his interests was in the micrometrical measurement of wide double stars and he undertook to re-observe all the pairs in the Dorpat and Pulkova appendix catalogues which are usually marked by the characters OΣΣ, Σ I and Σ II.

  The results, which were achieved with a Cooke telescope a little over 6-inches in diameter and a filar micrometer, appear in eight lists published in Monthly Notices of the RAS between 1914 and 1920.

  During the course of this work he swept up a number of new, wide systems. The WDS catalogue contains the pairs FRK 1 to FRK 13 but in fact there are only 10 pairs in this list now left in the catalogue.

  FRK 11 (22 30 06.50 +49 21 23.1) lies in Lacerta just over 1 degree south and slightly preceding α Lac. The primary is V = 6.7 and it is a K2 giant star. The companion, which is optical but also a K giant, is currently 67 arc-seconds distance in PA 91 degrees and is considerably fainter - magnitude 10.7. The interest attached to this pair is that the brighter star is an RS CVn binary with a period of 17.76 days and an amplitude of 0.12 in V.

  In the south-eastern corner of Grus are a number of double stars discovered by James Dunlop at Parramatta.

  Three degrees east of ε Gruis is DUN 246, a fine pair of yellow stars of magnitudes 6.3 and 7.1 currently 9 arc-seconds apart in PA 254 degrees.

  Move a further two degrees east and you will alight on DUN 248 (23 20 50.13 -50 18 23.0). The stars in this case are magnitude 6.2 and 6.6 and measured by me at 211.4 degrees and 16".90 in late 2013, but in 1947, Richard Rossiter discovered that the A star was a close pair.

  At that time the separation was 0".5 but in the 70 years since the separation has widened and the stars are now 1".3 apart. The companion is magnitude 8.9 so it will need probably 25-cm to see this close pair.

  Bob Argyle - Double Star Section Director

• August 2017 - Double Star of the Month

  Located about 1.5 degrees south-west of Albireo in the Cambridge Double Star Atlas (2nd edition) is a single point marked magnitude 7 to 7.5 according to the scale. This is the binary star STF 2525 (19 26 33.71 +27 19 21.9) which consists of stars of mag 8.2 and 8.4.

  Found by Struve at Dorpat at a separation of 1".3, over the next 60 years it gradually closed up until it was unresolved between the late 1880's and about 1895 when it began to widen again. It was therefore regarded as an optical pair undergoing a close approach until Thomas Lewis (1906) clarified the situation.

  In the previous decade the stars had made a very close approach and the similarity in magnitude disguised the fact that the companion was heading out back to where it came from.

  The stars were measured in 2016 with the Cambridge 20-cm refractor (291 degrees, 2".38) and are now visible in 15-cm. The uncertain period is 883 years and the maximum separation of 3".3 will be reached around 2320.

  Pi Capricorni (20 27 19.20 -18 12 42.1) is the easterly of three faint naked-eye stars which form a triangle some 4 degrees south and somewhat east of beta Capricorni (V = 3.1). The other two stars in the triangle, omicron Capricorni and rho Capricorni, have already been dealt with in this column in August 2015 and August 2016 respectively.

  Pi Cap. is one of S. W. Burnham's earliest discoveries using the 6-inch Clark, but it transpires that it was first seen by O. M. Mitchel in 1846 and not published until 5 years after Burnham noted it in print.

  From northern latitudes this is not a particularly easy pair. The stars are of magnitude 5.1 and 8.5 and the current position is 3".3 in PA 160 degrees; there has been very little motion since 1846. Hartung notes that the companion is white and the primary is a B8 star.

  The discovery of variable radial velocity in the A star later led to the discovery of a close companion which appears to have a period of the order of 40 years or so.

  For a real test, try the 14.1 mag star found by Burnham with the Washington 26-inch refractor. It is being left behind by AB and is currently 38" away in PA 40 degs.

  Bob Argyle - Double Star Section Director

• July 2017 - Double Star of the Month

  The northern star (STF 1575, 11 51 57.57 +08 49 48.0) is one of the few doubles to be found in Virgo north of the equator. It is a William Herschel discovery (H 4 49) and since 1782 there has been little change in separation which is currently 30".4.

  Hipparcos demonstrates that both stars have similar parallaxes and proper motions and Shaya and Olling in their 2011 paper have indicated that there is a near 100% chance of the stars being physical.

  The magnitudes are 7.4 and 7.9 and the pair can be found a little more than 5 degrees due south of Denebola (beta Leo) and it forms the vertex of an isoceles triangle of stars which include 4 and 6 Virginis.

  Thomas Lewis (1906) incorrectly calls this pair H III 51 and notes both components are white, which are the colours derived by F. G. W. Struve. In fact both stars have spectral type K0. Richard Harshaw using 20-cm finds orange and white.

  About 1.3 degrees below the bright star Kaus Australis (lambda Sgr) there is a triangle of 5th and 6th magnitude stars. Each of these is a visual pair which can be seen in 15-cm. At the peak of the triangle is WNO 6 (18 28 57.76 -26 34 55.0) with magnitudes of 6.7 and 8.0 and which appears to be unchanged in over 100 years. The writer found 182.0 degrees and 41".83 in September 2016.

  About 15 arcminutes preceding are the other two members of this triangle.

  The northernmost star is BU 133 found by the great Americam observer on 1873, July 6 using his famous 6-inch Clark refractor. He estimated that the stars were both 7.5 magnitude and indeed all the early estimates show no difference in brightness between the stars and yet the WDS gives 6.6 and 8.5. The writer recently measured it (September 2016) and found 233.2 degrees and 0".77 which would make the pair a stiff test in 15-cm.

  The remaining star is magnitude 6.7 and has a magnitude 8.7 companion at 54" and PA 135 degrees. This component was mentioned by Burnham and subsequently ignored.

  Bob Argyle - Double Star Section Director

• June 2017 - Double Star of the Month

  beta Serpentis (15 46 11.21 +15 25 28.9) is a magnitude 3.7 star of spectral type A2IV some 155 light years away.

  About 30" west of it is a magnitude 10 companion which is unrelated. This pairing was seen by William Herschel in 1781. The companion should be visible in 7.5-cm on a fair night as it is not too close to the glare of the bright primary.

  About 25 arcminutes further west is an 8th magnitude star which is moving through space with the same motion as beta and it is almost certainly physically associated. This star, in turn, has a companion and the system is known as ROE 75.

  E. D. Roe was an American astronomer who had access to the 12 and 40-inch refractors of Yerkes Observatory for double star measurement, but in this case he discovered number 75 on 1911 May 20 with his own 6.5-inch Clark refractor.The secondary is magnitude 10.7 at only 6" distance so a larger aperture may be needed to see this unequal pairing.

  Ara lies below Scorpius in the southern sky and the north-east of the constellation is rich in open clusters.

  NGC 6193 is part of the Ara OB1 association and is thought to be 4200 light years away. Buried in the cluster is the bright and wide pair DUN 206 (16 41 20.42 -48 45 46.7).

  This is an easy pair for the small aperture with the white components of magnitudes 5.7 and 6.7 being 10" apart.

  The brighter component is itself quadruple. Owners of 10-cm or more may be able to see the mag 8.4 star at 1".7 from A, but A itself is a massive triple consisting of three O stars, two of which form a close SB of 2,67 days period and they are in turn circled by a third star every 8.1 years. The total mass of these three stars is thought to be about 140 solar.

  The whole region is spectacular for the small telescope and there are fainter and more distant stars to be seen with small apertures.

  Bob Argyle - Double Star Section Director

• May 2017 - Double Star of the Month

  24 CBe (12 35 07.76 +18 22 37.4) sits just on the northern edge of the Coma Cluster of galaxies and about 10 degrees to the south of the coarse open cluster of stars in Coma.

  It is a fine pair for the small aperture and well worth looking for. The different spectral types of K2III and A9Vm promise a substantial colour contrast. Smyth notes A, 5.5 orange colour, B 7 - emerald tint,- the colours very brilliant. Hartung, from Australia finds deep yellow and white, whereas Sissy Haas records citrus orange and fainter royal blue.

  The WDS gives the visual mags as 5.1 and 6.3. Hipparcos gives similar proper motions but differing parallaxes - 7.24 mas for A and 19.29 for B but the latter comes with an error of +/-14.58 mas possibly due to the fact that this star is known to be a spectroscopic binary (SB). The relative positions are very useful for calibration purposes. At present the PA and separation are close to 270 degs and 20".

  Starting at beta Crucis and moving eastwards by about 3 degrees will bring you to a pair of barely naked-eye stars separated by about a degree in right ascension.

  The first of these is the fine pair R 213 (6.6, 7.0, 22 degs, 0".7) whilst the second is the multiple star I 424 (13 12 17.63 -59 55 13.9). To the user with 15-cm or more, here is an unequal pair of stars of mags 4.6 and 8.4, separated by 2". This has opened slightly and moved 20 degrees in increasing PA since being found by R.T.A. Innes.

  Two fainter and wider companions are also catalogued. C is mag 12 at 258 degs, 46" whilst D, a much greater test, is mag. 14.9 at 24 degs and 53".

  Whilst in the neighbourhood, about 7' north-east is the wide pair COO 152 (mags 6.2, 9.4, 146 degs, 25").

  If you are able to bring 50-cm aperture to bear then there are two further tests. I 424 A is the close binary SEE 170 - currently at 0".17 and with a period of 27.4 years; the mags are 5.3 and 6.2. One of these stars is also a SB - actually an eclipsing beta Lyrae-type system.

  The A star of COO 152 was resolved by Hipparcos. The components of this system are 6.4, 9.7 and the separation is 0".4. One of these stars is a SB of 4.23 day period and the 25" component is also physical meaning that COO 152 is a quadruple system.

  Bob Argyle - Double Star Section Director

• April 2017 - Double Star of the Month

  Iota Leo (11 23 55.37 +10 31 46.9) is a naked-eye star which sits below the hind legs of Leo. It is a relatively easy double star to resolve given 15-cm of aperture and it seems to have been missed by William Herschel during his double star surveys around 1780.

  The current orbit, which has a period of 186 years predicts a separation of 1".05 for 1780 but the pair is a very unequal one: the visual magnitudes are 4.06 and 6.71 making it somewhat easier than zeta Her which Herschel did discover. At the present time, the stars are separated by 2".1 and they are now almost back in the configuration in which they would have appeared to F. G. W. Struve in 1827.

  From the UK iota Leo is relatively low and I have never found them particularly easy to measure. The stars will continue to widen until 2060 when they are 2".7 apart and then closing to 0".63 in 2128.

  Admiral Smyth found the colours to be pale yellow and light blue. T. W. Webb noted white and tawny in 1870, whilst Hartung found yellow and whitish.

  Pz 3 Velorum (10 31 57.33 -45 04 00.1) consists of two luminous B-type giants which lie over 600 light years away. Pz 3 lies at the end of a 5 degree arc of third, fourth and fifth magnitude stars starting with bright binary star mu Vel.

  The arc also contains t Vel (HJ 4330 5.2, 8.6, 163º, 40") the primary of which is a recently discovered close pair, separation 0".4) whilst about 6 arcminutes east is HJ 4332 (mags 7.1, 9.8, 162º, 28").

  Both components of Pz 3 were observed by Hipparcos and the resulting parallaxes show agreement although the scatter in each case is large.

  With such a distant system, relative motion, if any, is very small and the current position angle and separation are 219º and 13".7 are little different from the first measures in 1826.

  Gould notes that the components of the double star itself are white.

  Bob Argyle - Double Star Section Director

• March 2017 - Double Star of the Month

  STF 1282 in Lynx (08 50 44.28 +35 04 15.4) is a neat pair which can be found 7 degrees north of iota Cnc and a little east. The components are magnitude 7.6 and 7.8 and in 2015 I found the relative position to be 282 degrees and 3".41.

  There has been little motion between the two stars since the pair was found by F. G. W. Struve. Also called S 582. Burnham records yellowish white and very white.

  A faint and distant star, mag 12.4 at 49", has a common proper motion with AB and is therefore likely to be a physical member of the system. It was found by the French observer G. Soulie. It, in turn, has a companion of magnitude 14.6 some 19" away which was found by Robo-AO, the first robotic adaptive optics system which is mounted a 60-inch telescope on Palomar Mountain.

  upsilon Carinae (09 47 06.12 -65 04 19.2) is a 3rd magnitude (actually V = 2.97) star which is brilliant white and is accompanied by a mag 6.0 companion which is also white.

  The separation between the two has barely changed since the early 19th century when the pairing was first noted by Rumker in Australia. It is almost certain that this is a binary system and both stars are massive, luminous and hot.

  The primary is an A8Ib supergiant with an absolute magnitude of -5 giving it a luminosity of about 9000 suns whilst the companion is somewhat earlier in spectral type (B3 or B4) and is about 600 times brighter than the Sun.

  Not surprisingly Sissy Haas includes it as a showcase pair and small telescopes should suffice in dividing the stars although the significant brightness difference might militate against using too small an aperture - the separation is currently 5".

  A paper written in 1986 speculates that the period of the pair might be close to 19,500 years, and gives the distance as 400 pc. In 2007 the Hipparcos satellite found 440 pc but with a formal error of 54 pc. E. J. Hartung notes the pair HJ 4252 (9.3, 9.5, 303 degrees, 12") about 5 arc mins south following.

  Bob Argyle - Double Star Section Director

• February 2017 - Double Star of the Month

  STF 958 Lyncis (06 48 12.23 +55 42 16.0) is a neat, bright pair in SW Lynx near the border with Auriga.

  Start by locating the bright trio of 12, 15 and 14 Lyn, all visual binaries of increasing difficulty, and move 2 degrees south of the beautiful triple 12. This brings you to 13 Lyn, an orange giant of magnitude 5.3. Move a further 1.5 degrees south and you will alight on STF 958.

  One of William Herschel's finds, it is doubtless a binary, and even although the movement since 1782 amounts a slight decrease only in the apparent separation, the WDS 6th Orbit catalogue contains two orbits computed for it. With magnitudes of 6.3 and 6.3 it is not clear which star is the brightest and even SIMBAD appears uncertain. The SB9 catalogue assigns spectroscopic duplicity to the A star and the period is given as 4.26 days. The WDS notes that the other component has variable proper motion, thus indicating that it is a physical quadruple system.

  Herschel called STF 958 A pretty double star and assigned colours of pale rose to both components. Struve, on the other hand, found both stars yellow, as did Webb in the 1850s, whilst Sissy Haas notes that both stars are khaki-white. A 11.2 magnitude star can be found 176" away in position angle 268 degrees.

  19 Pup (08 11 16.32 -12 55 37.3) is in a fairly sparse area of sky but it forms an approximate isoceles triangle with Sirius and Procyon.

  It is the bright star on the south edge of the galactic cluster NGC 2539 and not surprisingly it comes with a number of faint distant companions in the WDS catalogue.

  The primary is a G8 giant of magnitude 4.8 and small telescopes will easily show two distant companions, a magnitude 8.9 at 58" and a 9.3 a further 12" out from the primary.

  The WDS lists two more which are fainter but Burnham in 1899 found a very faint star at a distance of about 2" which became BU 1064 AB. It was also measured by Aitken but no further sightings seem to have been made since then. Van den Bos looked twice, in 1936 and 1939 without success. Steve Coe noted 19 Pup and recorded that it was a triple star with the primary yellow and the two brightest comites being white.

  Bob Argyle - Double Star Section Director

• January 2017 - Double Star of the Month

  The two pairs selected for this month are ostensibly relatively, bright and wide pairs; easy for the small aperture, or even binoculars. In fact, both are more complex.

  The northern object is S 503 in Orion (05 56 03.43 +13 55 29.7). The brightest stars are mags 6.7 and 8.4 and they are currently 76" apart in position angle 322 degrees. The WDS however lists 6 companions, most are field stars but at least one is travelling through space with S 503.

  Component C (mag 11.6 at 310° and 55".3) and D (mag 8.2 at 334° and 299") both also found by South are optical and should be visible in 10-cm aperture. Willem Luyten found a 13 magnitude star at 205° and 47" (E) which has the same space motion as A and which the WDS notes is a white dwarf.

  In late 2015, Marcel Fay, who has been publishing his CCD astrometry with a 283mm reflector in El Observador de Estrellas Dobles found two further stars of around magnitude 13 (F and G) at distances of 32" and 9" from A. The status of these is as yet unclear but further measures will show whether they are co-moving with A.

  In 2011 Shaya and Olling published a list of widely separated pairs of stars with common proper motions. This list includes pairs such as Capella and 50 Per, and gamma and tau Persei which have almost 100% probability of being physically related.

  Number 185 on this list is a wide pair of apparently unrelated stars in Columba: SHY 185 (06 36 54.07 -36 05 18.4). Star A is a G1 dwarf of visual mag 6.4. Some 288" away in PA 129 is a mag 7.3 star (B) of spectral type G0V. Both stars are close binaries of short period.

  A is RST 4816 which has a period of 14 years and is currently at 99° and 0".1 according to the orbit, whilst B was found by W. S. Finsen (FIN 19) and is predicted to be 339° and 0".3 at present. The Hipparcos parallaxes for A and B agree well within the quoted errors and the proper motions are similar.

  There are two other double stars to be found close by. One is UC 1454 (7.2 and 12, 181°, 33"), on the southern edge of SHY 185 and about 40 arc minutes south of the group is the fine pair BU 755 whose stars of mags 5.9 and 6.9 are now 1".5 apart and a faint third component which is mag 11.5 can be see 21" away (HJ 3875).

  Bob Argyle - Double Star Section Director

• December 2016 - Double Star of the Month

  pi Arietis (02 49 17.55 +17 27 51.5), also known as STF 311, is a very unequal triple and as such probably needs 10-cm to see all three well. The stars are magnitudes 5.3, 8.0 and 10.7, and the separations of AB and AC are 3".2 and 24".1.

  AB first came to the attention of the elder Herschel in 178x and is listed as H I 64. Sissy Haas notes that this pair was not seen in 12.5-cm, but Admiral Smyth is enthusiastic about this superb trio. Pale yellow, flushed and dusky is his conclusion on the star hues.

  The proximity of pi Ari to the ecliptic has resulted in lunar occultations occurring and it was during one of these events that a close companion to A was found. A also appears to be an SB1 but it seems unlikely that this is the occultation pair so A would appear to be a group of 4 stars.

  Looking much further out David Arnold finds a mag 10.5 star at 220" whilst Tofol Tobal has imaged stars of mags 14 and 15, at respectively 14".2 and 17".5 from A.

  DUN 16 = f Eri (03 48 35.88 -37 37 12.6) is a member of the Tucana/Horologium + Columba moving group - a cadre of bright stars with similar space motions which are between 37 and 65 light years away from the Sun.

  The pair consists of a magnitude 4.7 B8 dwarf paired with a 5.3 magnitude A1 dwarf. The separation has increased from 7" in 1826 to 8".4 in 2009 whilst the position angle has changed from 202° to 216° in the same interval.

  The significant proper motion of star A would have moved it further away from B had B been a field star, rather than a binary companion, as seems to be the case.

  One star is possibly a beta Lyrae-type eclipsing binary. The revised Hipparcos parallax gives a distance of 50.8 parcsecs. Ernst Hartung says that This beautiful pale yellow pair dominates a field of scattered stars and is a fine sight with 7.5 cm. Sissy Haas calls it a Showcase pair.

  Bob Argyle - Double Star Section Director

• November 2016 - Double Star of the Month

  Iota Cas (02 29 03.96 +67 24 08.7) can be found by extending the line between delta and epsilon Cas by the same distance again. It was originally observed by William Herschel in 1782 as a 7" pair (H 3 4); he missed the closer component B (1".5) on that occasion but found it in 1804 (H I 34). As it has widened considerably since then, B is now easier to see.

  Smyth gives the colours as pale yellow, lilac and fine blue whilst Webb restricts himself to yellow, blue and blue. Measures by the writer in 2015 give 232 degrees and 2".9 for AB and 120 degrees and 7".1 for AC. With a small aperture, B and C are delicate objects. The WDS gives magnitudes of 4.63, 6.92 and 9.05.

  Orbital motion has reduced the position angle of AB some 60 degrees in 200 years and an orbit for it indicates a period of about 620 years, although one recent report argues that the motion of B relative to A is linear. The separation of AB has not changed in a smooth fashion but rather B appears to execute a loop every 50 years with respect to A. In fact, it is star A which has a faint K-type companion and it was first directly detected in 1982. This star is visual magnitude 8.5 and is separated by about 0".5 from A. Another companion, this time to C, was found in 2006, also at a distance of about 0".4 and this is probably an M dwarf.

  Stars in the Dunlop catalogue, denoted by a capital Greek delta (Δ) are mostly very bright and wide and therefore constitute an excellent introduction to the double stars of the southern hemisphere.

  Number 7 in that catalogue, DUN 7 (02 39 39.84 -59 34 02.9), is a rather faint member of its class (the mags of A and B are 7.56 and 7.66 in SIMBAD) but for the larger telescope aperture it does boast a close star to B which was discovered by Robert Innes. The primary, A, is a late G or early K giant of visual mag 8.0 and has a mag 8.9 companion about 36" away in PA 97 degrees. Although the Hipparcos parallax for each star differs by an amount somewhat greater than the quoted errors, the similarity of the proper motions indicates that this is almost certainly a physical system, and is located about 700 light years away.

  In 1926, whilst measuring BC, W. H. van den Bos estimated the colour of A as between yellow and orange and considered this was equivalent to a spectral type of about K0. He gives the spectrum of B as A5. BC has mags of 8.0 and 8.9 and is currently separated by 0".4 and, although the system has not been measured for 20 years, there is significant angular motion.

  Bob Argyle - Double Star Section Director

• October 2016 - Double Star of the Month

  STF3050 (23 59 29.33 +33 43 26.9) is a beautiful pair of white stars forming a long period binary system to the north of the Square of Pegasus. More specifically, it is 5 degrees north of and 2 degrees preceding Alpheratz (alpha And).

  The components are mags 6.4 and 6.8 and the spectral type of star A is F8V, whilst that of B is likely to be similar. The first observation in the WDS is for 1777 representing the discovery of the system by Christian Mayer. It is the 80th and last entry in his pioneering double star catalogue. William Herschel later recovered it on December 13, 1787 and called it H N 58.

  Thomas Lewis in his book on the Struve stars published in 1906 said it was evidently a binary, whilst Burnham in his General Catalogue of the same year notes apparently rectilinear motion. Subsequent observations have proved Lewis right and the pair is now significantly wider than it was at the beginning of the last century when it closed to less than 2". An orbit produced in 2011 by W. Hartkopf predicts a period of 717 years, with a position in late 2016 of 340 degs and 2".4. This makes it an easy target for 75-mm apertures and above.

  Lewis noted that both stars were yellowish.

  Beta PsA, (22 31 30.33 -32 20 45.9) bemoans Jim Kaler on his Stars website, is a neglected object most likely all alone, apparently unloved by a companion, or, for that matter, by astronomers and he compares it unfavourably with the much more referenced star alpha PsA.

  Beta is certainly a fine double star and worth a visit, and the question of whether it is binary or not can be settled by considering the proper motions of the two stars.

  The Hipparcos satellite reveals than A (mag. 4.3) is 143 light years away and moves across the sky, mostly to the east, at about 60 milliarcseconds per year. Star B has an almost identical proper motion and radial velocity and so is physically connected to A. Over the last 200 years the separation has reduced from 35".3 to 30".6 and the PA is almost fixed at 172 degrees.

  The SIMBAD spectral types of A1 and G1 might suggest stars of white and pale yellow when in fact the observed colours (by Hartung) are given as pale yellow and white, although he notes that B sometimes appears reddish.

  Bob Argyle - Double Star Section Director

• September 2016 - Double Star of the Month

  Located in Cepheus, STT 461 = 15 Cephei (22 03 53.86 +59 48 52.5) offers a stellar grouping which more resembles an asterism than a multiple star. On the basis that a picture is worth a thousand words, an image from the POSS Quick V survey is appended showing the main components of the group.

  

  STT 461 is located close to the galactic equator and can be found about 2 degrees north-west of the orange supergiant zeta Cep. (mag. 3.4), the most south-easterly of the five bright stars in the pentangle of Cepheus. W. J. Hussey, who re-observed the complete STT catalogue using the Lick 36-in (1901) merely measured AB and the 9th magnitude companion C. The WDS lists eight stars to mag 14.3; the primary star is a hot B1 dwarf of magnitude 6.6 and has B (mag 11.4) 11" distant in PA 297. C which is a K0 giant is 90" away in PA 40. There is little relative motion in the group, which, if the stars are all at the same distance as A, lies about 1500 light years away.

  One of S. W. Burnham's earlier discoveries (BU 172), made with the 6-inch Alvan Clark refractor, was 51 Aquarii (22 24 06.87 -04 50 13.2). At that time Burnham's telescope was not fitted with a micrometer so his friend and colleague, Baron Ercole Dembowski in Italy measured the pair for him. In 1875 the two stars were at PA 20 degrees and separated by 0".46 so offered a tough test for Dembowski's 7.5-inch refractor.

  The pair then slowly closed through most of the last century, reaching a minimum distance of 0".11 around 1987 when the position angle was changing by 20 degs per year. A good set of speckle measures since then means that the orbit is now tolerably well-known. The period is 145 years and at the time of writing the separation was just under 0".5 and widening, with the companion almost at the point in the orbit where it was discovered last time around.

  A good 20-cm should show the pair but its fairly low altitude means the air needs to be steady. On 1782 Oct 2, William Herschel noted three faint and distant companions to 51 Aqr, but the duplicity of the primary star, then close to its maximum separation of 0".6, escaped him.

  Bob Argyle - Double Star Section Director

• August 2016 - Double Star of the Month

  STF 2690 (20 31 11.94 +11 15 37.7) was found by William Herschel in 1779 and he called it H III 16. Since that time the position angle has reduced 26° to 255° and the separation has increased from 15" to 17".6.

  Located in Delphinus it can be found just 30 arc mins preceding epsilon Del. Also in a low power field is 1 Del (BU 63) an unequal close pair which is a test for 15-cm.

  The components of STF 2690 are mags 7.1 and 7.4 and Sissy Haas called them both 'peach-white'. Whilst observing at George Bishop's observatory, at South Villa in Regent's Park, London, W. R. Dawes noted .. on the first night on which I measured this object with Mr. BISHOP's 7-inch refractor (1840, Oct 27) I perceived that B was unquestionably wedge-shaped, and succeeded in obtaining moderately good measures of it. Dawes also speculated on why Struve has missed this pair during the Dorpat survey, and suggested that pairs such as STF 2690, being wide and easy, were only measured on poor nights and hence the real nature of star B would have eluded him.

  DA 1, as the close pair became known, has a highly eccentric orbit and the separation ranges from 0".55 to 0".02, a distance which it attained in the last years of the previous century. It is now widening and in mid-2016, the separation is expected to be 0".34. The stars are mags 7.9 and 8.0 so this will be a severe test for 30-cm. A is also a very close pair being found in speckle survey of B stars in 1983. Motion appears slow and the period is likely to be a century or more.

  Rho Capricorni (SHJ 323 - 20 28 52.19 -17 48 49.2) is the northernmost of a bright triangle of naked-eye stars about 5 degrees south-south-east of beta Capricorni. Of the other two, omicron featured in this column in 2015, and pi will appear in 2017.

  Although discovered by the elder Herschel the pair now has the moniker SHJ attached to it - the name for stars catalogued by James South and Sir John Herschel.

  This is a binary of period 278 years which is now widening but will remain quite a difficult object for the northern observer. The writer has not yet observed it but with a magnitude difference of 2 between the brightest components and a separation of 1".8 then it requires a good night to be seen.

  There is a 13.3 star at 55" (distance increasing) and a 6.7 mag star some 259" away (and increasing) has, in turn, a companion of mag 10.6 at 54" and is catalogued as DOB 13DE.

  Sissy Haas notes that A and D are pretty - Bright Sun-yellow and pale rose-red.

  Bob Argyle - Double Star Section Director

• July 2016 - Double Star of the Month

  39 Dra (17 57 12.56 -30 22 24.80) can be found about 4 degrees following and slightly north of xi Dra in the 'head' of Draco. It is also known as b Dra.

  The binocular user will see a pair of mag 5.1 and 8.0 stars separated by about 89". Any aperture larger than 6-cm will show the companion to A which William Herschel found in 1780 and allocated to his class 1 i.e. between 0 and 2". Since that time the pair has widened with increasing position angle and in 1993 was found at by the writer at 351° and 3".9. The AB stars are magnitudes 5.1 and 8.1 so it requires good seeing to see B clearly.

  This is a quintuple system since all 3 bright stars are physically connected and both A and B are spectroscopic binaries. Surprisingly, although the motion in AB amounts to just 25 degrees in 230 years, a highly speculative orbit with a period of 3962 years appears in the USNO Sixth Orbit Catalogue.

  In October 2011, John Nanson found the primary was a distinct yellow watered down by a weak touch of white, I could see a slight tinge of blue in "B" and "C" was just weakly white. There is much more information on this system on the Starsplitters website.

  PZ 6 (17 59 05.28 -30 15 10.8) is a beautiful pair which lies about 1.5 degrees due west of gamma Sagittarii. In the 1st edition of the Cambridge Double Star Atlas it is given as PZ 4 but is corrected to PZ 6 in the second edition.

  An observation of this pair on the evening of 5 September 2013 using the Johannesburg refractor showed the colours of the stars to be deep yellow and lilac.

  The WDS gives spectral types M1Ib and G8II so this is a rare pairing of a supergiant and a giant. There is little motion between the two stars and the primary star sits a little over 1,000 light years away but with an uncertainty in the distance of 30%.

  The stars are magnitudes 5.4 and 6.0, and the current separation of 5".7 appears to indicate that the stars have been slowly closing since the first measure in 1826. A 13.2 mag star at 25" was measured by the writer in September 2013.

  By moving the telescope another 1.5 degrees further west, and crossing the border from Sagittarius into Scorpio, the observer will come across PZ 5 - another bright wide pair easily resolvable in a small aperture. The stars are mags 6.7 and 8.2 and both white - the spectral types are A3 and B9. Note, however, that PZ 5 is not labelled in CDSA 2 whilst it is in the first edition. The position in 2013 as determined by the writer was 104° and 5".7. On the same night a mag 11 companion was also noted at a distance of 49".

  Bob Argyle - Double Star Section Director

• June 2016 - Double Star of the Month

  16/17 Dra (16 36 13.72 +52 55 27.8) is a 90" binocular pair to be found about 15 degrees preceding the head of Draco and the stars are of magnitude 5.4 and 5.5. William Herschel noted that A itself was a rather unequal but easy pair 4" apart, and he included it is his 1782 catalogue as H I 4. He reported that It is the star to which a line drawn from nu through mu points, at nearly the same distance from mu as mu from nu. The brighter star was recorded as white whilst the fainter was 'white inclining to red'. In mid-2015 the writer found the position angle 106 degs and the separation 2".9. The angular motion is but 8 degrees and the stars have closed up from 4" at discovery. All three components appear to be physically connected.

  COO 197 (16 25 17.59 - 49 08 52.2) is a rather faint pair (mags 8.1 and 8.2) in southern Norma, near the border with TrA, which appeared in the catalogues of stars compiled at Cordoba Observatory.

  It was first measured as double by R. P. Sellors at Sydney in 1895 using the 11.5-inch refractor. It was found to be in slow direct orbital motion and in 1977 an orbit was calculated with a period of 311 years. In 2008 the writer observed it with the large refractor in Johannesburg and at that time the observed position angle and the calculated value differed by more than 20 degrees. Andreas Alzner then performed a re-calculation of the elements of apparent orbit and found that the period was much longer (1132 years). In mid-2016 the stars will be found at 93 degs and 2".3.

  In recent years, observations of this system with high resolution techniques and large telescopes have revealed that it is quite a complex multiple star. Using the NACO infra-red camera on one of the 8.2-metre VLT telescopes in 2004, Chauvin and colleagues found that B was again double at a distance of 0".1. The image in the journal shows the two stars clearly separated, but nearby star A appeared single. However, in 2014 Andrei Tokovinin also resolved A into two unequally bright stars separated by 0".1. It appears likely that there is also a spectroscopic sub-system in either Aa or Ab but which of the stars it can be pinned to is not yet determined. A 12th mag star now at 77 degs and 20" is being left behind.

  Bob Argyle - Double Star Section Director

• May 2016 - Double Star of the Month

  Virgo straddles the celestial equator and this month two pairs are included from this constellation - one north and the other south of the zero line of declination.

  The primary of STF 1764 (13 37 44.01 +02 22 56.5) is a K2 giant and the note in Sissy Haas' book says that the stars are yellow and blue and that the colours are vivid. This system has been left out of Hartung and the Rev. Webb dismisses it with the comment yellow and ash.

  The pair can be found about 3 degrees north and slightly following zeta Virginis. It forms an equilateral triangle with 84 and 78 Vir. There has been very little change since discovery. The stars are mags 6.8 and 8.6 and last year the writer measured the pair and found PA 32° and separation 16".2. The distance of A has been measured by Hipparcos but the resulting large value (2038 light years) is very uncertain. There are two further and fainter companions 10.4 at 139° and 172" (C) and D 10.7 at 143° and 207" (D), which together form the pair STF 1765.

  SHJ 162 (13 14 55.85 -11 22 07.3) misses the cut in both Webb and Hartung but Haas was obviously impressed by the colours of the two wide stars A and B - white and pale red. John Nanson, however, using a 6-inch f/10 lens and x84 in 2011, noted that they were yellow and white.

  It was measured by James South on 1823 May 7 using his 5-foot equatorial when the distance of AB was 45". The writer has not observed this pair but apart from AB there is another star in the system, close to A which was discovered by Richard Rossiter from Bloemfontein in 1937 using the 27-inch refractor. This has turned out to be a binary of period 122 years and in mid-2016 the companion (a) is at 161° and 0".55. It is about 1.5 magnitudes fainter than A but might be seen in 30-cm on a good night. The rapid proper motion of Aa (0".37 per year - distance 128 light years) is leaving behind star B which is now 112" away. There is a star of mag 13.3 at 67". The system is 3 degrees directly preceding Spica.

  Bob Argyle - Double Star Section Director

• April 2016 - Double Star of the Month

  STF 1517 (11 13 41.22 +20 07 44.9) is very easy to find. It is about 30 arc minutes south and slightly west of the 3rd magnitude delta Leonis. Found by F. G. W. Struve at Dorpat, this binary appears to be moving in an orbit which is very highly inclined. Motion is therefore mostly in separation and between 1820 and the late 1890s the stars closed from more than 1 arc second to about one-quarter of an arc second. After that the quadrant changed and they began to separate. The proper motion of the system is large enough that we can say the system is definitely physical. The USNO Sixth Orbit Catalogue gives a period of 924 years which is highly speculative as the motion has been virtually linear since discovery.

  The calculated position for early 2016 is 316° and 0".71 making it a good test for 20-cm. The stars are of similar brightness but a little on the faint side (mags 7.5 and 8.0). As part of his proper motion programme, Burnham added a star of mag 10.8 at 246" and 97° but the distance is increasing rapidly due to the proper motion of 0".4 annually in the bright pair.

  BU 28 (12 30 04.93 -13 23 35.0) is one of Burnham's most interesting discoveries. Its low number tells us that it was in Burnham's first list of 81 new pairs which he published in MNRAS in 1873. Burnham often underestimated the magnitude of the fainter star in very close and unequal pairs and in this case he assigned a value of 12 to the magnitude of B, calling the pair a very delicate and beautiful double star. The WDS gives 9.6 with the primary at 6.4. It is marked in the Cambridge Double Star Atlas 1st edition but without a label.

  This is a pair with a period of 151 years and the separation ranges from less than 0".4 to 2".2 which, fortuitously, is where it is at present, so now is the very best opportunity to observe these stars. It is found 3 degrees due north of delta Corvi, but the low declination from the UK means that a good steady night is required. The writer has not yet looked at this system but will put it on his 'to do' list. BU 28 is 80 light years distant and moving across the sky at more than 0".25 per year. It is leaving behind two distant comites (mags 10.6 and 11.6) which are currently both 71" distant.

  Bob Argyle - Double Star Section Director

• March 2016 - Double Star of the Month

  STF 485/STF 484 (04 07 51.38 +62 19 45.4) are really just part of the open cluster NGC 1502 (H VII 47).

  

  For the small telescope user this is a pretty sight but its appears to be a real headache for the double star cataloguers and there is an extensive entry in the WDS catalogue notes trying to sort out which stars are which, and who discovered what.

  The brightest pair is STF 485 AE which consists of stars of mags 6.91 and 6.94 separated by 17". There are a further 5 comites of magnitude 10 or brighter within 140" of star A. Two of these, mags 9.63 and 9.81, form STF 484 at 336° and 22".8 (visible on the drawing to the east of the cluster centre). Recently, Andrè Debackére measured a new component in one of the pairs in this group (see DSSC 24 on this website).

  For the astrophysicist, the most interesting star is E. This turns out to be a multiple star consisting of a 2.69 day Algol system with a third star (but also a close binary) 0".1 distant circling the two every 54 years. The total mass in this system is more than 50 suns. The GCVS gives the name SZ Cam to the variable but ironically the distance derived from the dynamics of the 54-year system place the stars over 300 pc further than the accepted distance of 800 pcs to the cluster. There is little colour in this grouping as many of the components are hot early B stars.

  15 Hya (08 51 34.44 -07 10 38.0) is an object which I have not yet observed but which in terms of separation and magnitude difference looks like a tempting target.

  The star first entered the double star catalogues as H V 120 when William Herschel noted on 1782 Dec 28 that it appeared Extremely unequal with the brighter star white and the comes red whilst the separation was 43 arc seconds.

  The WDS gives magnitude 11.4 for the fainter star but relegates it to component C as in 1878 S. W. Burnham found that the primary was a close double (0".47) whose components he estimated had magnitudes 5.7 and 7.2. Since then the close pair has widened significantly and is now more than 1" apart so it should be visible in 15-cm in the UK; the seeing needs to be good as the declination counts against it.

  Burnham adds a further faint star of mag 12.1 at 55". AB is clearly a long period binary and the position angle has decreased to 121 degrees in 2003 although there do not appear to have been any measures since then.

  Bob Argyle - Double Star Section Director

  We have had an observation submitted for STF 485. If you have observed this double star – or the cluster it is in – please let us know.

• February 2016 - Double Star of the Month

  38 Gem (06 54 38,63 +13 10 40.1) can easily be swept up since it directly follows the 3.4 mag xi Gem by a little over 2 degrees.

  The current orbital period, 1898 years, as determined by Brian Mason in 2014, is clearly very uncertain but the position for 2016.0 is 143° and 7".31 in close agreement with measures by the writer late last year. The stars are of visual magnitude 4.8 and 7.8 so the quadrant in which B lies is certainly the second whilst Sissy Haas puts it in the 4th.

  Admiral Smyth gives light yellow and purple, but E. J. Hartung sees yellowish and pale-orange, whilst to Sissy Haas the colours appear lemon-white and greyish.

  A third, much fainter star C of mag. 11.3 can be seen at a distance of 119" whilst Andrei Tokovinin noticed a 15.0 mag dot at 151". The primary, a dwarf star of spectral class F0 is 96 light years away.

  STF1121 (07 36 35.71 -14 29 00.3) is not a double or multiple star - rather it forms the bright core of the open cluster M47 in Puppis.

  This cluster was discovered by the Italian astronomer Giovanni Hodierna sometime before 1654. As well as finding a dozen or so deep-sky objects before Messier catalogued them, Hodierna also compiled a small list of double stars.

  The WDS contains 26 entries to cover this system and its large array of comites, but the small telescope user will easily be able to see AB (6.9, 7.3 at 300° and 6".5), whilst amongst the more obvious comites D is mag 9.5 at 72" (distance increasing), E is 9.9 at 70" (distance decreasing) and G is 7.7 at 82".

  It is perhaps best seen with a pair of large binoculars. A report on the Cloudy Nights website for 2004 notes that the AB pair can be split easily with Celestron 25 x 100s. M47 and nearby M46 can be swept up in a wide-field telescope by moving 20 degrees due south of Procyon.

  Bob Argyle - Double Star Section Director

• January 2016 - Double Star of the Month

  STT 147 (06 34 19.37 +38 04 33.6) is located in Auriga and is about 35 arc minutes south preceding the very red star UU Aur which is visual magnitude 5.3 and has an exceptional B-V colour index of +2.6. The field can also be found from the bright binary theta Aurigae by moving about 9 degrees due east. Burnham describes it as an almost equilateral triangle with the sides being about 40" long and the stars A, B and C being respectively 6.8, 8.7 and 9.9. Sissy Haas notes that A is bright orange and indeed the spectral type of this star is K0.

  When the pair was first observed by Otto Struve at Pulkovo he also noted that C was a very close and difficult pair. The WDS gives the magnitudes as 10.6 and 11.0 with PA 109 and separation 0".5. There seems to be little motion in this system. Burnham notes that it was 'difficult' in the 18.5-inch Dearborn refractor and no measures have been made since 1957.

  So here is a challenge for the larger aperture user and a real test of the seeing and transparency. Another neat pair (STF 928) can be found 27' north following, mags 7.9 and 8.6 at 131 degs and 3".5. These stars have common proper motion and appear to form a binary pair. An unconnected mag 12.4 can be found at 124 degs and 131" to B.

  HJ 3857 is in Columba (06 24 01.02 -36 42 28.4) about 2.5 degrees south following the 4.4 magnitude kappa. This is also an easy triple although star B did not appear to Dunlop when he recorded this pair as number 28 in his catalogue, but was swept up a few years later by John Herschel at Feldhausen.

  The WDS catalogue values for the magnitudes are 5.7 (A), 9.8 (B) and 6.9 for C. The Dunlop component would appear to be a field star and has moved 6" closer to the closer pair over the last 2 centuries or so. Gould, using 175-mm, notes that the primary is orange and the wider companion is bluish.

  Bob Argyle - Double Star Section Director

• December 2015 - Double Star of the Month

  Epsilon Arietis = STF 333 (02 59 12.73 +21 20 25.6) can be found 17 degrees following beta Arietis. It is in a rather sparse area of sky and for those without setting circles this is probably the easiest way to find it.

  It is a splendid binary star with both components of mags 5.2 and 5.6 being white to most eyes although W. H. Smyth fancied that he saw a pale yellow hue to the primary star and the secondary was 'whitish'. Smyth also noted that the Reverend Dawes first saw this pair at his observatory at Bedford.

  Struve noted is as being amongst the closest of his discoveries (0".5) and subsequent observations showed it widening with a small increase in position angle. Over the last century there has been little motion of note except that the companion now seems to have reached greatest elongation and is slowly heading back towards the primary star.

  In Webb Society Double Star Circular No 17 (2009) Ian Coster produced an orbit for STF 333 with a period of 313 years and three years ago Francisco Rica published one with a period of over 1200 years, so the future motion is almost entirely indeterminate. The pair will certainly remain an excellent test for 10-cm aperture for a few years yet, and the current separation is 1".34. A faint field star of mag 12.7 is 146" distant.

  John Herschel swept up the coarse triple HJ 3644 (04 21 31.29 -25 43 42.4) in 1836. It is located in an empty region in Eridanus and can be found by firstly locating the bright pair nu3 and nu4 Eri and moving about 9 degrees due north.

  Herschel noted the stars had magnitudes 6, 8 and 14 and only estimated angles and separations. Modern catalogues give the brighnesses as 6.2, 8.2(C) and 13.0(D) and the distances between AC and AD are now 41" and 44". Burnham found the A star to be a close double in 1879 when using his 6-inch refractor on Mount Hamilton in California, fortuitously as it turns out because the pair was then at its widest separation of 0".65.

  For a few years after discovery motion appeared rather slow but accelerated considerably in the second decade of the last century and the pair closed up to 0".2 by 1920. Modern computations give orbital period as 81 years and at the present time the stars are slowly widening. The position for 2016.0 is 222°, 0".40 so at least 25-cm is necessary to see the pair divided. The WDS lists the magnitudes of AB as 6.6 and 7.3.

  Bob Argyle - Double Star Section Director

• November 2015 - Double Star of the Month

  56 And (01 56 09.23 +37 15 06.5) is a very wide pair of stars which can be found 5 degrees south and a little preceding that spectacle of the autumn sky, gamma Andromedae.

  It is notable for the colours of the stars involved.

  Here we have a K0 giant (A) and an M0 giant (B) within 200" of each other but actually not associated at all. The M star is about 3 times further away than the other according to Hipparcos.

  Although the WDS gives visual magnitudes of 5.7 and 6.0, historical reports sometimes put star B as the brighter of the two - it is likely that either or both stars are liable to some small variation in brightness of a size such as to blur the difference between the two altogether.

  Burnham, who would have been interested in this system because of the large proper motion of A (0".2 per year), found a faint companion of mag 11.9 at 18" and 77 degrees from A.

  Also nearby (two-thirds of a degree preceding and a little north, according to the discoverer William Herschel), is STF 179, containing stars of magnitudes 7.6 and 8.1 separated by 3".5, and the open cluster NGC 752 is less than a degree north following.

  Bernhard Hildebrande Dawson was an Argentine astronomer who was born in Kansas City in 1891 and who worked initially at La Plata Observatory in Argentina. He was perhaps best known as the discoverer of Nova Puppis 1942.

  His double star discoveries are denoted by a small greek delta and number 31 is perhaps his most interesting find - a visual pair with a period of 4.56 years.

  He started using the 17-inch refractor at La Plata in 1912 to pursue a programme of re-measurement of the John Herschel pairs. The first star in his catalogue (DAW 1 at 02 27 57.34 -58 08 22.3) was found in 1916 and sits in a rather blank area of sky in Horologium about 13 degrees following the first magnitude Achernar. This is a triple system with the two widest components (17" apart) being found by Sir John Herschel in South Africa.

  In 1916 Dawson divided A and found a separation of 0".8. The current value is 1".2 so assuming it is still widening this would explain why Herschel did not see it during his sweeps.

  The WDS gives A and B as 8.0 and 8.5 whilst C is 9.6. The range of position angle of AB is close to 180 degrees, a sign that the magnitudes of the two stars must be close enough to cause uncertainty about the quadrant in which B sits.

  Ross Gould from Australia using a 40-cm sees A as a bright yellow star whilst x140 will show the Dawson companion.

  Bob Argyle - Double Star Section Director

• October 2015 - Double Star of the Month

  STF 2816 (21 38 57.62 +57 29 20.5) which sits within the cluster Trumpler 37 (and the HII region IC 1396) and is a multiple star with three stars visible to the small aperture.

  AC are stars of magnitudes 5.7 and 7.5 separated by 12" in PA 120 and with little motion visible. The primary is a very hot star of spectral type O6f. Some 20" away in PA 338 is a further star (D) of magnitude 7.8. This in turn has a 13.2 mag comes at 351 degs and 55". In 1889, using the Lick 36-inch refractor, S. W. Burnham added a 13.3 star to A at a distance of 1".7. At the highest angular resolutions A is double again with a companion at a distance of 0".1 and it is also a double-lined spectroscopic binary with a period of 3.7 days so it seems likely that this is a very close triple.

  For the binocular user, the stars that make up the wide pair of pi1 and pi2 Gruis make a splendid sight. They lie about halfway between alpha and beta Gruis and a degree north of that line.

  The brighter of the two is pi1 Gru (22 22 44.2 -45 56 52.61) at V = 5.62. This is a F3 star which is a giant or sub-giant and which has a large annual proper motion. Its distance is correspondly small - 130 light years. Some 269" east and slightly north is pi2 at V = 6.55 but extremely red in colour. It is a member of the rare spectral class S and has a (B - V) index of 2.1. Looking at it on the POSS images it appears slightly brighter than pi1 but when the 2MASS survey image is examined, its overwhelmingly bright image practically obliterates that of its companion.

  Both stars are double and both were discovered by Robert Innes. Pi1 is I 135 - the companion is a GO dwarf, magnitude 10.7, distant 2".5 in PA 200, although observations of the system with VLTI show a spiral-shaped arc of emission which may be due to orbital motion of this dwarf and indicates the possibility of a third star, much closer in with a period of less than 10 years. Pi2 is I 382 where the 11.4 mag companion has moved from a separation of 4".6 to 11" over the course of a century, due to the large motion of A.

  Bob Argyle - Double Star Section Director

• September 2015 - Double Star of the Month

  Nestling in the region about 4 degrees north following the upper left-hand corner of the Square of Pegasus, AC 1 (HIP1669) (00 20 54.10 +32 58 40.9) seems to have been a little neglected by the double star community. It doesn't even have a note in the WDS catalogue but things may change now that Henry Zirm has published an orbit for it.

  It was discovered by Alvan Clark using a 7.125-inch object glass of his own make in October 1856, and published in MNRAS a year later with additional notes by W. R. Dawes. At the time of discovery it was 0".4 apart and Dawes was of the opinion that it escaped the attention of F. G. W. Struve at Pulkovo because it was too close at that time.

  This view has been borne out - the stars are now easily measurable with a 20-cm refractor. The magnitudes are 7.3 and 8.3. Hartung notes that the stars are 'deep-yellow' and also points out the presence of a orange-red star some 4'.5 SW (HR 59 - spectral type K5III). Zirm's orbit with a projected period of 525 years is clearly provisional as the stars are close to maximum separation and only 12 degrees has been described in the apparent orbit. The author found 289°s; and 1".9 in autumn 2014.

  Psi 1, 2, and 3 Aquarii are three bright stars of visual magnitude 4.2, 4.4 and 5.0 respectively. They can be found about 3 degrees east of the centre of the line joining alpha Peg to Fomalhaut, in a region rather low down from the UK but filled with interesting visual doubles.

  Some 4 degrees directly below psi 3 is 94 Aquarii (23 19 06.51 -13 27 30.4) a fine, wide pair which is worth seeking out. Its mag 5.3 and 7.0 stars appeared yellow and orange to Hartung who was able to observe them close to the zenith whilst the Reverend Webb noted a reddish glare in A whilst B appeared greenish.

  The proper motion of the pair amounts to more than 0".3 per year so the fact that the change recorded in separation over 200 years amounts to only 2 arc seconds tells us that this is a physical pair, and it is located 69 light years away. In 1976 McAlister and colleagues discovered that B was a close pair (MCA 74) and it has subsequently turned out to be a binary of short period. It rotates every 6.3 years and the separation is never more than 0".2.

  Bob Argyle - Double Star Section Director

• August 2015 - Double Star of the Month

  Lambda Cygni (20 47 24.563 +36 29 26.7) is easily found. Just move 3 degrees due north from epsilon Cygni, the left-hand star in the cross. One of Otto Struve's discoveries from Pulkova the seeing needs to be good to get a measure of the companion. The stars are magnitudes 4.7 and 6.3 but have always been separated by less than 1" although the angular distance between the stars is now roughly double that at discovery. Since 1990 the writer has observed the pair in 12 seasons and in that time the position angle has decreased by about 10 degrees. The current orbit has a period of 391 years but this is complicated by the fact that Hal McAlister and colleagues found the primary to be an interferometric binary with a period of 11 years and maximum separation of 0".05. There is some evidence that one of the three stars is also a single-lined spectroscopic binary. Lambda Cyg has a spectral type of B5Ve and is a rapid rotator surrounded by a circumstellar disk. Sir James South adds a faint companion, mag 9.7, at 106° and 83".

  The sprawling constellation of Capricornus sits near the meridian on a northern summer night but locating stars in it apart from the third magnitude alpha and beta needs the help of a star atlas. However, starting with the brightest star of all, beta, by moving 3 degrees south and slightly east a trio of stars is encountered, all enclosed by a 1 degree field. Each of these is a visual double star and the subject of this column is omicron Cap = SHJ 324 (20 29 53.91 -18 34 59.4) the most southerly of the three. Smyth calls it omicron2 and notes that both components are to be found in Piazzi's catalogue. The WDS gives magnitudes of 5.9 and 6.7 and the separation is currently 21".9, down from 25" when found by William Herschel with a small decrease in the position angle, currently 238°. The Hipparcos catalogue gives the distance as 216 light years but with a formal error of 27% this is an indicator that there could possibly be another star nestling in the system.

  Smyth calls both stars bluish, and whilst Sissy Haas regards them as almost equal, the report by Hartung notes that they are an 'unequal white pair'.

  Bob Argyle - Double Star Section Director

• July 2015 - Double Star of the Month

  lambda Ophiuchi - STF2055 (16 30 54.84 +01 59 02.8) lies 20 degrees due south of the south-easternmost star in the Keystone of Hercules, beta Herculis. During the 19th century, measurements showed the two stars slowly separating and reaching a maximum distance of about 1".9 in the early 1880's. In 1906, Thomas Lewis, in his book on the Struve stars, considered that the period was about 130 years which as it turns out was a good attempt. The current period from observations of almost 2 revolutions gives a period of 129 years and a predicted position for mid-2015 of 42° and 1".43. The pair is not particularly easy to observe, partly because of its altitude in the sky - it is barely above the celestial equator - and partly because of the magnitude difference between the two components (4.2 and 5.2). Both stars are A0 dwarfs, according to the WDS, and lie at a distance of 173 light years. There are two much fainter stars at 120" and 308" which appear unconnected to the system.

  DUN 219 (17 58 55.69 -36 51 30.2) lies not far from the spectacular open cluster Messier 7 and follows G Sco by about 2 degrees. It has not been included in either Hartung or the book by Sissy Haas but is nevertheless is a splendid sight. The stars are magnitudes 5.8 and 7.8 and the separation derived by Dunlop in 1836 of 47".1 has now increased to 53".5, (with the position angle changing from 265° to 252° over the same interval of time), making it a striking object object in small telescopes. The USNO include the pair in their linear elements catalogue, confirming that the two stars are entirely unrelated. A third star of mag 11.3 can be found at 40" from A. Just 16 seconds of time following and 8 arc minutes south is HJ 5000 AB, stars of magnitudes 7.1 and 8.9 separated by 7".3 but apparently in orbital motion. The writer measured this pair in 2010 and noted pale yellow and pale blue - pretty.

  Bob Argyle - Double Star Section Director

• June 2015 - Double Star of the Month

  Two bright double stars, each with its own observational difficulties, feature this month.

  In the constellation of Draco, eta Draconis (16 23 59.51 +61 30 50.7) is circumpolar for observers in the UK but as a classical refractor user I find it is easiest to observe when in the north-west in the early evening. The primary is a G8 giant (V = 2.8) which is 92 light years distant and as a relatively nearby object it has attracted the attention of the astronomers wishing to determine its diameter. The result is that it is a shade under 10 million miles in diameter or about 11 solar diameters. The mag. 8.2 companion was first noted by Otto Struve at Pulkova in 1843 and he found it at PA 150° and separation 4".4. Since then it has moved retrograde by 11 degrees and the separation has slightly increased. I measured the pair on 4 nights in 1994 but in recent years have not been able to see the companion. The primary is also recorded as being variable so here is a pair to keep an eye on.

  epsilon Lupi (15 22 40.89 -44 41 22.5) first attracted the attention of James Dunlop from Paramatta as a fine unequal and wide pair (DUN 182 AC). The stars are magnitudes 3.6 and 9.1 and today are separated by more than 26", a considerable increase on Dunlop's figure of 19". When Ralph Copeland, who later became Astronomer Royal for Scotland, visited the Andes with a 6-inch refractor for site-testing purposes, he found the primary star was a close and unequal pair which has since turned out to be a binary of period 319 years. Star B, mag 5.1, ranges in distance from A between 1".2 (as in 1834) to 0".18 (in 2026); in 2015 the pair are separated by only 0".26 and needs a large aperture. More recently, in a survey of stars in the Sco-Cen-Lup-Cru association, Rizzuto and colleagues, using the Sydney University Stellar Interferometer (SUSI) found another star of magnitude 5.1 but closer in at a distance of 0".05. It is not clear whether this is the known spectroscopic binary component of A (which has a period of 4.55 days), but epsilon appears to be at least a massive triple system.

  Bob Argyle - Double Star Section Director

• May 2015 - Double Star of the Month

  STF 1694 (12 49 13.80 +83 24 46.3) is one of the most northerly of the bright double stars and lies in a rather sparse area of the sky. It is worth making the effort to find it as the planetary nebula IC 3568 lies 1.5 degrees south-west. Both stars are in the Hipparcos Catalogue and though the parallaxes seem significantly different (8.14 mas for A and 5.57 mas for B), the large error on that of B means that they are the same within the quoted uncertainties. However, B is a spectroscopic binary and this may well account for the Hipparcos satellite's problems with defining its distance accurately. The primary star (V = 5.3) is an A1 giant whilst the companion (V = 5.7) consists of a pair of A0 and A2 dwarfs. The current position angle and separation is 324° and 20".9, little changed from the epoch of discovery. The proper motion of RA of both stars is significant and similar so it would appear that this is a common-proper-motion pair. In 1944 Wallenquist found a third star of magnitude 11.5 at 223° and 73" but both these values are decreasing quite quickly. Sissy Haas calls the primary 32 Cam and notes that both stars are lucid-white.

  4 Centauri (H N 51) is located at 13 53 12.54 -31 55 39.4. Also known as h Cen, this is one of the pairs found by William Herschel in his last concerted campaign of double star observation and was observed on 1787, Mar 15. It is in an arc of faint naked-eye stars some 5 degrees north-west of theta Centauri and is a splendid sight in a small aperture. The stars, magnitudes 4.7 and 8.5, were measured by the writer in 2013 and were found to be at 185 degrees and 14".8. Hartung notes that the colours are pale yellow and ashy, and that both stars are spectroscopic binaries. The primary star is a subgiant of spectral class B4 which Hipparcos puts at a distance of 637 light years.

  Bob Argyle - Double Star Section Director

• April 2015 - Double Star of the Month

  STF1555 (11 36 17.94 +27 46 52.7) is in Ursa Major in a fairly sparse part of the constellation down by the Bear's front foot. It is perhaps most eassily found by moving 5 degrees south-east from the bright binary xi UMa (STF1523). This was an easy pair for the small aperture at discovery at 1".4 but during the next one hundred years the two stars approached each other until minimum separation of about 0".1 was reached in the 1930s and since then they have been separating. The nature of this system is a little unclear but observations over the next decade or so will show whether it is an optical pair (as classified in the WDS, or a highly inclined binary system, as suggested by Docobo in 2007 when he derived an orbit of 916 years for it. The orbit predicts star B beginning to turn back towards A with the separation slowly decreasing again. In spring 2015 B can be found at 150°, 0".67. The stars are magnitudes 6.4 and 6.8, and a third component of magnitude 11.2 which is listed as HJ 503, can be seen at 158° 22".5, both values are slowly increasing.

  RMK 14 (12 14 02.71 -45 43 26.1) can also be found by reference to a nearby bright binary star. It forms an isoceles triangle of side about 5 degrees with gamma and delta Centauri to the south. Unfortunately, as of early 2015, gamma is near closest separation and needs a large aperture to resolve but RMK 14 (D Cen) is a beautiful pair which is worth searching out. The primary is a K3 giant of visual magnitude 5.8 and is also known to be a spectroscopic binary. The companion can be found at 243° and 2".7 having closed up from 4" at discovery. This is a distant pair - Hipparcos lists the parallax as 5.71 mas which, as it happens, translates to 571 light years. The colours seem to be well determined. E. J. Hartung gives orange and white whilst more recently, Richard Jaworski finds yellowish-orange and white. Sissy Haas notes it as a showcase pair.

  Bob Argyle - Double Star Section Director

• March 2015 - Double Star of the Month

  With Cancer now moving inexorably towards the western horizon, its stars are becoming more comfortably accessible to the classical refractor user and one of its finest doubles is 57 Cnc (08 54 14.70 +30 34 45.0).

  According to Webb, it is a pair of crocus-yellow stars which are not quite equal in brightness (magnitudes 6.1 and 6.4). There has been little motion of any kind save a small increase in both position angle and separation with the result that the separation is now well over 1".

  It is an ideal test for a 7.5-cm refractor. Webb was able to divide it at x144 on his 3.7-inch Tulley OG, and the writer found 1".55 with the Cambridge 8-inch in 2006.

  The pair is almost certainly binary in nature as the proper motion of the system would have separated them a long time ago. A faint component can be found at 55" and PA 202° which appears fixed. The WDS gives its magnitude as 9.2.

  57 Cnc is about 2.5 degrees north following the spectacular wide pair iota Cnc.

  Gamma Sextantis, also called 8 Sextantis (09 53 30.47 -08 06 17.7) was discovered on 1852 Apr 7 by Alvan Clark with a 12-cm aperture. As it happens he observed the star at a suitable part of its orbit but this discovery is a good indication of the quality of Clark's telescopes.

  He said, reporting his finds in Monthly Notices for 1857, Notwthstanding the moderate meridional altitude of of 8 Sextantis at Dorpat (about 24°), it may reasonably be doubted whether its duplicity would have been left to be discovered with a 4 3/4-inch object-glass, however perfect, if no change had occurred in its appearance since Struve's scrutiny of that part of the heavens.

  The separation of this close pair is never wider than 0".6. It reached maximum separation in its 77.8 year cycle in 2002 and is now closing again. By Spring 2015 it will be at 44°, 0".54 and the low declination means that it will be a significant test for 20-cm and its more likely that a good, steady air will be essential to resolve it, especially as there is a considerable difference in brightness between the components - A is 5.4 and B is 6.4. Hipparcos places the system at 278 light years distance and the primary is an A1 dwarf.

  To find gamma Sextantis, locate alpha Hya and move 5 degrees east.

  Bob Argyle - Double Star Section Director

• February 2015 - Double Star of the Month

  In mid-February, Gemini passes the meridian at 10pm and offers an excellent opportunity to view some of the fine double and binary stars in this constellation. One of the less-well known systems is 20 Gem (06 32 18.52 +17 47 03.4) which is eaily found about 3 degrees north preceding the bright star gamma Gem. Wil Tirion's Cambridge Star Atlas shows another double star about a degree south of 20. This is STT143 (6.2, 10.4, 103°, 7".6 with a fainter third component at 345° and 47"). Also known as STF924, 20 Gem offers a beautiful pair of stars which Webb noted as topaz yellow and cerulean blue, but as these are the exact hues noted by Smyth in the Bedford catalogue it may be assumed that Webb was merely repeating the Bedford colours. More recently Sissy Haas finds both stars to be gloss-white.

  There is little relative motion between the stars and the current situation is that the PA is 211° and the separation 20".2. Hipparcos found the determination of the parallax of star A difficult and finds a distance of 262 light years with an error of 30%. This is no doubt due to the motion induced by the duplicity of A confirmed by both occultation observations and also directly as a spectroscopic binary.

  Looking about 1° slightly north preceding the V = 4.4 star q Puppis, one will alight on the wide pairs DUN 67 (08 13 58.31 -36 19 20.2) and DUN 68 about 1.5 arc mins south and west of it. DUN 67 consists of stars of magnitude 5.0 and 6.0 which are currently separated by 66 arc seconds and PA 174°. The separation is slowly decreasing. Hartung chooses not to include them is his book 'Astronomical Objects for Southern Telescopes' but Gould with 175-mm finds both stars to be pale yellow and notes the existence of several fainter pairs in the area. The components of DUN 68 are both mag 7.3 but form a wider pair than its neighbour. The PA is 25° and the separation is 125" and increasing.

  The whole region is fine with the large scattered star cluster NGC 2546 about 1.5° further south adding further reason to take in the area with a pair of binoculars as well. Both pairs are at the same distance from us within the stated errors of the Hipparcos parallaxes.

  Bob Argyle - Double Star Section Director

• January 2015 - Double Star of the Month

  STF 644 in Auriga (05 10 18.81 +37 18 06.7) is a pair noted for the contrasting colours of its components. The spectral types of the two stars are respectively B2II and K3 and when W. H. van den Bos observed the pair with the Lick observatory 12-inch refractor in 1962 he noted the colours were white and reddish. E. J. Hartung noticed this colour disparity too - he found orange amd white with 10.5-cm aperture adding if merely elongated the appearance of the ellipse is striking with one end orange and the other white. Sissy Haas on the other hand with 125-mm at x200 found both stars to be yellow-white.

  This is a close pair which has remained virtually motionless since it was discovered by F. G .W. Struve. The magnitudes are 7.0 and 6.8 with the orange component being slightly brighter visually. G. van Biesbroeck added a distant mag 10.5, K3 dwarf at 192 degs, 72". To find STF 644 look about 1.5 degree south of mu Aurigae. It is the preceding of two mag 6 stars - the other being the unequal wide pair SEI 105 (6.5, 11, 27 degs, 35"), about 1 degree following.

  HJ 3683 (04 40 17.72 -58 56 39.5). Very fine noted John Herschel from South Africa in 1836 when he first laid eyes on this pair of stars and gave them both magnitude 8. The WDS catalogue says they are 7.3 and 7.5 but the main interest lies in the nature of the apparent orbit of this 326 year binary. The high inclination means that motion is restricted to a narrow range of position angles and so mainly manifests itself in separation, whilst the very high eccentricity (0.95) means that the maximum separation is 4".3 whilst the minimum is only 0".03 which, given the Hipparcos parallax of 32.77 mas, indicates that the two stars are only 3.7 AU apart at periastron but 144 AU apart at apastron. When at their nearest separation the position angle changes by 1 degree per day, almost 5 times faster than that managed by the two components of gamma Virginis at periastron.

  The pair is currently at 3".7 and so is an easy object for the small telescope. Gould with 175-mm calls both stars yellow and the pair can be found about three degrees south and slightly following alpha Doradus.

  Bob Argyle - Double Star Section Director

• December 2014 - Double Star of the Month

  The double stars being featured this month require apertures around the 30-cm mark. Both are close binaries with periods in excess of 200 years and both are slowly widening, but at the time of writing each pair is at or below 0".5 separation.

  52 Arietis (03 05 26.69 +25 15 18.7) is also known as STF 346. At discovery this was an 0".7 pair and motion during the remainder of the century was slow but by the early 1930's the pair was out of reach of all telescopes and circling around each other at the rate of 1 degree per week.

  Since then the companion has been heading out towards the discovery position but even now it is still a difficult pair. The ephemeris for 2014.6 gives 258 degs and 0".49, and the stars are almost equally bright, both being around 6.2 visual. The writer was just able to measure it for the first time last autumn with a 20-cm Cooke refractor and will attempt to get some confirming measures later this year.

  The two stars are accompanied by a 10.8 at 5" distance which is physically attached to the system. Another star of mag 13.2 at 105" appears to have been found by Smyth in 1835 and is mentioned in the Cycle of Celestial Objects. Smyth gave the magnitudes of C and D as 15 and 13, no doubt reflecting the difficulty of seeing C rather close to the bright binary components.

  At the turn of the nineteenth century, Walter F. Gale was an active amateur astronomer living in Paddington, New South Wales and found a few double stars with an 8.5-inch reflecting telecope made by George With. He published a short list of discoveries in Astronomische Nachrichten in 1896, consisting of 5 double stars and a ring planetary nebula (IC 5148 - the Spare Tyre Nebula). Two of the pairs turned to be already known so the WDS now contains but three of his double stars.

  The second object on the list was a close pair in Reticulum now called GLE 1 (04 16 20.92 -60 56 54.8). The stars, whose visual magnitudes are 6.8 and 7.5, passed through periastron in 2002 and are now slowly widening again although the current separation (0".35 at 218 degs) does require at least 30-cm and a good night. This system also contains the star TT Ret which is an alpha CVn variable with small amplitude and period of 2.8 days.

  Bob Argyle - Double Star Section Director

• November 2014 - Double Star of the Month

  10 Arietis (=STF 208, 02 03 39.26 +25 56 07.6) is a marginally naked-eye star at the preceding end of a string of similar stars some 2.5 degrees north of alpha Arietis. Found by F G W Struve in 1821, when the separation was 2", the motion of the companion over the next 80 years appeared more or less linear but it became progressively more curved as the pair closed to 1". The companion then made a closest approach at 0".3 in 1920 or so and since then has widened and can be found at 347 degs, 1".49 in 2014.8. The currently accepted period is 325 years but this could well be an underestimate as nothing is known about the motion at apastron. The pair should be divided in 15-cm but a good night is needed because the magnitudes of the stars are 5.8 and 7.8. A 13.5 mag. can be glimpsed 95" distant in PA 150 degs.

  kappa Tucanae (01 25 45.50 -68 52 34.5) is located about 5 degrees north following the Small Magellanic Cloud and Burnham's Handbook spends a lot of time on the latter but only notes in passing the former which is a splendid object for both binoculars and telescope. For the binocular user there is a mag 7.8 star which is 319" from the mag 5.00 primary. John Herschel found A to be a fine pair (HJ 3423, mags 5.0 and 7.7) - 'very beautiful' was his verdict. E J Hartung gives the colours as yellow and orange. Two measures at Feldhausen yielded separations of 2" and 2.5" and a mean position angle of 11.7 degrees. Recent measures in 2009 show that the separation has doubled to 4".8 and the position angle is now 319 degs. This is a binary system with a period of about 850 years.

  In 1895 Robert Innes found that the distant 7.8 mag star was also double but much more equal in brightness (7.8 and 8.4) and also a binary of fairly short period. He was using a reflecting telescope owned by Mr. F. Dixon Edmonds, an early member of the BAA, and in presenting his list of 16 new pairs Innes said that the telescope had enabled the 'discovery of more new double stars than all the rest of the silver-on-glass reflectors ever made'. I 27 CD revolves every 85.2 years in a circular orbit only slightly tilted to the line of sight so the separation ranges between 0".9 an 1".1 and the angular motion is a fairly constant 4 degrees or so. Innes also noted that both AB and CD have similar proper motions so this is likely a quadruple system.

  Bob Argyle - Double Star Section Director

• October 2014 - Double Star of the Month

  About 2 degrees north of iota Cephei can be found the pair STF2947 (22 49 00.68 +68 34 12.2). This neat 4".6 pair can be well seen in 15-cm and the magnitudes are 6.9 and 7.2. Sissy Haas notes that iota is golden colour and STF2947 is a pair of yellowish-peach stars. Hipparcos does not appear to have observed this pair but it does appear in the 1952 Yale Catalogue of Parallax where the distance is given as 120 light years but with an uncertainty of 20%. A third star of magnitude 12.5 can be found at 208 degrees and 121" but it does not share the space motion of the close pair. About two degrees south is STF2948 (7.3, 8.6, 4 degs, 2".6))

  Theta Gruis (23 06 52.77 -43 31 17.2) is the brighter component of the very wide pair SHY 366. The nomenclature refers to Shaya and Olling who in 2010 made a study of wide pairs in the Hipparcos catalogue for which the proper motions were very similar. In the case of Theta Gruis they concluded that the likelihood that A and C (mags 4.5 and 7.8, 292 degs, 159") were physical was 100%. The distances to A and C are respectively 131.9 and 130.4 light years. Jacob then discovered that A itself was a close pair with star B of magnitude 6.6, being found at 114 degrees and 1".5 in 2009. William Stephen Jacob was an Army engineer with a deep interest in astronomy an during secondment on duty in India in the 1840s managed to make some observations of double stars. He used the 6.3-inch Lerebours refractor at Madras to make some micrometric measurements and also discovered a number of new pairs. The WDS contains 24 pairs bearing his discovery number which also includes the binary JC 8 and the Antares lookalike-pair 21 Sgr = JC 6 (see the column for Sep 2008).

  Bob Argyle - Double Star Section Director

• September 2014 - Double Star of the Month

  H 1 48 (21 13 42.46 +64 24 15.1) is a very rare example of one of William Herschel's close discoveries which retains its original designation instead of being absorbed into the catalogue of F. G. W. Struve, as many of his pairs were. It is also remarkable as being a fairly short period system which has both high eccentricity and high inclination. It is characterized by periods of rapid angular motion at very small separations and then stretches of decades when it is visible to the medium aperture. The brightness of the stars (7.2 and 7.3) means that it is never an easy object in small telescopes but may well be visible in 15-cm after 2020 or so when the separation slowly increases to 0".9. The ephemeris for the 81.7 year orbit by Marco Scardia and colleagues gives a separation of 0".72 and PA 243° for 2015.0. The pair can be found 1 deg south preceding 6 Cephei which is, in turn, 3 degrees north of alpha Cephei.

  BSO 15 (21 48 15.75 -47 18 13.0) is a naked eye star in Gruis about 4 degrees preceding and slightly south of alpha. It was found to be double by Thomas Brisbane in the early 1830s. The primary is a GO dwarf of V = 5.6. Hipparcos places this star at a distance of only 52 light years and as a consequence it has a fairly substantial proper motion of almost one-third of an arc second per year. The mag 8.8 companion is not connected and is being rapidly left behind by the proper motion of A. The pair was first accurately measured by John Herschel in 1836 when he found B at a separation of 30.3" in PA 14°. Last year the writer measured the pair again and found a distance of 78".9 in PA 350.2°, in good agreement with the prediction given in the USNO Linear Elements Catalogue. This pair has not been observed by either Hartung or Haas and the writer didn't note any significant colour in either component. Recent observations by the infra-red Herschel telescope show there is a large proto-planetary dust ring around this star stretching from about 100 to 180 AU.

  Bob Argyle - Double Star Section Director

• August 2014 - Double Star of the Month

  STF2375 (18 45 28.36 +05 30 00.4) is a pair of 6th magnitude stars found about 2.5 degrees north preceding the beautiful pair theta Serpentis. At discovery in 1825 F. G. W. Struve found 108° and 2".2. Orbital motion, for it appears to be a binary, has been rather slow. By 2010 the position angle had advanced to 120° and the separation to 2".6. Interest in the system was renewed in 1952 when Dr. William Finsen was observing the pair with his newly constructed eyepiece interferometer on the 26.5-inch refractor in Johannesburg. When an apparently single star is examined with the interferometer if it is in fact a close double there will be formed a set of fringes which disappear when the instrument is rotated so that the slits are parallel to the line joining the stars, in other words the position angle. Finsen was somewhat surprised when he found that there were fringes on both stars and they disappeared at exactly the same angle of the interferometer. It transpires that both stars were equally close pairs with identical position angles. It led Finsen to call them Tweedledum and Tweedledee. Since then orbital motion has destroyed the symmetry of the pairs and Aa-Ab is currently at 0".13, whilst Ba,Bb is now only separated by 0".08. Aa,Ab has a period of 27 years whilst that of Ba,Bb is 38 years.

  DUN 224 (18 54 01.4 -47 16 27.4) is a beautiful triple star in the north of Telescopium, about 2 degrees south of the border with Corona Australis. Dunlop found the wide pair in 1826. The stars are mags 7.1 and 7.3 and currently separated by 87" so its likely that they could be seen in binoculars of sufficient aperture. R. T. A. Innes found the primary to have a closer, unequal companion - mag 9.1 at 1".8. There has been little change in separation since then but the PA has moved on to 192°. The two bright stars are unassociated. Both have parallaxes determined from Hipparcos; star A is 225 light years distant whilst B is 626 light years away. Ross Gould notes colours of yellow and white.

  Bob Argyle - Double Star Section Director

• July 2014 - Double Star of the Month

  zeta Her (16 41 17.46 +31 36 07.0) is part of the Keystone of Hercules, the south preceding component of the four stars in the pattern. Only 35 light years from us, it has long been known to be a close and difficult pair, but at the time of writing it is opening up and will soon be as easy to resolve as it gets. The ephemeris for the 34.5 year period orbit shows that in summer 2014 the stars are at 140° and 1".20. The difficulty comes with the large difference in magnitude - in the visual the components are magnitudes 2.95 and 5.40. The writer has followed this pair since 1990 and has been able to measure it each year since apart from 2001 - 2004 inclusive when it was too difficult for the 20-cm Cooke refractor at Cambridge. Over many years there have been suggestions of a sub-period due to one of the stars being a close, unresolved pair, and in 1983 a third component was detected in the infra-red but since then no confirming observations have been made and at present it is assumed that zeta is a simple binary star. The primary star is of spectral type early G and sometimes appears orange to observers with the companion appearing green by contrast.

  eta Oph (17 10 22.66 -15 43 30.5) was one of S W Burnham's later discoveries and is also known as BU 1118. This bright, twin pair of white early A stars of visual magnitudes 3.05 and 3.27 was separated by 0".4 at discovery in 1889. Like zeta Her above this pair is now close to its maximum separation and is actually starting to close. In summer 2014 it will be found at 232° and 0".57. This needs 30-cm on a night of very good seeing as it is low from the UK. The orbital motion accelerates rapidly as periastron approaches in 2024 at which time the stars are 0".006 apart and moving at 15 degrees per DAY. The period of this highly inclined and very eccentric (e = 0.95) orbit is 88 years. There are two faint comites of magnitude 11.2 and 12.4 both about 100" distant. Eta (combined magnitude 2.4) can be found about 15 degrees north following Antares.

  Bob Argyle - Double Star Section Director

• June 2014 - Double Star of the Month

  STF1883 (14 48 53.22 +05 57 15.9) is in Virgo near the northern border with Boötes. In `Celestial Objects' however it appears in Boötes. In Thomas Lewis' work on the Struve stars he gives the magnitudes of both stars as 7.0. The WDS however lists 7.02 and 8.95. The current value for B must be somewhere between these limits as the writer has resolved this pair with the 8-inch Cooke at Cambridge which would have been much more difficult if B was near magnitude 9. The WDS notes than one star is variable - this is presumably B and the amplitude must be considerable if the stars were deemed to be equally bright a century ago. The star has only the designation SV ZI1089 and does not appear to be a fully-fledged IBVS variable star. STF1883 is a binary star of period 216 years which has a highly inclined and eccentric orbit. Near 1".2 at discovery in 1830, it closed to around 0".25 in the early 1930's before widening to 1" where it is today. This value will not increase very much before the stars begin to close again in about 30 years time.

  HJ 4788 is d Lupi (15 35 53.25 -44 57 30.0) which can be found as one of triangle of naked-eye stars some three degrees following epsilon Lupi. This pretty pair consists of pale and deep yellow components in a well-occupied field (according to E. J. Hartung). The stars are mags 4.60 and 6.51 and the separation has slowly decreased from 3".1 in 1836 to 1".9 in 2013 whilst the position angle has increased from 349° to 13° over the same time interval. The primary is an early B-type dwarf which is 428 light years away and the WDS notes it is a spectroscopic binary.

  Bob Argyle - Double Star Section Director

• May 2014 - Double Star of the Month

  The northern target for this month's column is one of the most popular of all double stars - zeta UMa (13 23 55.42 +54 55 31.5). Known equally well as Mizar, this beautiful pair of early A stars has the longest history of all telescopic pairs. First seen by Castelli in 1617, the two stars, whose magnitudes are 2.2 and 3.4 and which are separated by 14".5 were also the first double star to be photographed by Bond in 1857, and Mizar A was the first star to be shown to be a spectroscopic binary in 1889. Mizar A was also high on the list of pairs observed by Michelson with the interferometer on the 100-inch telescope at Mount Wilson in the 1920s, and was duly resolved. Some 706" distant is Alcor (mag 4.0) which forms a naked-eye pair with the Mizar system. It seems likely that the three stars are moving through space together in common with other members of the UMa moving group. In addition to Mizar A, Mizar B is also a single-lined spectroscopic binary and Alcor was also suspected a number of times of being a close binary but no proof has yet been put forward. There is, however, a faint, low-mass star only 1".1 distant from Alcor, discovered in 2009 which does seem physically connected, making the whole group a sextuple system.

  Another easy pair of B stars can be found in 3 Cen (13 51 49.58 -32 59 38.6). Discovered by William Herschel it was also picked up by James Dunlop in Australia and appears in his catalogue as Dun 148. With 32.5-cm Gould finds the colours are pale yellow and white. The stars have closed slightly since discovery and are now separated by 7".8 in 104° as measured by the author in 2013. The parallax of A, determined by Hipparcos, puts the star at a distance of 343 light years and the system is part of the Sco/Cen moving group, as is the neighbouring 4 Cen which is barely 1° north following. Another Herschel discovery, in 2013 these stars were 14".8 apart in PA 185°. Hartung notes the colours as pale yellow and ashy. Hartung notes both stars are spectroscopic binaries but the WDS only mentions the duplicity of the brighter component which has a period of 6.93 days.

  Bob Argyle - Double Star Section Director

• April 2014 - Double Star of the Month

  STT 235 (11 32 20.76 +61 04 57.9) is in UMa close to the bowl of the Big Dipper and about 5 degrees slightly south preceding alpha UMa. The pair has a period of 72.7 years and is presently opening, reaching maximum separation of 1" in 2027. At the time of writing the stars are separated by 0".88 so this is a good opportunity to resolve this pair. The components have visual magnitudes of 5.7 and 7.6 so pick a night when the seeing is good and use at least 20-cm, although 15-cm, if the optics are particularly fine, would probably show the object as double. The star appears in the Hipparcos catalogue as HIC 56290 and it has an annual proper motion in declination of about 0".1 towards the south. The mag. 11.3 star some 195" away would seem to be travelling through space with a similar motion, and was noticed by Helmut Abt. STT 235 has a parallax of 35.73 mas putting it at a distance of 91 light years.

  Far down in the southern sky, epsilon Cha (11 59 37.58 -78 13 18.5) is the brightest member at the centre of a small cluster of stars some 111 pc distant. Its nearby co-moving companion, HD 104237 (mag 6.6) is also called DX Cha and is the nearest Herbig Ae star. This is a stellar quintet with most of the companions being very young stars. In 1836, John Herschel divided eps Cha itself into two components 1".6 apart, and the pair is known as HJ 4486. The WDS gives magnitudes of 5.3 and 6.0 but orbital motion has taken the fainter star to within about 0".4 of A.

  The author made a measure of this pair from Johannesburg in 2008 and obtained 210° and 0".37 very similar to the last measure in the WDS dated 1997. A substantial aperture will be required to see this pair and it would be interesting to have a confirmatory sighting.

  Unlike many of the stars in the cluster and a wider association which are spectral class M, eps Cha is a late B star.

  Bob Argyle - Double Star Section Director

• March 2014 - Double Star of the Month

  24 CBe (12 35 07.76 +18 22 37.4) sits at the north edge of the great Coma cluster of galaxies about 3 degrees following M88, but it is in a poor naked-eye star field so that locating it is not straightforward. One way would be to find beta CVn (V = 4.3) and move 23 degrees south. The effort is worth it - this is a very pretty, bright and easy pair for the small telescope. The contrasting colours have been noted by many observers. Webb thought the stars yellow and very blue, Smyth found orange and emerald and Sissy Haas made them citrus orange and fainter royal blue. The pair also serves another purpose - as a scale and orientation calibrator. At present the position angle is 270 degrees and the separation 20" (actually 270.3 and 20.14 for 2015.0). Many observing guides list the pair as optical but the evidence is not very persuasive. The Hipparcos satellite gives the parallaxes as 7.24 +/- 2.74 milliarcseconds (mas) for A and 19.29 +/- 14.58 mas for B. The proper motions are small but very similar. A is a K2 giant and B is a metallic-lined A9 dwarf which is also a spectroscopic binary.

  x Vel = DUN 95 (10 39 18.39 -55 36 11.8) is in a rich area of the southern Milky Way just 5 degrees north of the Eta Carina Nebula, NGC 3372. This pair was found by John Dunlop at Paramatta in 1826 and is a glorious sight in small telescope. The stars (V = 4.38 and 6.06), according to Ross Gould using a 35-cm reflector, are yellow and deep-yellow and the low power field contains two small asterisms. Andrew James, on the other hand, is an experienced Australian observer with a very extensive website devoted to double stars and especially those of Dunlop. He reports that Russell in 1873 made the colours straw-yellow and greenish blue and around 1980 members of the AS of New South Wales reported orange and pale blue. Given the spectral type of B is B8V, the reported deep yellow is rather unexpected. There has been little motion between the two components over the last two centuries. In 2000 the position angle was 105° and the separation 51".7. The primary is an early G-type supergiant which is also a semi-regular pulsating star. Hipparcos puts the primary at a distance of 840 light years. In 1834 John Herschel found a faint companion to B, V = 11.9, some 15" away in position angle 178°. The distance has widened to 20" today.

  Bob Argyle - Double Star Section Director

• February 2014 - Double Star of the Month

  STF 1126 (07 04 06.99 31 51.9) is easily to locate - it lies in the same low power field as Procyon, preceding the bright star. Since discovery by Herschel in 1781 the stars widened slightly but at the mid-point of the 19th century they slowly began to close. Motion is very slow and at present the companion can be found at 174° and 0".9. With magnitudes of 6.6 and 7.0 this is a relatively easy object for 20-cm although being fairly low in the sky it is not often seen to full advantage. Observations by the writer with the 20-cm at Cambridge were made in 1992, 2002 and 2012 and over that period the position angle increased by 10 degrees with no change in separation. Either A or B is a spectroscopic binary according to the WDS. A third star of magnitude 11.4 can be found at 251°, 43". The primary star is an AO giant, and the catalogue value for the parallax is given as 12.6 13.7 milliarcseconds.

  STF1146 (07 47 56.71 -12 11 33.8) is also known as 5 Pup. It lies near the extreme northern border of Puppis about 3 degrees north following M46 and M47. During a winter evening of very good seeing in early January the writer made a rare foray around the stars of Puppis and saw this star well resolved with the 20-inch Thorrowgood OG. After discovery by F. G. W. Struve when separated by 3".3, the pair started to slowly close. In the 1960s Ernst Hartung noted that it was 'a fine object in a starry field and 75-mm shows it well. In recent years, however, it has been closing more quickly and now is separated by barely 1". This is a long period binary with a highly inclined orbit and the stars will reach a separation below 0".5 before widening again. Thomas Lewis gave the colours as yellowish and blue whilst Webb thought the fainter star to be ruddy in 1851. The primary star is a F5 dwarf, and this pair lies 93 light years away according to Hipparcos.

  Bob Argyle - Double Star Section Director

• January 2014 - Double Star of the Month

  The two stars to be highlighted this month are both systems of higher multiplicity although the closest visual components are at the limit of amateur instrumentation.

  2 Cam (04 39 58.03 +53 28 23.7) is a Struve pair (No 566) which has somehow eluded me for more than 40 years. I first saw it in 2012 with the Cambridge 8-inch Cooke and the companion was quite well seen and measurable. Since it was first measured in 1828 at 312° and 1".5, the two stars have slowly closed and moved in a retrograde manner and last year I obtained 169.7° and 0".89. A preliminary orbit gives a period of 425 years for this unequal (5.6 and 7.5) pair of stars. In 1901, using the Yerkes 40-inch OG, Burnham noted that the primary was a close pair in itself with the new component (V = 7.4) being found at 317° and 0".2. This turned out to be a rapid binary and the currently accepted period is 26.89 years, whilst the separation of the stars never exceeds 0".3. Dembowski adds another pair in the field (D 4) and it can be found about 4 minutes preceding - mags 9.0, 10.3 and separation 5".8. Whilst in this area look at 1 Cam = STF 550, a fine bright pair.

  eta Orionis (05 24 28.62 -02 23 49.7) This fine, bright pair of white stars is a good test for the 20-cm aperture. It was first resolved, when separated by 0".9, by W. R. Dawes who noted 'This close and beautiful object was discovered by me on Jan 15, 1848, with an aperture of only 4.25-in which I happened to be using on my 6.33-in refractor'. It is one of 13 pairs in the WDS under his discovery code. The current separation is around 1".8 and the stars have been slowly separating since discovery. Notwithstanding the fact that they are similarly hot, young and massive stars, the WDS notes that A and B form an optical system; the more distant mag 9.4 star at 114" is also believed to be unconnected with the bright stars. DA 5 was not known to Smyth but Webb notes colours of white and purplish (this may be the observation of Dawes) whilst others see only two white components. The A star is actually a massive triple system. Many years ago it was found to be an eclipsing spectroscopic binary of 7.88 days period with both stars possessing about 12 solar masses. More recently, speckle interferometer observations by H. A. McAlister and colleagues found a third companion with a period of 9.9 years, whose mass is about 1.5 solar.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - December 2013

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  88 Tau (04 35 39.23 +10 09 39.3) is a binocular pair whose visual magnitudes are 4.3 and 7.8, which can be easily found 6 degrees due south of Aldebaran. Smyth referred to it as a `star with comes' and blithely asserted that after the year 2000 the stars would start to approach each other in the southern part of their orbit. He based this on a difference of position angle of 4.5 degrees between 1800 and 1822 when perhaps the most obvious inference is that the two stars are unconnected. In fact, Piazzi's position for 1800 is not very accurate and there has been little change since 1822. The two stars actually have similar proper motions and may have been co-eval. Smyth noted that the colours were bluish-white and cerulean blue whereas T. W. Webb (yellow-white and yellow-red) and, more recently, Sissy Haas (vivid lemon yellow and silvery cherry) seem to differ from this. In fact 88 Tau is a sextuple system. The A star was resolved by speckle into two components separated by 0".12 and mags of 4.4 and 6.6. The period is 18 years. Both of these stars are spectroscopic binaries with periods of 3.57 and 7.59 days. Distant B is also an SB with a period of 1349 days.

  BU 391 (04 26 56.93 -24 04 52.8) can be found in a very sparse part of Eridanus, about 8 degrees N of nu3 and nu4 Eri. It is one of Burnhams's earlier discoveries - made with the 6-inch Clark. Burnham was not convinced that there was much change in the system when he compiled his Catalogue in 1900 but subsequently the stars began to close and periastron was passed around 1980. The pair is currently at 151° and 0".4 which makes it a challenge for a medium aperture from the UK. The magnitudes are 6.7 and 7.1 so it should be divided on a good night with 30-cm. The orbital period is 596 years (Scardia 2003) but note that the WDS catalogue notes give a much shorter and incorrect value.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - November 2013

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Three degrees north of alpha Ari is a line of four stars of marginal naked-eye visibility. The most westerly of these stars is 10 Arietis, a close visual binary. The third star is 14 Ari - a coarse triple. 10 Ari (02 03 39.26 +35 56 07.6) was found by Wilhelm Struve at Dorpat and during the remainder of the nineteenth century the star closed slowly, leading Burnham to comment (in 1906) 'Probably orbital motion, although the measures are well represented on the hypothesis of rectilinear motion'. From a distance of 1".98 in 1833, the companion passed by the primary around 1920 at a distance of about 0".3 and has been widening since. The current catalogue period is 325 years and gives a position of for 2014.0 of 346° and 1".48. It is a nice pair in the Cambridge 8-inch but needs a night of good seeing to see the companion of magnitude 7.9 close against the mag 5.8 primary.

  omega Fornacis (02 33 50.71 -28 13 56.4) was swept up by John Herschel at Feldhausen in 1834 and is catalogued as HJ 3506. He noted that it was a 'very fine star but ill-defined'. The stars are magnitudes 5.0 and 7.5 and there has been little motion since discovery, the latest position in the WDS gives 10".8 and 245°. Sissy Haas calls this a 'showcase pair' and gives colours of goldish-white and smoke-grey whilst Magda Streicher with a larger aperture notes yellow-white and light grey-blue. Hartung does not pronounce on colours but notes a similar pair about 2' west. It seems likely that the two stars are physically connected. Hipparcos gives a distance of 484 light years and the primary is a subgiant of spectral type B9.5.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - October 2013

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Delta Cephei (22 29 10.25 +58 24 54.7) is the prototype pulsating variable the period of which holds the key to its distance. Thanks to Hipparcos this distance has also been determined geometrically and it comes out at 865 light years with a formal error of 37 light years. Regrettably few Cepheids were in range of the satellite during its operation between 1989 and 1993 but its successor, GAIA, will sweep up many more when it launches later this year. Delta is also a most attractive double star and its mag 6.1 companion is probably visible in regular binoculars. It can be found 41" distant from delta in PA 191°. This is formally star C in the system as S. W. Burnham found a much fainter (mag 13) and somewhat closer star (B) using the 18.5-inch refractor at Dearborn. Matt Heijen using a 30-cm Orion records the colours on his StarObserver.eu website and notes 'yellow, almost orange and bluish white'. There are three more distant comites between mag 13.5 and 14. C was noted as a spectroscopic binary by Belopolsky more than 100 years ago but little seems to be known about it now and it does not appear in the Ninth Catalogue of Spectroscopic Binary Orbits. Recent radio observations of delta reveal an extended nebula of ionized hydrogen surrounding the star which infers that mass loss is taking place at a level of around 10-6 solar masses per year. This may go some way to explaining the discrepancy between the mass derived from stellar evolutionary models and that obtained from stellar pulsation and dynamical techniques.

  STF2944 in Aquarius (22 47 50.19 -04 13 44.8) is a nice triple star which contains a visual binary system. It is 2.5 degrees following the pretty, coarse pair kappa Aqr which has an orange primary star. The closer AB pair consists of stars with magnitudes 7.3 and 7.7 which have closed up considerably since discovery in 1782. In the same time the position angle has increased by almost 60°. For the end of 2013 the stars can be found at 303° and 1".86. C which is mag 8.6 is 60" away in position angle 86°. This is not physically connected to AB and is being left behind by the rather considerable proper motion of the close pair, some 0".4 per year. It is large enough to have attracted the attention of Willem Luyten and both components are in the NLTT (Not Less than Two-Tenths) Catalogue. The colours in these stars are not too obvious given that they are rather faint, but at low power Sissy Haas gets beige white and arctic blue for AB combined and C. Hipparcos confirms the proximity of the system to the Sun - the parallax yields a distance of 105 light years.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - September 2013

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  As telescope makers of renown the Clarks (Alvan and his son Alvin George) were able to point large new telescopes at bright stars in order to test them and, if lucky, they discovered new companions. Such was the case of Sirius but here there was already significant evidence that the star was double based on Bessel's discovery of variable proper motion.

  In the case of tau Cygni (21 24 47.35 +38 02 39.6), however, the discovery of duplicity was entirely serendipitous. Discovered by A. G. Clark using a 26-inch refractor, it has turned out to be a system of considerable interest. The period is 49.8 years and the separation varies from about 0".5 to 1".1 but the difficulty for the observer is the significant difference in brightness between the two stars. The WDS gives V mags of 3.8 and 6.6. The writer has seen the comes with the 8-inch OG at Cambridge and the system is widening again at present - in the autumn of 2013 the position will be 213° and 0".9 and so should be seen in 20-cm on a night of good seeing. The pair is easy to find as the southernmost of a trio of brightish stars 10 degrees following gamma Cygni and it is 1.5 degrees south following 61 Cygni. Recent investigations by the astrometric-based Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES) have pointed out the possibility of a sub-stellar companion to one of the stars. The period may be 826 days and the mass may be 12.3 Jupiters but this is very much preliminary work.

  About 10 degrees preceding the 3rd magnitude star beta Aquarii is 3 Aqr. Just south following is a pair of stars the preceding of which is 4 Aqr and the following 5 Aqr. William Herschel noted that 4 Aqr (20 51 25.69 -05 37 35.9) was double on Sept 3 1782 and listed it as number 44 is his Class I stars. Wilhelm Struve measured it at Dorpat in 1825 and at present the stars are in almost exactly in the same place having undergone a whole orbital revolution since then. This 187-year-period binary is not particularly easy from the UK due to its low declination and it is now closing again. For 2014.0 it will be at 30° and 0".7 making it a target for a superior night. The stars are mags 6.4 and 7.4.There are two distant and unconnected comites - C is 13.3 at 74" from AB and D is 9.7 at 136". In each case the separation is increasing.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - August 2013

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Those who have read last month's version of this column will have noted that the northern object was 26 Dra, one of Burnham's very unequal pairs. This month's selection is another from that stable, BU 648 (18 58 01.47 +32 54 05.8) can be found in Lyra, in the same low power field as gamma, and 20 minutes or so north-west of the bright star. Burnham assigned magnitudes of 6 and 9.5 to this discovery which was only separated by 0".6 at the time. The WDS gives magnitudes of 5.3 and 8.0 and it can certainly be seen with 20-cm on a good night. At the time of writing, the separation is 1".23 and the position angle 247° at mid-2013. This star has also recently been in the news because it is accompanied by a planetary object whose mass depends on which of the two stars it is orbiting. This is not known at present but if, for instance, it accompanies star B, then the mass is estimated at 1.5 Jupiters. This discovery is unusual because it was made astrometrically, rather than by radial velocity measurement, using the PHASES (Palomar High-precision Astrometric Search for Exoplanet Systems) part of the Palomar Testbed Interferometer instrument. There are 4 distant comites ranging between magnitudes 11.0 and 12.6.

  H N 126 (19 04 21.53 -21 31 53.7) belongs to William Herschel's last double star catalogue which was published in 1823, a year after his death. In this case the N stands for 'New'. This 'small yellow binary' says E. J. Hartung, can be found in a 'field profusely sown with stars' and it is very close to omicron Sagittarii. Burnham's Celestial Handbook also attributes the label HU 261 to the system but it has now reverted back to its original discoverer. This is a pretty pair of long period and the two components are moving slowly retrograde. At this time the position is 186°, 1".25. The writer has not observed this pair from the UK but it should be well seen in 20-cm provided the seeing and transparency are sufficiently good at this low declination.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - July 2013

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  S W Burnham discovered 26 Dra (17 34 59.58 +61 52 28.4) using the 18.5-inch refractor at Dearborn Observatory in 1879. It was soon apparent that this was a binary system because the large proper motion of the primary star (almost 0".6 per year) was clearly shared by the faint and close companion. Along with many of the other similar pairs he discovered, Burnham substantially underestimated the brightness of the companion, and gave the magnitude of the stars as 5.5 and 10.1 is his General Catalogue of 1906. The WDS gives 5.28 and 8.54 and I saw the comes perfectly well with the 8-inch Cooke refractor at Cambridge in summer 1999. At that time the separation was 1".6 but the pair is now closing quickly and it will take a larger aperture to see them in 2013 when the separation is 0".65. The large proper motion is a consequence of the proximity of this star system. The Hipparcos catalogue gives a revised parallax of 70.47 mas which corresponds to 46.3 light years with a quoted error of 0.24 light years. To find it, draw a line between beta and nu Draconis in the head of the Dragon and extend the line twice as far again.

  See 342 (17 53 23.47 -34 53 42.5) is also a close pair requiring a reasonable aperture to resolve, but it does have the additional attraction of being embedded in the open galactic cluster M7, close to the tail of Scorpius. Since discovery in 1897, the pair has moved in retrograde fashion by 80 degrees whilst the separation has remained close to 0".4. A provisional orbit by Andreas Alzner puts the period at 700 years and its parallax places it at the distance of M7. A good chart for identifying See 342 is given in Burnham's Celestial Objects Volume 3, page 1712. It is #1 and is located at the SW corner of the cluster. Robert Burnham quotes magnitudes of 6.5 for each star whilst the WDS lists 5.85 and 7.89 which also seems too unequal. Recent measures by Tokovinin give a delta m of 0.3 to 0.4. The primary star is a K giant and a spectroscopic binary.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - June 2013

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Situated in the north of Boötes, STT 298 (15 36 02.59 +39 48 08.7) is one of the more rapid binary systems found by Otto Struve at Pulkovo and it now embarking on its fourth orbit since discovery. Look for the naked-eye pair nu1 and nu2 Boötis some 6° following beta Boötis and STT 298 can be found just south preceding phi Boötis. The star is nearby (the distance is 73 light years) so the orbit is relatively large in angular terms. The stars are almost near peristron and at 182°, 1".18 in mid-2013 they offer a good opportunity to see a pair with a period of only 55.6 years. The system moves across the sky at almost 0".5 per year and is accompanied at a distance of 121" by star C which is mag 7.8 and also a K dwarf. For the telescopic observer there are two fainter but unrelated stars of mags 12.1 and 13.9.

  DUN 178 (15 11 34.82 -45 16 39.0) is an orange KOIII giant star of mag 6.3 accompanied at a distance of 30".6 by a mag 7.3 white star according to Richard Jaworski using a 100-mm aperture in Australia. Both these stars appear in the Hipparcos catalogue but do not appear to be connected in any way. A is 510 light years away whilst B is 400 light years distant. At discovery in 1826 the pair were separated by 40" so the change is purely due to different proper motions. This pair is located in the heart of Lupus, a constellation rich in visual double stars, and can be found in the same low-power field as lambda Lupi. In 1929 Willem van den Bos, using the 26.5-inch refractor in Johannesburg, found a companion of mag 9.6 some 1".1 distant from A. There has been little change in the position of this star in the intervening 80 years and it offers a challenge to a 30-cm telescope in a good location.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - May 2013

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  There are two pairs for the price on one in the northern half of this month's column. In the extreme north-west of Boötes following eta UMa is a coarse group of bright stars. The two closest together are kappa and iota and both are worth seeking out with telescopic aperture. Kappa Boötis (14 13 29.00 +51 47 23.8) is a long period visual binary with both stars possessing the same proper motion and at a similar distance from us (162 light years). There has been little motion over the past 200 years beyond a slight widening of the separation to 13".6 and a small diminution in the position angle. In 1850 T. W. Webb made the stars pale yellow and bluish whilst in 1915 William Franks recorded yellow and purple. B is a spectroscopic binary with a period of about 5 years. More recently Tokovinin has recorded a 16.9 mag comes at 108" to A which appears to have shared proper motion with AB. Some 25 arc mins south following is iota Boötis (STFA 26 - 14 16 10.07 +51 22 01.3) which is a binocular pair which will benefit from telescopic aperture. The stars are mags 4.8 and 7.4 and are 38" apart. The Irish amateur, Isaac Ward, who used a 4.3-inch Wray refractor, found a comes of mag 12.6 at 92". The spectral types are A7 and K0 and Webb found whitish yellow and lilac.

  When William Herschel was accumulating his catalogue of double stars he searched every part of the sky and he must have had a good southern horizon. One of the lowest of his finds is Y Cen (13 53 32.75 -35 39 51.2) which at -35 degs could only have been a mere 5 degrees above his horizon even at culmination. He acknowledges that the pair is `too low for accuracy' giving a distance of 54" (currently 68"). The stars are mags 5.5 and 8.7 and it’s not surprising that Herschel missed the duplicity of the primary, later found by Howe. Currently at 1".0 it was nearer 0".7 in the 1780's but as the two components are almost equal it is a fine object for 15-cm at a suitable latitude. The period appears to be 258 years and the position angle is increasing. Burnham and Innes respectively added fainter companions - 12.3 at 28" and 14.8 at 38". Hipparcos puts the system at 167 light years.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - April 2013

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Near the centre of the coarse Coma star cluster is STF1639 (12 24 26.81 +25 34 56.7) which forms an equilateral triangle with 12 and 13 Com and is the faintest of the three. It is a binary with a highly eccentric orbit (e = 0.93) and which is now resolvable in 10-cm aperture given a night of reasonable seeing. The stars are magnitudes 6.7 and 7.8 and appear to be dwarfs of spectral types A7 and F4. It was discovered by Struve and in 1827 the separation was 1".2. This decreased steadily until the 1890s when the pair was unresolvable in even the largest apertures. By the end of that decade it was again measureable and since then has increased in separation to 1".8 at the time of writing. The orbit currently in the USNO 6th Orbital Catalogue gives a period of 575 years which gives a distance of 0".09 for 1892 and the maximum is reached around 2175 when the stars will be 2".35 apart. The 10-cm telescope might also make out a distant third star of mag 11.5, 92" away in PA 160.

  Modern star atlases show the star N Hya (11 32 16.90 -29 15 39.7) firmly in Hydra and some 3 degrees north of the mag 3.5 xi Hya - itself a very unequal and wide double star with a 10.7 mag companion at 68", the distance of which is increasing. The WDS shows no measures of this pair since 1928. William Herschel found N Hya on 1783 Jan 10 and it is number 96 in his third category. He called it 17 Crateris and noted that both stars were reddish white. A few years later it appeared in Piazzi's Catalogue as 17 Hya. Modern telescopes tend to yellow tints - Hartung gives both yellow and Sissy Haas called them grapefruit colour. The stars are almost identical - both are spectral type F8V - and have changed little in separation and position angle over the last 200 years. The current position is 210° and 9".4. This is a nearby system which is certainly binary - the proper motion is significant.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - March 2013

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  6 Leo (09 31 57.58 +09 42 56.8) can be found about 2 degrees preceding omicron Leo, and is in the same low power field as omega Leo - a splendid binary (see the column for March 2008). Not unsurprisingly it was first catalogued by Sir William Herschel on 1781 Feb 21 when he noted that the `Large' star was red and the `Small' star `dusky'. It then seems to have appeared in everyone else's catalogue (SHJ 107, STTA 101).The primary is a K giant star and has visual magnitude 5.22. It is accompanied at 75° and 37".5 by a magnitude 9.3 star whose relative position has changed little over 200 years. T. W. Webb noted colours of deep orange and green with his 3.7-inch Tulley refractor and again, in 1882, presumably with the 9.3-inch where he notes pale orange and blue. Hipparcos places the primary star at just over 500 light years away.

  J Velorum (10 20 54.81 -56 02 35.6) is near the southern border of Vela with Carina and is located almost on the galactic equator. It is 2 degrees north of the Smile nebula (NGC 3199), a cloud of gas some 75 light years in diameter and 12000 light years distant formed by the interaction of a hot Wolf-Rayet stellar wind and the surrounding interstellar medium. On sweep 435 with his 18-inch reflector at Feldhausen, Herschel described it as `A very large and very remarkable nebula, which is brighter to the S.f. part, and dies off to the N.p., having a curved form and forked tail. In the head of it is a double star. The nebula is pretty bright, very large, figure irregular, 8' long 4' broad'. The double star mentioned is HJ 4302 (10.9, 12.1, 116°, 22".7). Whilst J Velorum was first observed as a double by Rumker (it is RMK13) he missed the brighter but closer B component and recorded only A and C. It was John Herschel who noted the star as triple and referred to it as T Velorum. AB has mags of 4.5 and 7.2 which are currently at 102° and 7".1. C, which is V= 9.2, is 36" away in PA 191° and the distance is slowly widening. Andrew James calls it a spectacular triple. The colours are blue, white and yellow.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - February 2013

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  It was near delta Gem (07 20 07.39 +21 58 56.4) that Pluto was first seen on a plate taken by Clyde Tombaugh at Lowell Observatory in February 1930. By a strange coincidence, Herschel found the mag 8.2 companion to delta on March 13 1781, the very same night that he discovered Uranus and later recorded the system as H 2 27. The primary star is a late A9 giant, of visual magnitude 3.55, and at a distance of 60 light years according to Hipparcos. To see the companion comfortably requires 15-cm and a night of reasonable seeing. Some disagreement attaches to the colour of the secondary. It is given as K3V in the WDS and E. Hartung noted it as reddish, as it did T W Webb but some years before he had recorded the hue of the star as purplish. The primary is also a single-lined spectroscopic binary and has also been seen double at lunar occultation’s but attempts to resolve it directly using speckle methods have so far failed.

  BU 332 (07 27 51.66 -11 33 24.7) is a multiple star which appears coincident with NGC2396 on Map 8 in Norton, close to the point where Monoceros, Canis Major and Puppis meet. Just after completing the notes on this system I read the Sky and Telescope for February 2013 and found that it featured in Sue French's column (page 57) under the name STF 1097. AC has mags 6.2, 8.7 and the components seem relatively fixed at 313° and 20". In 1865 Baron Dembowski suspected that A was double and it was later confirmed by Burnham using his 6-inch Clark refractor. Star B is magnitude 7.35 and there has been very little motion - amounting to 7° retrograde in PA, and the stars are possibly closing up. Small telescope users should be able to see the more distant D (V = 9.7) at 157o, 23". A 12.7 mag star at 32" will need at least 10-cm. Hartung notes that it is a beautiful field and that the close pair is deep yellow and white.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - January 2013

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  26 Aurigae (STF 753 - 05 38 38.10 +30 29 32.8) is one of William Herschel's class III double stars, and can be found about 3 degrees north-following beta Aurigae. It is a bright and easy pair - the two main stars being visual magnitude 5.4 and 8.4 and the companion 12".2 distant from A. Smyth found the stars to be pale white and violet whilst Sissy Haas notes straw yellow and atlantic blue. It seems likely that this is a physical pair but the same cannot be said of the unassociated star of V = 11.5 which is slowly increasing its distance from A and can be found at 113 degrees and 35". Burnham found it independently in 1872 but noted that it had already been found by Morton using the 7.75-inch refractor at Lord Wrottesley's observatory some 10 km north-west of Wolverhampton. In 1892 Burnham found that the primary was an almost equally bright close pair and indeed it turns out to be a binary with a period of 53 years (BU 1240). At present it is just beginning to close and at 0".2 will require considerable aperture and excellent seeing.

  Five degrees following epsilon Col is a wide (13 arc min) pair of stars which make up an easy binocular double and may just be visible to the naked eye. This is gamma1 (V = 4.7) and gamma2 Caeli (V = 6.3). There is much more here for the telescopic observer to note, because both these stars are again double. Gamma1 (05 04 24.40 -35 28 58.7) somehow missed the attention of John Herschel and was swept up by Captain Jacob in 1847. The companion at magnitude 8.2 is currently 3".2 distant in position angle 305 degrees. There is slow retrograde motion and the distance is gradually increasing. The primary is a K3 giant and indeed Hartung notes the colour to be orange, with the companion white. Gamma2 was discovered to be a close unequal pair by the Hipparcos satellite and is now called HDS658. The companion (mag 9.7) is 0".9 away in position angle 195 degrees - there has been about 12 degrees of motion since discovery. This star should be visible in 30-cm on a good night. Both gamma1 and gamma2 are in Hipparcos but gamma1 has a parallax of 17.90 mas whereas that of gamma2 is 10.15 mas.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - December 2012

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  The conventional long-focus refractor user is at a disadvantage when it comes to examining that part of the sky near the celestial poles. This is a pity since there are a number of systems north of +75 which are worth looking out - amongst the binaries are STF2 and pi Cep and the beautiful optical pairs 19 Cam, STF1694 and kappa Cephei, not to mention the Pole Star itself.

  The writer measured STF460 in Cepheus (04 10 02.74 +80 41 55.2) on three nights in 1994 but has not examined this slow moving binary since. The period appears to be about 415 years so it has moved almost half an orbital revolution since discovery in 1828. The position angle is increasing and anyone observing it in late 2012 should see the companion at about 149° and 0".69. Having reached a maximum apparent separation in the 1920s the pair is now closing and should reach 0".65 in around 2030. The stars are visual mags 5.6 and 6.3, and Webb gives colours of yellowish and bluish whilst Sissy Haas notes only that the primary is straw yellow.

  32 Eri (93 54 17.49 -02 57 13.0) is another of William Herschel's discoveries, but being close to the equator is comfortably within reach of many latitudes and the smallest telescopes. The stars are mags 4.8 and 5.9 and the colours ascribed to stars by Hartung deep yellow and white seem to chime perfectly with the given spectral types of G8III and A2V. Only the earliest measures of the elder Herschel seem to disagree with the general finding that the separation is around 6".5. Is this an error by WH or evidence of rather swifter orbital motion which has not manifested itself since? Between the measures of Struve in 1822 and today there seems to have been no significant movement in separation or position angle but as the stars have similar proper motions there seems no doubt that they form a binary pair. At 165" there is a 10.5 magnitude third star - it too shows no apparent motion over 150 years, so is it also a member of the system?

  Bob Argyle - Double Star Section Director

• Double Star of the Month - November 2012

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  STF 162 (01 49 15.54 +47 53 49.0) sits in an obscure corner of Perseus about 6 degrees north and 2 degrees preceding gamma And. It is also about 1.5 degrees south following 51 And. It is a multiple star, four components of which can be seen in 7.5-cm. The closer pair, which consists of components of magnitudes 6.5 and 7.2, was found by Struve in 1828 when the separation was 2".2 and the position angle 227 degrees. At the current epoch these have both decreased to 1".9 and 298 degrees. A third star which remains fixed with respect to A can be found 20" away almost due south, and another at mag 10.1 is 139" distant. The WDS reveals that either A or B is itself an interferometric binary with a separation less than 0".2 and B is also a spectroscopic binary.

  Epsilon Sculptoris (01 45 38.65 -25 03 08.8) is a fine pair located in the extreme north-east corner of Sculptor and which can be seen from the UK but which first caught the attention of John Herschel at Feldhausen when he noted 'large * w(hite) small star dull red' an gave the magnitudes as 6 and 10(modern values 5.4 and 8.5). Since then the companion has moved slowly retrograde from the discovery position of 72 degrees to its current value of 20 degrees. In 1969 de Freitas Mourao looked at the observations and decided that they could be fitted by a face-on circular orbit with a separation fixed at 4".65 and a period of almost 1200 years. In recent years, however, the companion has begun to stray ever further from the primary and the separation is now over 5". More observations are needed but the real orbit will not be obvious any time soon, and in fact the apparent motion to date looks more linear than curved. However, epsilon Scl is relatively nearby (92 light years) and the annual proper motion of 0".16 would take it almost 30" from B since discovery if the latter was optical. Hartung called it an easy, bright yellow pair whilst Gould with 175-mm called the primary pale yellow.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - October 2012

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  This month's targets are a coarse but interesting multiple star in Cassiopeia and an unequal pair in Piscis Austrinus. In both cases the primary stars are brighter than 5th magnitude.

  1H Cas - STT 496 (23 50 01.92 +58 32 56.1) challenges the writer's ability to describe a picture in less than a thousand words so for the first time in this series he has resorted to an image to help out. (See Fig 1). The field diagram comes from Burnham's General Catalogue of Double Stars, Part 2, page 1030. 1H Cas - so called because there is no Bayer or Flamsteed letter assigned to this mag. 4.9 star, first came to be catalogued by William Herschel - and it is number H VI 25 in his catalogue, but appears as SHJ 355 in the Cambridge Double Star Atlas and the book by Sissy Haas. It is located by extending the line from gamma Cas through beta Cas by the same distance again. Herschel also noted stars F, G and H which are given magnitudes of 10.6, 11.1 and 13.0 in the WDS. For the moderate aperture there are two close binaries in the group. The first is A itself which was divided by Otto Struve. The companion B, of magnitude 9.3, is only 0".8 distant and appears to be in direct motion. Somewhat earlier, Dawes doubled C which is a little wider (1".4) and less unequal, the magnitude of the companion, D, being 9.1. Again there is slow retrograde motion. Finally, a magnitude 10 companion at 207 degrees and 231" called I in the WDS was observed by Burnham with the 36-inch Lick refractor, and also turns out to be a close binary currently separated by 0".3.

  

  gamma PsA (22 52 31.53 -32 52 32) is about one degree south-west of delta PsA which, in turn, is 3 degrees south of Fomalhaut. Delta is a fine, unequal pair found by Howe which is almost a perfect copy of gamma which was thought to have been first seen as double by John Herschel whilst at the Cape of Good Hope in 1835. In fact, Brian Warner, writing in the Monthly Notices of the Astronomical Society of Southern Africa in 1977 (Vol 36, page 134) makes a case for the duplicity of gamma PsA having been found by Fearon Fallows, the first Astronomer Royal at the Cape, a few years before. Fallows used a Mural Circle and a Transit instrument and made a couple of lists of double stars which he had happened across during the course of routine observations. Gamma is a very unequal system, magnitudes 4.5 and 8.2, with a companion which is slowly widening and is currently 4" distant. The primary is an AO giant some 217 light years away although Hartung (using a 30-cm reflector) saw the colours as pale and deep yellow whilst Gould with 175-mm also from Australia called the primary `off-white'.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - September 2012

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  beta Cep (21 28 39.58 +70 33 38.5) was observed as a double star by both William Herschel and Piazzi. The mag 3.2 primary is accompanied by a companion of magnitude 8.6 some 14" distant in position angle 251 and there has been little change in this relative position in more than 200 years. Beta Cep is a distant star - Hipparcos puts it at almost 700 light years. It is clearly also a very luminous star, the WDS catalogue marks it out as a giant of spectral class B3. Interest in beta was reawakened in the 1970s when Antoine Labeyrie, the father of speckle interferometry, found the brighter star to be a close but unequal binary. Paul Couteau the famed visual observer tried to resolve it with the telescope at Nice but failed and subsequent observations have shown the magnitude difference in the visual to be more than 3. The orbit of Aa is very highly inclined and motion is almost all in distance - ranging from 0".33 at greatest separation (to be reached in 2035) to less than 0".01 near periastron. AB is noted as a showcase pair by Sissy Haas and she gives the colours as brilliant white and green. Smyth notes white and blue and also that there is a coarse but very minute double star preceding. Beta Cephei is probably more well-known as the prototype of a class of pulsating variable stars similar to Cepheids but with lower amplitude and range of magnitude. In this case the period is about 4 hours and the star varies by about 0.03 in V.

  MLO 6 (21 27 01.62 -42 32 52.5) is located in Microscopium - close with the border with Grus and sadly too far south to be seen from the UK. Hartung gives colours of deep yellow and white which is surprising given that the primary is an A star but with metallic lines in its spectrum. The distance is 184 light years and the small change in relative position (146°, 4".2 in 1879, to 150°, 2".9 in 2008) suggest that this is a long period physical system. The magnitudes of 5.63 and 8.15 means that a good view of both stars would be gained with apertures of 15-cm or more.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - August 2012

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  The two pairs selected this month are almost copies of one another. They have the same separation, and position angle, similar magnitudes and even the colours are close.

  Eta Lyrae (19 13 45.49 +39 08 45.5) appears as H 4 2 in the great astronomer's first list of double stars published in 1782, but it was first listed by Christian Mayer in 1779. The primary is a luminous object more than 1,000 light years away. The companion appears to be an F star and with the primary being an early B subgiant, one might have expected colours to be white and yellow when in fact Hartung records yellow and ashy. The WDS gives magnitudes of 4.38 and 8.58 with current distance and PA being 28".5 and 79°. A fainter and more distant star (mag 11.4) can be seen at a distance of 161". Along with theta, also a wide, unequal pair, it follows Vega by about 7 degrees.

  Beta1 Sgr (19 22 38.29 -44 27 32.1) is not only not the second brightest star in Sagittarius but it is actually about 12th in the list. The naked eye can make out both beta 1 and its close neighbour beta 2 (mag 4.27) some 20 arc mins away. Dunlop listed beta 1 as a telescopic double in 1826 and and since that time the position angle has increased slowly to 76° and the separation decreased slightly to 28".7. Hartung gives the colours as pale yellow and ashy white - the spectral types are actually B8V and F0V perhaps reversing what one might expect from the colours. However, Ross Gould using a 175-mm refractor disagrees with this judgement and gives the colours as white and yellowish. The stars are magnitudes 3.98 and 7.21 and again this is an optical system. Beta1 lies in the far south-western part of Sagittarius, close to the border with Corona Australis and Telescopium.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - July 2012

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  With the summer constellations now becoming dominant, the two pairs selected this month are to be found in the constellations of Hercules and Ophiuchus.

  STF2194 (17 41 05.50 +24 30 43.2) sits in the preceding edge of a glorious 1 degree field which includes the bright stars 83 and 84 Herculis. The pretty pair itself is an optical system and since 1783 it has widened gradually from 14".3 to 16".3. The main stars are magnitudes 6.5 and 9.3 and the current position angle is near 7°. Webb noted them as orange and blue in 1850, using his 3.7-inch refractor whilst Franks early in the last century found them to be yellow and lilac. The primary star is a K0 giant which is also a spectroscopic binary. About 169" away in PA 163° is a third star, nominally the same brightness in the visual as B.

  STF2173 (17 30 23.78 -01 03 46.2) is a visual binary with a period of 46.4 years. Its highly inclined apparent orbit means there are two opportunities for small apertures to see the two stars before orbital motion takes them beyond the range of most amateur telescopes. The writer measured this pair in 1992 and 1997 with the Cambridge 8-inch Cooke telescope when the separation was respectively 1".1 and 0".7. The two stars are now separating reaching a separation of 0".8 in the south-eastern quadrant in 2012, they then close rapidly and open out again, reaching 1".1 in the north-west in 2037. Both stars are yellow and close to V = 6.0 so this pair forms a beautiful test object at present for 15-cm aperture. Whilst in this area, check out another target some 4 degrees slightly preceding this pair - 41 Oph, a close unequal binary which is one of R. G. Aitken's last discoveries. It needs at least 15-cm on a good night.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - June 2012

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  The two systems featured this month are both unequal and difficult pairs which need at least 30-cm to see well. Both stars are also at the same distance from us - about 140 light years.

  gamma CrB (15 42 44.57 +27 17 44.3) is number 1967 in F. G. W. Struve's catalogue of 1837. This has always been a hard object to see in the 8-inch OG at Cambridge but on nights of good seeing the companion can be measured, most recently in the summer of 2010 when the separation was found to be 0".7. The stars are in a very elongated and inclined 91 year orbit so that most of the motion is in distance. At the time of writing the pair is closing again and the separation has reached 0".56 at PA 111° but the stars are magnitude 4.0 and 5.6 which makes resolving the pair more difficult. Check the nearby eta CrB first before attempting this pair. It is slightly wider and more equally bright. If this does not jump out as a clear double then its unlikely that gamma will!

  See 258 (16 03 32.22 -57 46 29.5) is also known as iota 1 Normae and it sits half way between zeta Arae and beta Circini. It has a short period for a visual binary - 26.9 years and at the middle of 2012 can be found at 213° and 0".33 but after this the stars close further. The two components are relatively bright (5.2 and 5.8), even so a night of good seeing will be necessary to see any sign that this is a double star. For the smaller telescope user there is a third star which forms HJ 4825 and which is currently at 241° and 11".2 having slowly decreased from 251° and 15" at discovery in South Africa in 1835. John Herschel measured this system again in 1837 and on the latter date the close pair would have been 0".42 apart but Herschel made no mention of it. Hartung notes that C is reddish compared with AB which are probably a pair of late A stars.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - May 2012

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Nine degrees south of Arcturus is 15 Boo - a naked eye star which is a difficult double star for the small aperture but which can be well seen in a 20-cm OG on a good night. Track another 3 degrees south-east and the eye alights on a crooked line of 3 stars with the brightest of them being the most northerly. This star is STF1835 (14 23 22.74 +08 26 47.8) and because it is almost half a magnitude brighter than 15 Boo it is unusual in that it has no Bayer number or Flamsteed letter. For the small telescope the pair offers a very pleasing sight with an AO primary being accompanied by an F2 secondary some 6" away to the south. Hartung makes the colours white and deep yellow, Sissy Haas has goldish-white and powder blue whilst W. S. Franks in 1916 made them white and lilac. Angular change has amounted to 7 degrees in the last 230 years but it seems certain that the two stars are physically connected. In 1889 Burnham found that B was a close double (BU 1111) and indeed it turns out to have a binary period of 40 years. The separation ranges from 0".15 to 0".3 so could be seen in 30-cm or more when at its widest in 2022. For 2012.0 the distance is 0".2.

  About 3 degrees following beta Crucis, although actually in Centaurus, is a group of bright double stars - R 213, I 424 and CorO 152 - the latter two of which are in the same field. I 424 (13 12 187.63 -59 55 13.9) is a very unequal and rather close pair which needs 20-cm on a good night to see well, but which Hartung could just catch with 10.5-cm. The magnitudes are 4.8 and 8.4 and a recent measure in 2008 by the writer put the fainter star at 13° and 2".0. For the larger aperture, the primary is the binary See 170 - discovered by Thomas Jefferson See and which is only 0".22 apart in mid-2012. This 27 year pair has components of 5.3 and 6.0. Hartung notes that the primary star is yellowish but it has the spectrum of a B5 dwarf. Some distance from AB-C are two faint companions of magnitudes 12.6 and 14.9. About 8' NE is CorO152 - a 25" pair with colours of orange and reddish according to Hartung. R213 is a pair of white stars of magnitude 6.6 and 7.0 which has moved little in PA since discovery by Russell but the separation has tripled to 0".9 and this makes it an excellent test object for a 12-cm aperture.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - April 2012

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  2 CVn = STF1622 (12 16 07.55 +40 39 36.6) is a beautiful pair with components of contrasting hues which sits near the western border of Canes Venatici with Ursa Major and about 5 degrees following beta CVn. The primary star, an early M giant is accompanied by a late F dwarf and many writers have commented upon the colours to be seen here: - Webb called them very gold and blue, Dembowski thought them yellow and azure, Franks made them orange and blue and more recently Sissy Haas recalled brick red and silvery sapphire. The stars are magnitudes 5.9 and 8.7 and are currently separated by about 11".4 which makes them an easy target even for the small telescope.

  mu Crucis (12 54 35.66 -57 10 40.4) is simply one of the most beautiful doubles in the sky. A pair whose components of visual magnitude 3.9 and 5.0 share common proper motion and distance, this system belongs with the Scorpio OB2 association of young hot stars, and Hipparcos places both stars about 412 light years away. Shatsky and Tokovinin used the ADONIS near infrared adaptive optics system on the ESO 3.6-metre reflector to search for faint, close companions and they found two objects within 5" of component B. No magnitudes or proper motions are available so it is too early to say if these are physically connected but the two bright stars certainly form a very long period binary. Mu Crucis was found by Dunlop in 1826. The spectral types are both B and Hartung record them as both white whereas Richard Jaworski sees a tinge of yellow in the fainter star. The current separation of 35" and the brightness of the stars almost makes this a southern equivalent of Albireo but without the colours.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - March 2012

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  STT 215 (10 16 16.05 +17 44 24.6) is one of Otto Struve's discoveries at Pulkova and is a slow moving binary. In 1844 the stars were just 0".47 apart in position angle 256°. Since then, slow retrograde motion has brought them to 177° and 1".5, sufficiently wide to be well-seen in a small telescope although the relative faintness of the two stars does require a night of good seeing and transparency to get a good view - it is not the easiest of pairs to measure with the Cambridge 8-inch OG. Surprisingly, the star is not in the Dover edition of Webb's `Celestial Objects' but it is described in Hartung and Haas. The current orbit projects a period of 670 years with the separation continuing to increase slowly. Hipparcos puts this star at a distance of 375 light years and the WDS gives the spectral type of the primary as A9IV.

  I11 (09 15 14.64 -45 33 19.8) is in Vela, currently prominent from southern latitudes, and which is full of interesting double stars. It is located about 2 degrees south following λ Vel and was one of Robert Innes' first discoveries with a borrowed 6-inch refractor from Sydney in the last decade of the 19th century. Set in a fine field Hartung found both stars to be yellow, although the spectral type of the primary is B8V. This is undoubtedly a binary, albeit of very long period. Having closed slightly since discovery with increasing position angle, it was found at 290° and 0".8 in 1997 when last measured. It is a very distant system, only just giving a significant parallax form the observations made by Hipparcos which place it at a distance of almost 1900 light years.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - February 2012

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  15 Lyn (06 57 16.60 +58 25 23.0) is on the western edge of a coarse cluster of double stars which also includes the close binaries 4 and 14 Lyn. Discovered at Pulkova by Otto Struve at a distance of about 0".5 the two stars slowly widened reaching around 0".9 at the turn of the last century before the companion made a close (0".1) approach to the primary and is now slowly widening again. This is a good opportunity to see this pair which needs 20-cm on a good night because there is also a substantial difference in brightness of about a magnitude. Andreas Alzner's orbit from 2000 gives the ephemeris position of 232° 0".67 for 2012.0. It is missing from Sissy Haas' book which is a little odd given that 4 Lyn is included and is similarly difficult to resolve. A good night may also show the faint companion discovered by Burnham. This 12.5 mag star can be seen at 346° and 29" whilst a more distant 9.5 is some 187" away and the distance to AB is decreasing due to proper motion of the bright pair.

  Dun 39 (07 03 15.12 -59 10 41.1) is in Carina, lying about 3 degrees north preceding alpha Pictoris and forms a beautiful white pair which is easily resolvable in a small aperture. When discovered by Dunlop from Australia in 1826 the stars were separated by 2".8. The revised version of Hartung (1995) by Malin and Frew repeats the note in the original edition that it can just be seen with 7.5-cm aperture, but the two stars are closing and the last measure in 1997 put the separation at 1".4. In 2006 Graeme Jenkinson and Tin Napier-Nunn of the Astronomical Association of Queensland noted that x320 was needed on a 15-cm f/8 OG to see it clearly so it will be interesting to see if it is still accessible to 7.5-cm now. The pair is clearly binary and lies almost 500 light years away.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - January 2012

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  35 Cam (06 04 29.10 +51 34 24.2) was observed by Madler in 1843, and became number 128 in the Pulkovo Catalogue of Otto Struve, although it had been discovered by John Herschel in 1830. Later authorities including Aitken in the ADS added the word rejected because the separation was too wide for the pair to be a likely binary system. The rejected tag seems to have been removed in the current WDS but there is no evidence that the two bright stars are connected. The pair is a fine sight in a small telescope - the primary appears white and V = 6.4, whilst a V = 9.26 mag. bluish companion sits about 40 arc seconds away in PA 14°. For the user with say 30-cm there is the additional interest that the companion is a close pair - HU 559 which appears to be moving in retrograde binary motion - some 30 degrees since the discovery by Hussey in 1902. The WDS gives magnitudes of 9.6 and 10.6 and the current separation is 0".6 so this is as much of a test of the atmosphere as it is of the telescope.

  The constellation of Columba lies to the south of Canis Major and a triple star worth seeking out is HJ 3858 (06 25 30.01 -35 03 50.5). It forms an isosceles triangle with kappa and delta Col and is exactly 5 degrees south of zeta CMa. This is both an easy binocular pair and a fine triple in the small telescope. The brightest stars have magnitudes 6.4 and 7.6 and are separated by 131" a distance which is slowly increasing. The primary star, of spectral type K3 appears deep yellow to Gould with 175-mm, whilst the companion appears white, reflecting its spectral type of A4. When John Herschel noted it on sweep 663 at Feldhausen, he described its three stars as 'Fine object. A star 6 mag precedes to the south'. The close pair (BC) is currently 3".8 apart and also slowly separating with little change in position angle, and the magnitude of C is 8.2.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - December 2011

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  With the Pleiades now high in the sky there is the opportunity to get a good look at this young star cluster some 420 light years distant.

  James Mullaney and Wil Tirion in their Atlas note nine double stars within a radius of 3 degrees of the cluster centre. Most western of these is 7 Tau (033426.62 +242752.1) and although it lies at the same distance as the cluster its proper motion indicates that it does not belong. William Herschel noted the wide companion of V = 9.9 at a distance of 22" and it resides in the WDS as H 4 88. Thomas Lewis considered it was an optical companion but the WDS says that it is physically connected. It was Freidrich Struve who resolved the primary into a close pair of stars of mags 6.6 and 6.9 at a distance of 0".6. The pair closed to 0".2 about 90 years ago and has now widened again to 0".7 and PA 353 (2012). In a 20-cm OG it is usually a difficult pair and requires a steady air for a good view. The period appears to be around 520 years and both stars appear white to the writer; Smyth made them white and yellowish.

  Some 20 degrees below the tip of the `V' of Taurus can be found the naked-eye pair omicron 1 and omicron 2 Eri. Omicron 2 is the nearby triple star STF518 which contains two dwarf stars but this month the spotlight falls on 39 Eri (041423.69 -101521.2) which is 3 degrees south of omicron 2. Another discovery by William Herschel this beautiful pair is a slow binary having moved some 10 degrees retrograde since discovery and is now somewhat closer than then also (2009, 6".7). The significant proper motion in declination of 0".16 per year would have separated the stars by more than 30" since discovery if the pair were not connected. Modern catalogue values for the magnitudes are 5.03 and 8.53 although Smyth noted star B as mag 11 in the `Celestial Cycle'. The primary is a K3 giant so both Hartung and Gould, observing from the southern hemisphere where the star is high in the sky, noted an orange hue whilst the faint secondary appeared white in both cases. Webb, observing from the Welsh border made them yellow and blue.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - November 2011

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  The constellation of Andromeda now rides high in the evening sky stretching across some 50 degrees of right ascension. One of its most famous double stars is gamma (see DSOTM for Dec 2006) but some 6 degrees north of gamma and slightly following is the binary system STF 228 (021402.43 +472903.3). The spectral types of the stars are given as F2V and F7V in the WDS with visual magnitudes of 6.56 and 7.21. The period is 144 years and at present the separation is beginning to decrease with the relative position for 2012.0 given as 295°, 0".8. By 2032 the distance will have reached 0".32. Hipparcos gives the parallax of 25.23 milli-seconds of arc (mas) with an error of 0.66 mas which places the system at a distance of 129 light years.

  Whilst Andromeda is one of the largest constellations, Reticulum (The Net) is one of the smallest. Extend a line from alpha through delta Reticuli a little more than the separation of those two stars and you come to Dunlop 14 (033810.24 -594635.0). Visible in binoculars it repays examination through the telescope. The two stars, magnitudes 7.00 and 8.34, are both main sequence F stars and yellow in colour. Their separation was 45" at discovery in 1826 and by 2008 this has increased to 57".4. However, looking at the Hipparcos Catalogue reveals the fact that both the proper motions and parallaxes of the two stars are identical to within the given errors which makes this a bound system and it is located about 247 light years away. In Sissy Haas's book Double Stars for Small Telescopes, Ross Gould observing from Canberra with a 7-inch refractor, noted a faint third star of magnitude 12.8 close to B and which is not yet in the WDS.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - October 2011

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Located in northern Cepheus and about 15 degrees from the NCP, pi Cep (230753.8 +752315) is an awkward target for the user of the classical refractor and the writer has only measured this binary once, in 1994, when the position angle was 347° and the separation 1".1. Since then direct motion of 10 degrees has occurred with virtually no change in separation. It has now almost reached the point where Otto Struve first measured it in 1846 and the current period is given as 163 years. Around 1920, a number of observers recorded the star as single but the current orbit predicts separation in the 0.5 to 0.6 arc second range so it is not clear why the star should have been unresolved. Is the companion perhaps a variable? The primary is an early G star and both Webb and Smyth thought it very yellow or deep yellow, respectively, with both men agreeing that the secondary appeared purple. It is also a spectroscopic binary with a period of 556 days and it is speculated that the invisible companion is itself a short period binary. John Herschel found a 12.2 mag. comes at 58". Pi Cep is an attractive object but the stars are more than 2 magnitudes apart and it needs good optics and a steady air to resolve it cleanly. It will remain in range of 15-cm or so for a few decades to come.

  Sculptor contains a few nice pairs and one of these, kappa1 Scl (000921.0 -275917) is a good test for a 10-cm as the stars are magnitudes 6.1 and 6.2 and currently 1".3 apart. It was found low in the Chicago sky by Burnham using his 6-inch refractor when it was considerably closer than it is today and a preliminary orbit gives a period of over 600 years. Willem Luyten, in his proper motion survey work, found an 18th magnitude companion at a distance of 73" moving through space with the bright pair, making this a physical triple. Hipparcos puts the bright pair at a distance of 256 light years, some 10 light years more distant than pi Cep. The low power field also contains kappa2 Scl, a 5.4 mag. K giant, and most curiously, this too has a very faint (mag. 21) cpm companion found by Luyten which appears in the WDS as LDS2099.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - September 2011

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  kappa Pegasi (21 44 36.4 +35 47 39.5) is a famous visual binary, having been discovered by Burnham in 1880 using the 18.5-inch refractor at Dearborn. Apart from observations by the Greenwich double star observers around the turn of the century, and later at Herstmonceux and Christopher Taylor with his 12.5-inch it has eluded attempts at resolution from the UK. The separation barely exceeds 0".2 at any point in the 11 year orbit. Of more interest to the smaller aperture user is the Struve companion (STF 2824) which sits about 14" distant at PA 288. The writer has measured it with a 20-cm OG but, it has to be said, with some difficulty, owing to the faintness of B. The WDS gives 10.8 with the primary at V=4.1.

  pi Gruis (22 22 44.2 -45 56 53) would be a naked-eye pair if it were a little brighter. There are 2 stars of mags 6.5 (pi 1) and 5.6 (pi 2). Both are double stars in each case with companions about 4 magnitudes fainter and 3 to 5 arc seconds apart and both were discovered by Robert Innes. The easier of the two is pi1 (I 135) which also has the distinction of being a member of the rare S class of stars and varies between visual mag. 5.8 and 6.4. Hartung notes that the primary is bright orange. Whilst the binary nature of pi1 is not yet clear, pi 2 (I 382) seems to be physically connected. For the galaxy observer the edge-on spiral IC 5201 is about 15' SW of pi 2.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - August 2011

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Delta Cygni (19 44 58.4 +45 07 51), like Alpha Pav, is a B subgiant, if a little later in the spectral class. Its duplicity was discovered by the elder Herschel when the separation was around 2".3. Over the next half century or so, the mag 6.3 companion moved closer to A (mag 2.9) thus making the pair more difficult for early micrometrists. Indeed Webb reports that in the mid C19 the separation was such that the B star sat on the first diffraction ring of A making it difficult to spot and some reported that it was much easier to see in the twilight sky before sunset. Reports came in of brightness variation of up to 2 magnitudes in the companion but it may well be that this was due to the large difference in magnitude and close separation. At present the distance has increased slightly since Herschel's time and the companion has traced out about 150 degrees. An orbit of 780 years period currently occupies the USNO 6th orbit catalogue and predicts 219°, 2".70 for 2012.0. Its not an easy pair for the small aperture and can occasionally evade the 20-cm user if the seeing is not at least reasonably steady. Recently, Jim Daley in the US has added four faint field stars, arranged as two 3" pairs on either side of delta and distant 42" and 148" from it.

  Alpha Pavonis (20 25 38.9 -56 44 06) sits in an empty part of the southern sky but at mag 1.9 it is unmistakeable. From Feldhausen in South Africa, John Herschel examined the star and noted a distant wide pair - both stars of which are much fainter than alpha. It entered the catalogue as HJ 5193 and although there is probably no physical connection between any of the stars the system, caught the attention of the writer last year whilst using the 26-inch refractor in Johannesburg. The primary is a brilliant white, star B is reddish (John Herschel thought it `very red' and its closer companion C appeared blue - an unusual and rather patriotic combination of hues. The distance AB is 245" whilst C is 17" from B, and the magnitudes of BC are given as 9.14 and 9.17 in the WDS. It would be interesting to know if these colours are apparent in smaller apertures.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - July 2011

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Alpha Herculis or Rasalgethi (17 14 38.86 +14 23 24.9) is a red supergiant star (M5Ib/II) and the leader of Hercules. It has long been known as a visual binary and is particular attractive to small telescope users because of the contrast offered between the primary and its G5 giant companion some 4".6 distant. The primary is usually seen as orange/red but the companion has variously been reported as white (Hartung), bluish-turquoise (Haas) and sage green (Perez). The primary is a semi-regular pulsating variable whose brightness varies from about magnitude 3.1 to 3.9, and the main period of oscillation appears to be 1343 days. The companion is about 5.4. The motion is orbital but extremely slow and the period is thought to be about 3600 years, but this is complicated by the fact that each component is double again. The primary has been resolved occasionally by speckle interferometry at the 0".02 level whilst the secondary has been known as a spectroscopic binary for many years; the period is 51.6 days. There is also another companion to A with a period of 10 years which does not appear to be the speckle companion so alpha is at least quintuple. For the larger telescope user two faint companions mag. 11.1 at 79", and mag. 15.5 at 19" appear to be merely optical.

  Located in Ara, just on the border with Scorpius, HJ 4949 AB (17 26 51.98 -45 50 34.7) is the primary, of the wide pair DUN 216 (5.6, 7.1) which was found by Dunlop in 1826 at 30O and 33".1. By 1999 the pair had widened to 102" thanks to the proper motion of C. AB is a beautiful pair barely 2" apart which has moved about 20 degrees retrograde since being found by John Herschel in 1834. The stars, of magnitudes 5.6 and 6.3, are both late B in spectral type and hence appear white and afford "a beautiful contrast with the surrounding starry field" according to Hartung. Hipparcos places them at distance of 613 light years.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - June 2011

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Beta Serpentis (15 46 11.21 +15 25 18.9) lies about 11 degrees south of the crown of Corona Borealis. The primary is a mag 3.7 star of spectral type A2IV and many observers find it to be yellowish. The faint (mag 10.0) companion was found by the elder Herschel on 1781 Aug 13, and he did not allocate it a colour, noting it only as `extremely faint'. Today 15-cm aperture will show it clearly enough - a slighter harder test is the 10.7 mag star some 200" away in PA 212°. A very recent study has been using Hipparcos data to physically link apparently unrelated pairs of stars and in Beta's case the study found that there is a very high probability that the faint wide pair ROE 75 (mags 8.2, 10.7, PA 327°, separation 6".2) which can be found some 20 arc mins south preceding beta Ser is physically connected to Beta by dint of having a very similar parallax and proper motion.

  The small constellation of Ara is seen against the Milky Way near the tail of the Scorpion, and Hartung lists numerous objects of interest within its borders. The coarse cluster NGC 6193 is involved with the emission nebula NGC 6188 and in addition the brightest star MLO 8 (16 41 20.42 -48 45 46.7) is a multiple system of some interest. In 1878 observers at Melbourne divided A (mag 5.1) and found a mag 8.4 companion at 2".1 distance. The distance between the two has been closing since and the last recorded measure in 1938 placed the companion 1".6 from the primary, although Hartung reports seeing the star clearly resolved with 10.5-cm in 1963. It is not clear if B is a background star or an orbital companion - the next positive observation will help to decide this question. For the small telescope user there is star C (mag 6.8) some 10" away and medium apertures should easily find two fainter (mags 10.5 and 11.4) and more distant comites found by Herschel and another of mag 12.5 discovered by Innes.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - May 2011

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  STT 288 (14 53 23.35 +15 42 18.7) can be found about 3.5 degrees due south of xi Boötis as a star just below the usual limit of naked-eye visibility. The components are magnitudes 6.9 and 7.5 and revolve around one another in about 313 years. About 100 years ago, the Greenwich Observatory observer William Bowyer considered the two stars were optical in nature. To him they appeared to be steadily increasing in separation since discovery in 1845 when Otto Struve found them separated by 0".45. A few years after Bowyer's observation, the stars began to slowly close again. The orbit is fairly eccentric and the stars will continue to close to 0".51 in about 50 years time. Hipparcos places them at a distance of 155 light years. In the meantime at PA 160°, 1".06 they form an excellent test for the 10-cm aperture.

  Lupus is full of fine pairs and this column regularly features stars in that southern constellation. This month is the turn of Kappa Lupi = Dunlop 177 (15 11 56.07 -48 44 16.2), a beautiful pair of pale yellow stars according to E J Hartung. More recently Richard Jaworski notes them as yellow-white and plain white whilst the WDS gives the spectral types as B9.5V and A5V whilst the brightnesses are listed as 3.8 and 5.8. To the small aperture this is one of the easiest and brightest pairs in the sky - the current separation is 26".2 and this has decreased about 3" since 1826. With very similar proper motions, it is highly likely that the stars form a binary system, but Hipparcos has had some trouble in deciding the distance of the fainter component.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - April 2011

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  The two stars in this month's column both have early A type primary stars and are each located around 125 light years from Earth but as objects for observation, one can be seen in stabilized binoculars, the other needs at least a 30-cm telescope.

  alpha CVn (12 56 01.67 +38 19 06.2) is the leader of the northern constellation of the Hunting Dogs and sits in an desolate part of the sky to the naked eye, below Ursa Major and above the faint coarse grouping of stars which form Coma Berenices. It was found by William Herschel and the magnitudes are 2.8 and 5.5. Hipparcos, which has measured both components, has had trouble with the distance to star B but even though the error in the trig. parallax is some 30% the distance still agrees with that of star A within the mutual errors. The clincher here is the proper motion of each star - around 0".2 annually and in the same direction. The separation has reduced from 22" in 1777 to around 19".1 today - slow enough to be used by the writer as a standard for micrometer calibration. Both stars are brilliant white and A is the prototype of the alpha2 CVn variables - it possesses lines of rare earths in its spectrum and the amplitude of variability is some 0.14 mag in a period of 5.5 days. The WDS states that both stars are spectroscopic binaries but neither appears in the 9th catalogue of SB orbits.

  gamma Cen (12 41 31.20 -48 57 35.6) can be found by extending the line between alpha and gamma Crucis by about the same distance again. In 1847 John Herschel recorded his observation of the star in sweep 553 - "A star 4m. which I am very much inclined to believe close double, but could not verify it owing to bad definition. Tried 320 but it will not bear that power". He noted that there was indisputable evidence of rapid orbital motion (5°.4 in just over one year to 1836.28). The stars then closed rapidly to about 0".13 over the next 11 years and reached their closest point again in 1933. With a period of 84.5 years these two almost identical A stars are closing rapidly again and at the time of writing can be found at 311°, 0".26 - wait another year and the distance will be 0".18.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - March 2011

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  STF1333 (09 18 25.97 +35 21 51.3) appears with a brief description in Chambers revision of Smyth's Celestial Cycle but not in the original publication. Both stars appear `very white' in this tome but a more recent observation by Sissy Haas calls them `pure lemon yellow' but she also notes that they are in a very low power field with the orange star alpha Lyncis and also close to the bright, unequal pair 38 Lyn. The primary is spectral type A8V and the secondary probably similar although the WDS gives no spectral type. Discovered by William Herschel in 1782, Struve measured it in 1827 and found 40° and 1".5. The pair has slowly widened to 1".9 with little change of angle, so it is likely to be a long period binary but the relatively slow movement makes it a fine test for a small telescope with the components of magnitude 6.6 and 6.7. The distance according to Hipparcos is 308 light years.

  Mu Velorum (10 46 50.36 -49 25 12.8) is a magnitude 2.8 yellow giant located in a relatively blank area of sky to the naked eye some 11° north of the eta Carina nebula. In 1880, Russell, in Sydney, found it to be double with the companion some 2.7 magnitudes fainter located at PA 55° and a separation of 2".8. The pair turned out to be binary and by 1949 it had closed to 0".2. The currently accepted period is 138 years so the system is now almost at the point in its orbit where it was discovered. The relatively large difference in magnitude makes this star a southern equivalent of zeta Herculis although the separation range is much greater thanks to an eccentricity of 0.84. It will be near widest separation for many years and thus easily accessible to small telescopes on nights of steady seeing. Hartung notes that both stars are yellow, the companion being a G2 dwarf and therefore almost identical to the Sun. A is also a spectroscopic binary.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - February 2011

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  With Gemini high in the sky in the early Spring sky, a number of binaries are on display for the small telescope user. One of the more difficult is STF1037 (07 12 49.08 +27 13 30.2), a pair of yellow stars which are locked in a highly eccentric orbit. They present a good test object because at present the separation is 1".00 according to the 116 year orbit which is given in the USNO 6th orbital catalogue. The magnitudes are listed in the WDS as 7.2, 7.3 and with the 8-inch refractor at Cambridge this pair has always been more difficult to see and measure clearly than the parameters would suggest. It is a good time to observe STF1037 - the pair is closing up again and will reach well below 0".1 in about 30 years time, and it not be this wide again until around 2063. In the 19th century, the German observer Madler was convinced that B was double again and the volume by Lewis on the Struve stars does show a loop in the apparent motion of B but no convincing evidence for a third component has come to light. Madler, and Dembowski failed to see the faint star C (V~13) found by Otto Struve. It is located at 78°, 14" but may be variable.

  Puppis is a glorious constellation for the double star aficionado and one of the best objects is k Puppis (07 38 49.88 -26 48 14.0), a third magnitude star some 8° east of delta CMa. Discovered by William Herschel (H III 27) the stars are both hot blue dwarfs of spectral type B6 and might be expected to appear white in the eyepiece. Malin and Frew, in their revision of Hartung's book thought so but noted that Hartung himself had them as pale yellow. From his observatory in Victoria, Australia this object would have passed almost overhead. Haas also calls them white but gives the star name as kappa. There has been some angular motion since 1800 - star B has moved about 8° retrograde and can now be found at 318° and 9".9. Burnham added a faint, distant star, mag 13.7 at 7" from A, but as this has not been measured since 1927, it would appear to be a difficult object.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - January 2011

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Rigel = beta Orionis (05 14 32.27 -08 12 05.9) is perhaps the most intrinsically luminous star in a double star system which can be seen in a small telescope. Whilst the measured luminosity of other bright stars such as Canopus have varied wildly over many years, the advent of the Hipparcos mission has been able to pin down the geometric distance to these objects much more accurately. The current trigonometric parallax for Rigel from Hipparcos is 3.78 milli-arcseconds with an uncertainty of about 10%. This translates to a luminosity of 48,000 times that of the Sun. The faint companion star whose estimated magnitude has also fluctuated over the years might be dominated by the light of the dazzling primary star 9 arc seconds distant but it is an equally interesting object in its own right. In 1871, Burnham, using his 6-inch Clark refractor, suspected an elongation of this star, and having then examined it with the 18.5-inch OG at Dearborn was convinced that there was a `real and measurable' elongation. Around 1900, Aitken, Hussey and Barnard using the 36-inch refractor at Lick all recorded separations in the 0.09 to 0.16 arc second range for BC. Occasional sightings have been reported since then, including as late as 2005 but no orbit exists and in fact the reality of the companion to B must still regard as unproven. Rigel B is known to be a spectroscopic binary but this cannot be BC. It needs a good night to see Rigel B clearly in the glare of the primary. The magnitude is near to 6.8 rather than the 10.4 first assigned to it by Burnham.

  19 Cam (05 22 33.53 +79 13 52.1) is a mag 5.1 late F dwarf star some 68 light years distant. A companion star of mag. 9.2 was first noted, apparently by Piazzi, in the compilation of his Palermo catalogue and later catalogued by Struve as STF 634.This is a good example of an optical double. In 1825, South found star B at 346° and 37", by late 1926 it was at 62°, 9" and by 2008 it had reached 133°, 27". Both stars have significant proper motions (0.18 and 0.16 arc seconds per annum) but almost in opposite directions on the sky. This is an attractive pair for small telescopes although rather difficult to find being in a sparse area near the north celestial pole - the colours were given as light yellow and pale blue by Smyth.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - December 2010

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  STT 95 Tauri (05 05 32.1 +19 48 24) was found by Otto Struve using the 15-inch refractor at Pulkovo. It is a long period binary (760 years) and is currently to be found at 297 degrees and 0".96. The magnitudes are 7.0 and 7.6 so it should be easily resolvable in 15-cm. The apparent orbit is such that the pair will widen to about 1".2 over the next 350 years so this system will gradually become easier for the small telescope user. The primary belongs to the metallic lined A star group, both stars appear white, and the system is at a similar distance from the Sun as theta Ret (below) is. The star lies halfway between iota and 104 Tauri which in turn lie halfway between zeta Tauri and Aldebaran.

  Theta Ret (04 17 40.3 -63 15 20) is a beautiful pair of stars (magnitudes 6.4 and 7.7) in the small constellation of Reticulum, more specifically, it is in the same low power field as the stars alpha (mag. 3.4) and eta (mag. 5.4) Ret and 30 arc minutes south of the galaxy NGC 1559. Discovered by Rumker (the pair is RMK 3) the two stars have changed little in position angle and the separation has slowly reduced from 6".4 in 1835 to 4".3 in 2000 when the pair was last measured. Although the primary has a spectral type of B9.5 III both stars appeared deep yellow to E. J. Hartung whilst Ross Gould with 35-cm thought that the primary was a lighter shade of yellow than the secondary. This is almost certainly a binary star with the small proper motion of A not quite accounting for the change in separation. Hipparcos gives the distance to A as 7.02 mas (465 light years) and the primary star is consequently some 50 times brighter than the Sun. In his revision of Smyth's Celestial Cycle, Chambers quotes B. A. Gould as having noted a variation of 0.3 magnitude in the primary. In that volume, the stars are given as 5.5 and 9.0.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - November 2010

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Psi Cas (01 25 55.90 +68 07 48.8) is a pretty triple star found about 8 degrees due north of delta Cas the second left-hand-most star in the 'W'. AC (mags 4.7, 9.2) was found by William Herschel in 1783 and has closed up considerably since then. In 2007 the position was 128° and 20".3, a change which is due entirely to the proper motion of A. Both stars are double again and with a small telescope CD is quite difficult since the components are magnitude 9.4 and 10.0. Burnham found a 14th magnitude companion (B) to A which was last measured in 1970 at separation 2".4 and must be considered beyond the range of most amateur instruments. Chambers gives colours of orange tint, blue and reddish for A, C and D. In 1850 Webb found A orange and C blue.

  Beta Phe (01 06 05.11 -46 43 06.6) is a bright visual binary which has been under-observed since it was discovered by R. P. Sellors in 1891 with an 11-inch refractor. It widened to about 1".4 in the mid C20 but then started to close again and was measured occasionally as it closed up with a measure in 1999 giving 258.8°, 0".29. This was the last observation until 2008 and during that time the star passed unobserved through 140 degrees of position angle and more crucially, periastron passage. In 2002 Andreas Alzner calculated the first orbit but more recent measures by Rainer Anton (see JDSO) indicate that a further revision is necessary. This is now in progress. The star is now widening and at the time of writing is around 0".40 so that it should be seen as double in 25-cm on a good night. Steady air is essential since the stars are very bright - magnitudes 4.0 and 4.2. When looking for this pair, take time to look for Slr 2 some 30 arc minutes to the east. It is somewhat wider and fainter but it is doubtless also a long period binary

  Bob Argyle - Double Star Section Director

• Double Star of the Month - October 2010

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  The pairs featured this month are both close but bright visual binaries which afford excellent tests of optics and seeing for the possessors of 20 to 25-cm aperture.

  72 Peg (23 33 57.19 +31 19 31.0) is equidistant from the top two stars of the square of Pegasus, alpha and beta, and about 3 degrees above the line joining them. Discovered in 1878 with the 18.5-inch refractor at Dearborn by Burnham, this yellow pair has slowly widened with increasing position angle. At the present the companion can be found at PA 105 degrees and separation 0".53. This is the maximum separation which this system will attain but the period of 260 years means that it will be some years before it appears to be significantly closer. The magnitude of the two stars, 5.7 and 6.1, and the declination of +31 means that in the autumn this pair is high in the mid-northern sky and thus provides an excellent resolution test.

  Lambda 1 Scl (00 42 42.89 -38 27 48.6) is a similar system to 72 Peg although somewhat fainter (6.7 and 7.0) and rather wider at the present time (21°, 0".7) having widened somewhat since discovery by Harvard College Observers in Peru in 1901. The primary is A0V and therefore appears white to the observer. The brighter star lambda 2 sits about 18 arc minutes following lambda 1 and provides the binocular observer with a fine sight. The parallaxes of both stars as determined by Hipparcos seem to rule out a physical connection.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - September 2010

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  The two pairs this month are both long period binaries, whose physical association was in some doubt for many years.

  zeta Aqr (22 28 49.8 -00 01 12) is not strictly a northern double star being some 72 arc seconds south of the equator but it is such a bright and easy pair for the small telescope that it is worth seeking out. It is one of the bunch of bright stars which are 5 to 8 degrees following alpha Aquarii.

  The stars that form zeta are magnitudes 4.3 and 4.5 and the spectral type of early F suggests that the colour of the primary should be yellowish. Webb in 1851 finds pale yellow, Hartung also notes yellow and Sissy Haas has pale citrus-orange.

  According to Thomas Lewis, Zeta was found to be double by Christian Mayer in 1777 but it does not appear in his pioneering catalogue of 80 double stars published in 1780 according to Jurg Schlimmer. William Herschel observed it soon afterwards and includes it as H II 7. At this time the companion was about 4 arc seconds due north and for most of the nineteenth century the motion appeared linear, but in the first quarter century of the twentieth century the pair closed up and the companion began to swing around A. At present the position angle and separation are 170° and 2".1 and the stars are now separating and will continue to form an easy pair for centuries to come.

  53 Aqr (22 26 34.3 -16 44 31.9) can also be seen from northern latitudes but it requires a night of good seeing to separate the two stars cleanly. These are almost identical GO dwarfs of mags 6.3 and 6.4 and when the pair was first found by South/Herschel the separation was more than 10 arc seconds. Over the last 200 years the stars have slowly closed, mostly in an apparently linear fashion but about 50 years ago the companion began a slow majestic swing around the primary.

  Hale in 1994 computed an orbit with a period of 3500 years so our knowledge of the binary motion is cursory at best but regular and accurate measures over the next 50 years or so will define the periastron part of the orbit. If Hale's work is right, the maximum distance between the stars will be almost 27 arc seconds in 1800 yea’s time, but as luck would have it minimum distance of 1".27 is reached in 2014, so this is a very good time to watch this pair.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - August 2010

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  mu Cyg (21 44 08.57 +28 44 33.4) is a fine binary, currently well-seen in 10-cm aperture. It is number 72 in Christian Mayer's 1780 catalogue of double stars and since that time has been well followed by double star observers with the WDS listing more than 700 measurements. The apparent orbit is quite eccentric (0.66) and the companion spends about one-third of the orbital period of 789 years near periastron and the remaining two-thirds near apastron. A close approach occurred in 1936 when the distance dropped to about 0".5 and so the pair having reached an elongation will now start to close slowly, reaching around 1".2 in 40 years time and then widening to 7".2 in the 24th century.

  According to the WDS the stars are both F dwarfs with the companion somewhat earlier than the primary. This is an unequal pair with the stars being magnitudes 4.8 and 6.2. The primary is a double-line spectroscopic binary and both the distant companions are field stars.

  One of the finest sights in a small telescope in the southern hemisphere is gamma CrA (19 06 25.14 -37 03 48.5) whose binary nature was discovered by John Herschel from South Africa in 1834. With the separation ranging between 1".3 and 2".5 and a period of 122 years this is a good system to follow over a period of a few years. At the moment the companion is at 9 + and 1".35 with the position angle reducing at the rate of about 4 degrees per year. Like mu Cyg (above) it is an unequal pair of magnitudes 4.5 and 6.4. Hartung does not give any colours but Sissy Haas mentions that Gould finds pale yellow.

  gamma CrA is 56 light years distant according to Hipparcos and located near the border with Sagittarius. The bright globular NGC 6723 appears in the same wide-field finder view when acquiring the star.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - July 2010

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Located close to the head of the Dragon, mu Dra (17 05 20.12 +54 28 12) is a long period binary system, first found by William Herschel in 1781. With a period of 672 years, the apparent separation of the two stars ranges from 2".0, which last occurred about 40 years ago, and 5".7. At the present time it is almost 2".4 apart at position angle 5°. With components of magnitudes 5.66 and 5.69 the star is easily visible to the naked-eye and is an excellent target for small telescopes so it is included in James Mullaney's One Hundred Showpiece Double and Multiple Star list, where he gives the colours as yellowish-white. Sissy Haas also notes the stars as goldish-white, but Smyth sees them only as white. There is evidence for a spectroscopic companion to B which may be bright and wide enough to be resolvable in the optical, and there is a mag 13.7 star at 12" may also be part of the group. The system lies at a distance of 90 light years.

  Nu Scorpii (16 11 59.27 -19 06 53) is the southern equivalent of the Double-Double in Lyra, although the brighter pair is more difficult to divide than its northern equivalent, and as a consequence at least 150-mm is needed to see the four components clearly. The wide pair catalogued by Herschel as H V 6 consists of white stars of magnitude 4.2 and 6.0 separated by 41". In 1846 Mitchel, using an 11-inch refractor in Cincinnati, resolved the companion into two stars about 1".3 apart. Burnham then discovered that the primary was also double with his 6-inch Clark in 1873 when the separation was around 0".6. Since that time both pairs have slowly widened and the current values are 1°, 1".3 for A and 55°, 2".3 for B. This is a physical system of high multiplicity since A is double again at the sub-0".1 level and there is also a spectroscopic component of 5.5 day period. The writer has measured both bright pairs from the UK with 20-cm but it needs a night with very steady air to do this.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - June 2010

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  To the small telescope user STF1964 (15 38 12.96 +36 14 48.3) is a pair of 7.9 and mag 8.0 white stars separated by about 14". Readers of Celestial Objects for Common Telescopes will note that the stars are given as mags 6.8 and 7.3, a considerable difference to the quoted WDS magnitudes above. STF1964 lies about half a degree south preceding zeta CrB, a much brighter and somewhat closer pair. STF1964 is, however, not without its attractions. Each of the two stars is a visual double - the fainter component actually known as C was found by Struve and the writer managed to measure this 1".4 pair last year. The brighter star A (=HU 1167) is somewhat closer but the companion (B) is fainter and this system is a challenge to those with 25 to 30-cm of aperture. In 1970, professional observer Richard Walker of USNO reported that star A had another closer companion at a distance of 0".1 but there has not yet been confirmation of this star.

  HN 28 (14 57 27.99 -21 24 55.8) is also known as Hh457 in Webb. Star A is number 570 in the Gliese Catalogue of Nearby Stars. It is 19.1 light years distant and with both stars moving through space at more than 2" per year, it is a physical system. In 1806 Piazzi measured both stars and at that time the position was 251 degs and 9".4. In 2009, the writer found 306 degrees and 25".3. The stars are 5.8 and 8.2 and offer a fine colour contrast. Star A is spectral type K5V and orange in colour whilst B is type M2V and red, the colours being noted by Hartung. Sissy Haas calls it 33 Lib and notes that Smyth made it ``straw coloured and orpiment yellow". The USNO 6th orbit catalogue includes it with a period of 2130 years. In 1990 B was found to be a close binary with a period of 308 days and a separation of 0".2. To the small telescope user there are a number of fainter companions ranging from mag 9.5 to 13 or so all of which are being left behind by the principal stars.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - May 2010

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  78 UMa (13 00 43.59 +56 21 58.8) was found to be double by S. W. Burnham in 1889 using the then new 36-inch refractor at Lick Observatory. The pair can be found just following epsilon UMa. Although the stars are very unequal in brightness (the WDS gives 5.02 and 7.88 and Baize and Petit list the system in their catalogue of double stars with variable components), it was apparent from the proper motion of A and the relative motion of B that the pair formed a binary system. A recent orbit gives a period of 106.4 years but at no point in its apparent orbit can the pair be called `easy'. On good nights with the 8-inch OG at Cambridge the companion was measured several times in the last two decades but since then the separation has reduced and in mid-2010 the companion can be found at 99 degs, 1".18. The motion is relatively slow however with minimum separation of 0".48 not occurring until 2026. Sissy Haas notes an observation with 275-mm which gives colours of gold-white and bluish-turquoise.

  Another double star with a variable component is the wide southern system alpha Circini (14 42 30.69 -64 58 28.5). Catalogued by Dunlop (Delta 166) this beautiful pair is one of the finest systems in the southern hemisphere. The primary is a member of the rapidly oscillating peculiar A stars which has a basic period of 6.8 minutes and an amplitude of a few millimagnitudes. Other frequencies of oscillation have also been found. The visual magnitude of A is 3.18. B (mag. 8.47) is a K0 dwarf now separated some 15 arc seconds from A and the two stars form a very long period binary system since the proper motions of both are substantial and identical. Hipparcos places the stars at a distance of 54 light years. Since 1826, the companion has widened from a distance of 10" and the position angle has decreased by 38 degrees to about 225 degrees now. Several observers make the colours yellow and red including Hartung some 50 years ago and more recently Richard Jaworski, both from Australia.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - April 2010

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  35 Com (12 53 17.77 +21 14 42.1) is a visual binary with a period of 359 years meaning that since the first measures were made in 1829 we have observed exactly half its orbit. For 2010.5 the companion can be found at 196 degs and 1".03 but as the difference in magnitudes is almost 2 (the stars are 5.15 and 7.08), it is not an easy pair for the small telescope. A third star of magnitude 9.76 is some 27" away and is physically connected to AB. 35 Com lies to the north of the main Coma group of galaxies but is only 1 degree south preceding the galaxy M64. It is about 280 light years away and the primary is a giant star of spectral type G7. Gould, with 20-cm, estimates A and B are both orange-yellow with the distant C "possibly blue".

  The appearance of theta Muscae = Rmk 16 (13 08 07.16 -65 18 21.7) as a wide pair of bright stars belies its true nature. There are four hot young stars in this system which is so far away that Hipparcos was unable to get a handle on the distance. Estimates from other sources put the group 2,000 to 2,500 light years away. A is a spectroscopic binary of period 19 days consisting of a WC6 star with an O6 companion whilst some 0".04 distant is a B0 supergiant. The companion as discerned by the small telescope user is about 5".4 distance and is probably another Wolf-Rayet star. Deep-exposures of this star show very faint concave nebulae facing the stars which are probably the result of old shell ejection phases. This is a splendid object for the small telescope; Hartung gives the colours as yellow and white, the magnitudes are 5.63 and 7.55 and the pair sits in a rich field.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - March 2010

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  The subjects of this month's column have several things in common. Both are binaries with unequal components, both contain F stars and in each case both contain a variable component according to the 1989 paper by Baize and Petit.

  sigma2 UMa = STF1306 (091023.53 +670803.3) lies in NW UMa not far from the galaxies M81 and M82. The apparent orbit of this 1140 year system shows that at closest approach the stars were about 1"1 apart in PA 153 degrees as happened in 1912. With the magnitudes of 4.87 and 8.85 this would have been a difficult object. Since then the pair has continued to widen and the current position is 350 degrees and 4.15 arc sec. Widest separation occurs around 2520 when the pair are 11.3 arc sec apart. Suspicion of variability of B was voiced by Webb who noted that both Sadler and Dembowski had recorded this, the latter giving the range of B as 8.0 to 10.0. Smyth gives colours of flushed white and sapphire blue whilst Webb noted greenish and orange. The distance to this system is 66 light years.

  psi Velorum = Copeland 1 (093041.97 -402800.2) lies on the Vela/ Antlia border. A close and occasionally very difficult binary of short (33.95 years) period it escaped the attention of John Herschel (it was only 0".3 in 1835/6) and was discovered by Ralph Copeland, later Astronomer Royal for Scotland, in the early1880s whilst separated by 1". One of the brightest systems in the sky, with components of magnitudes 3.91 and 5.12, psi is now widening and offers a

  chance, in the next few years, for those with small to medium apertures to see it divided. The ephemeris is as follows:

  • 2010.0 101.3 0.85
  • 2012.0 110.1 1.00
  • 2014.0 117.0 1.08
  • 2016.0 123.2 1.11
  • 2018.0 129.4 1.08

  The stars are both subgiants of spectral type F0 and F3 respectively and the distance to this system is 61 light years.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - February 2010

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  STF1127 (07 47 00.43 +64 03 07.3) is a coarse, relatively bright triple star in Camelopardalis about 90 arc minutes south of 51 Cam. The magnitudes are given in the WDS as 7.0, 8.5 and 9.7 about 0.5 mag fainter in each case than the corresponding figures in Celestial Objects for Common Telescopes. Webb notes the colours as very white, ash and orange but no other observers venture a colour estimate. The primary is A2V. Component B is 5".4 away in position angle 340 degrees almost unchanged since the early 19th century whilst C is 11".7 distant from A in PA 177. Between STF1127 and 51 Cam the neat pair STF1122 can be seen.

  HJ 3928 (07 05 32.05 -34 46 40.1) The companion star was discovered in 1836 on sweep 809 of John Herschel's 20-foot reflector, along with two more distant, much fainter (10.8 and 13.5 mag) and probably unrelated stars. AB is a binary star with the position angle decreasing from 157 degrees at discovery to 145 degrees now and the separation closing from 4.0 to 2.7 arc seconds in the same interval. The magnitudes are 6.47 and 7.81 according to the WDS so the pair should be well seen in 100-mm. Star A is 175 light years away according to the revised Hipparcos catalogue.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - January 2010

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Lying about 4 degrees following iota Aur, the fainter of the stars in the horns of Taurus, is 14 Aurigae = STF 653 (05 15 24.39 +32 41 15.3) a bright and easy pair (magnitudes 5.01 and 7.33) currently separated by 9".8. Good nights in small telescopes may also reveal the fainter (11.0) component C at a similar distance and first seen by F G W Struve, although Sissy Haas did not see it in 12.5-cm. Lewis in his work on the Struve stars (1906) lists a number of colour estimates for the two components - Dawes: yellow and blue, Smyth: pale yellow and orange, Dembowski: yellow and blue, Duner: yellow and azure, and Perrotin: white and orange. More recently, Haas notes it as bright straw yellow and royal blue. The system as a whole is a complex one. A is also a spectroscopic binary and a delta Scuti variable known as KW Aur. C is thought to be physical, whilst B merely reflects the proper motion of A. However, C itself is a spectroscopic binary and HST observations have recently established the presence of a white dwarf companion (Cb) 2" away whose connection to Ca has not been established. 14 Aur A lies at a distance of 286 light years.

  kappa Leporis (05 13 13.88 -12 56 28.7), whilst certainly in the southern hemisphere, tantalizes the northern observer as the constellation below the feet of Orion can be seen briefly as Orion culminates. Many of the doubles noted in Lepus by Webb are unequal and this pair is no exception - A and B are respectively 4.43 and 6.99 according to Makarov and Fabricius in their analysis of Tycho-2 magnitudes) but also the relative close separation (now just under 2", having been closing since discovery in 1832). The writer has measured it with 20-cm but the seeing more than 60 degrees from the zenith is rarely good. Hartung reports that it easy with 7.5-cm from Australia. The primary is a late B dwarf but Smyth recorded pale white and clear grey for 1835 whilst Webb notes colours of yellow and grey in 1851.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - December 2009

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  80 Tau (4 30 08.60 +15 38 16.2) is a an unequally bright (5.70, 8.12) binary star with a highly inclined orbit in the cluster of bright stars close to Aldebaran, more specifically about half a degree south following the bright binocular pair theta1,2 Tauri. Found by Struve in 1831, the companion headed towards the primary until by 1878, the great Italian observer Dembowski, in one of his last observations, failed to see B in his 7-inch Merz dialyte and by the 1890s the companion was invisible in the largest apertures. The companion passed close to A in the following years and since then has been continuing to widen, reaching maximum separation (1".75) in around 1987. By 2010.0 the star is at 16 degs, 1".64 and should be visible for several decades in 20-cm but choose a good night because the large difference in magnitude makes this a far from easy pair. A itself is a spectroscopic binary and the distance to this system is 150 light years.

  Iota Pic (04 50 55.31 -53 27 41.5) is, according to Hartung, "an excellent object for small telescopes" and recorded the colours as yellow. The stars are magnitudes 5.61 and 6.24 and of spectral class F0IV and F4. Each component appears in the Hipparcos catalogue as a separate entry and the parallaxes, whilst each possessed of rather a large error, indicate that the stars are at the same distance from us i.e. about 130 light years and the similar proper motions confirm that this is a true binary system. The WDS catalogue notes that the PA of 59 degs and separation of 12".6 for 2002 is virtually unchanged since the pair was discovered by Dunlop in 1826. The star can be found in western Pictor, about 3 degrees north following alpha Doradus.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - November 2009

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Iota Tri (02 12 22.28 +30 18 11.1), also STF 227, is another of the elder Herschel's discoveries and was included in his second class as number 34. There is a similar if considerably fainter pair called H II 35 following about half a degree distant which became STF 232, both being found on 1781 Oct 8.Interestingly, iota Tri does appear in Piazzi's 1814 catalogue (but not as a double) whilst Baily in his Catalogue of Stars for the British Association in 1845, gives it only as 6 Tri. Whatever it is called, iota is an attractive pair of yellow and blue stars according to Webb (Smyth - topaz-yellow and green) which forms a very slow binary system which has changed by only 11 degrees in over 200 years with the stars closing very slowly. The position for 2008 is 68 degs, 3".7. Both stars are spectroscopic binaries with periods of 14.73 and 2.24 days for the brighter and fainter visual stars respectively. A is also called TZ Tri, an RS CVn binary which has been resolved using the Palomar Test bed Interferometer. This reveals that the angular size of the semi-major axis of the orbit is but 2 milliarcseconds.

  Tau Scl (01 36 08.50 -29 54 26.5) lies in a large rather empty area of sky north of Phoenix with only the galaxy NGC 613 some one degree north following for immediate company. This `close yellow pair', as Hartung calls it, is nevertheless worth seeking out. Separated by 3".9 in 1835 when John Herschel listed it as 3447 in his catalogue the pair has closed ever since and is currently just past minimum separation (2010, 186 degrees, 0".81), according to a recent orbit which gives the period as 1503 years. The magnitudes of the stars are 6.0 and 7.4, and the Hipparcos satellite puts them at a distance of 227 light years.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - October 2009

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Perhaps to be classed more as a small cluster than a multiple star the system of 8 Lac (22 35 52.3 +39 38 04) was first noted by Herschel. They form an arch was the comment he added in the Philosophical Transactions paper in 1784. The small telescope user can see four components with relative ease: A=5.7, B=6.3, C=10.4 and D=9.1. AB is 185 degrees, 22".2, AC is 168 degs and 48".3 and CD is 116 degs and 42".4. A fifth star I, is mag 11 and 228 degs and 9".7 from D. It was found by T. Espin in 1906. The remaining four components according to the WDS (where it is known as STF 2922) are either very faint, very close or extremely distant.

  Hipparcos had great trouble with the distance to stars A and B which were the only two that it observed, but it seems that they are so remote that the parallax is to all intents too small to measure and means that the main stars are before at least 2000 light years away. The spectral types are early to mid-B so they are probably hot young stars. Thus might indicate that the predominant colour would be white. Chambers makes A and B white but C greenish and D blue. Webb notes that A and B might be tinged with yellow but also makes D blue.

  DUN 246 ( 23 07 14.8 -50 41 12) lies in a thinly sprinkled star field according to Ernst Hartung. The stars are magnitudes 6.3 and 7.1 whilst the separation has been slowly decreasing since discovery by Dunlop in 1825. The current position is about 255 degs and 8".7 and the similar proper motions of both stars indicates a long period binary system. A pair of late type dwarfs is reflected in Hartung's colours of yellow for each component. More recently Gould with 35-cm noted the colours are both pale yellow.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - September 2009

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Two high declination systems are the subject of this month's column.

  Cepheus is a rich hunting ground for the northern double star enthusiast and Webb lists about 80 pairs in this constellation, many of which are suitable for the small aperture. xi Cep (22 03 47.2 +64 37 40) did not attract much interest from Smyth as the binary nature of the system was not then apparent, the change in angle having amounted to only 3 degrees from the observation of Herschel some 80 years previously. Since then the curvature of the apparent orbit has tempted the production of an orbit of period 3800 years and in 2010 the companion can be found at 274 degrees and 8".34. The system is relatively close by (30 parsecs) and the main interest for the small telescope observer are the colours of the two components. Webb called them white and tawny or ruddy whilst Smyth thought them both bluish. Sissy Haas considers them lemon white and royal blue. The spectral types are A3 and F8. For the large telescope observer, the A component is a close interferometric and spectroscopic binary of period 2.254 years and the separation never exceeds 0".06.

  In the far south Octans straddles the celestial pole. Lambda Oct (21 50 54.5 -82 43 08) precedes beta Oct by a few degrees in a rather sparse area of the sky but the effort of finding it is certainly worthwhile. It is one of John Herschel's discoveries from South Africa (HJ 5278) and is one of the more attractive ones. Hartung calls it a `bright elegant close pair, deep yellow and white.' It is clearly a physical pair and the position for 2002, when it was last measured according to the WDS, is 63 degrees and 3".5. This is a distant pair, more than 400 light years away according to Hipparcos, and the spectra are G8 and KOIII which makes Hartung's comments about colour all the more interesting. The magnitudes are 5.6 and 7.3. For more observations of Octans pairs, see the article by Magda Streicher in Deep-Sky Observer 145, 2008.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - August 2009

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  100 Her (18 07 49.6 +26 06 04) is a bright and wide pair of white stars in eastern Hercules, about 15 degrees south preceding Vega. It forms a fine sight for the small telescope. The original separation derived by Herschel in 1777 showed the stars to be 17".0 apart whilst a more recent measure by the author in 1995 indicates that the stars have closed to 14".2 with very little change in angle.

  These stars caused the Hipparcos satellite some difficulty as the errors in parallax, even in the revised version, are some ten times worse than might be expected but they still show that the parallaxes of the two stars are the same within the (large) errors and thus indicate that the two stars are probably physically related. In 1985 the CHARA team from Georgia State

  University discovered that component A was a close binary. Subsequently, the period turned out to be less than 16 years. Perhaps the same situation might apply to star B, as an explanation for the large error in parallax. Both stars are noted as white by several observers.

  HJ 5014 (18 06 49.9 -43 25 30) was another product of John Herschel's fecund search for new double stars at the Cape of Good Hope. Unfortunately there were few observations in the following 50 years when the pair moved through almost 180 degrees of position angle, widening considerably as it did so. Wierzbinski produced some orbital elements in 1958 with a period of 191.2 years. It was clear around the beginning of this century that the real period was much longer and Andreas Alzner produced an orbit in 2002 increasing the period to 450 years.

  This is a beautiful pair of white stars (both A5 dwarfs) each of visual magnitude 5.7 which can be well seen in 10-cm aperture. The position for 2010.0 is 2.4 degrees and 1".72, and it will continue to widen until 2170 or so.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - July 2009

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Both systems featured in this month's column are close to the Sun but vary considerably in difficulty of observation. One is a real test for a medium aperture whilst the other can be seen in a small telescope.

  Mu Herculis = STF2220 (17 46 27.72 +27 43 21.0) was found by William Herschel in 1781 and is a wide and very unequal pair of magnitudes 3.42 and 9.78. The current position angle and separation is 248 degrees and 34".9 values which have increased only marginally since Struve measured the pair in 1831. The large proper motion of A and the small change between A and B over time mean that the two stars form a physical system. In 1854 Alvan Clark found that the companion was double. It turns out to have a period of 43.2 years and is a severe test of resolution and light gathering power. The components are mags 10.2 and 10.7 and the separation varies between 0".5 and 1".5. At the time of writing the stars are 1".1 apart and will close until 2018 when a separation of about 0".6 is reached. This is a good opportunity to see this system as a triple star. The revised parallax from Hipparcos is 120.33 mas putting the group at a distance of about 27 light years. The spectral type of A is G5IV and that of the close pair appears to be dwarf M, not withstanding the fact that Chambers in his revision of Smyth's Bedford Catalogue, assigns to it a colour of cerulean blue.

  As a nearby solar-like star, mu Her A is a good candidate for hosting a planetary system and in 1994 two independent series of radial velocities were taken. What they showed was a slow drift which indicated a possible period of 30 years. In 1998, a star of V magnitude 12.7 was seen 1.4 arc seconds from A using adaptive optics on the 100-inch reflector at Mount Wilson. This object is close to the sub-stellar mass limit and further observations will be needed to establish its physical connection to A. The WDS lists another star, mag 11.5 at 256", but it far from clear that this also belongs to the system.

  About 4 degrees north preceding the 3rd magnitude star alpha Arae is BSO 13 (17 19 02.95 -46 38 11.4). Picked up as early as 1824 in the mural circle at Paramatta (Sydney) it is a similar system to eta Cas - a nearby, long period, unequal pair with a G-type primary and M secondary, in this case G8V and M0V. The stars, which are only 28.7 light years distant, are now moving slowly apart in their 693 year orbit and are currently at 257 degrees and 9".9 so they are visible in 75-mm aperture with ease, although larger apertures will show the colours to greater effect. Hartung notes deep yellow and orange. The WDS gives magnitudes of 5.61 and 8.88 but van den Bos in a series of measures in Johannesburg in the 1920s consistently estimated the magnitude differences as 2.5 or so, so there may be some real variation in the brightness of the companion.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - June 2009

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  The two pairs in this month's column are similar in that they contain stars of spectral type A but there the similarity ends. Zeta Boötis is a close, bright binary and SHJ 179 is much fainter and considerably wider.

  zeta Boötis (14 41 08.92 +13 43 42.0) is a white mag. 3.7 star south following Arcturus by about 8 degrees. Its binary nature was discovered by William Herschel on 1796, Apr 5 when he said that the pair was `very nearly in contact; I can, however, see a small division'. He had previously seen a mag. 11 optical companion in 1782 which became H VI 104. The proper motion of AB is taking it away from C and the distance has increased from 99 to 103 arc seconds over the last century or so.

  The main pair is a bright, equal binary of high inclination and extreme eccentricity - in fact it appears to be the current record holder, surpassing even the value for 41 Dra (see Astronomy Now for June 2009). Having spent most of the last half century near PA 310 degrees and separation 1", it is now noticeably closing and the writer found it separated by 0".6 last Spring. If the 122.98 year orbit by Andreas Alzner is correct it will pass 0".5 in 2011 and then dip below 0".01 in the summer of 2021 when the angular motion will be 10 degrees per DAY. From there it will rapidly return to the 4th quadrant again the following year. The stars are both A0 dwarfs and the revised Hipparcos parallax is 19.00 mas.

  SHJ 179 (14 25 29.91 -19 58 11.8) First measured in 1798 and placed in the catalogue of stars found by James South and John Herschel, this attractive wide pair of magnitude 6.6 and 7.2 stars makes an excellent target for the small telescope. Sissy Haas marks them as reddish white whilst Hartung from Australia notes them as pale yellow. They are low in the sky for the northern observer so this may explain the reddish tinge, as the WDS gives the spectral types as A2V and A4V. The proper motion of A seems to be shared by B and the relative position has changed little in the last 200 years, the current values being 205 degrees and 34.7. These are distant stars, the parallax of A placing it about 457 light years away.

  In 1867 Burnham using his 6-inch Clark refractor found that B was double again and in the intervening period, the companion has moved about 15 degrees in a retrograde direction, remaining close to the discovery separation of 1".2. This pair is BU 225 BC in the WDS but also bears the appellation HDO 138 indicating that it was found later from Peru by the Harvard observers at Arequipa but who were unaware of Burnham's observation. The difference in magnitude is about 1.7 so this pair requires at least 100-mm of aperture to see well.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - May 2009

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  At this time of year the constellation of Lupus is nearing the meridian in southern latitudes and offers a number of beautiful and sometimes difficult pairs for the small telescope user. Pi Lupi (15 05 08.16 -47 03 04.3) was found by John Herschel in South Africa and since that time (1836) it has been slowly widening so that it is now within range of a 75-mm aperture. It seems to have given early observers no little difficulty and values for the observed separations vary wildly which in a bright equal pair is difficult to explain. The current position is 67°, 1".6 and as this value of separation has not changed significantly for about 30 years, we may conclude that this is a binary in a highly inclined orbit which is close to apastron.

  The WDS gives the magnitudes as 4.56 and 4.60 and the negative value of (B-V) for both stars confirm that these are hot blue dwarf stars probably around spectral type B5. The revised parallax is 7.33 mas putting them at a distance of about 445 light years. A subsidiary note in the WDS says that both stars are spectroscopic binaries.

  Eta CrB (15 23 12.23 +30 17 17.7) is one of William Herschel's most important discoveries and one of the shortest period visual binaries visible in a small telescope. A recent orbit gives the period as 41.556 years with an error of 5 days so the pair has made five revolutions since discovery. The tilt of the apparent orbit means that the pair will stay rather close for a number of years. From the current separation of 0".56 the stars widen to 0".67 in 2014 before closing to 0".38 in 2020 and then widening to 1".0 in 2032.

  Eta CrB is also a relatively nearby system with a revised Hipparcos parallax of 55.72 mas which equates to 58.5 light years. The WDS gives the magnitudes as 5.64 and 5.95 and spectral types as FOV and GOV so that the stars appear yellow to the visual observer. There are three comites listed in the WDS, the first two, called C and D are field stars, but star E is a physically connected L8 dwarf some196 arc seconds distant from AB (equivalent to 3600 Astronomical units) and apparent visual magnitude 17. Imaging this star would be an interesting project for the well-equipped CCD astronomer.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - April 2009

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  25 CVn = STF1768 (13 37 27.70 +36 17 41.4) is a beautiful, unequal pair situated about 11 degrees south of M51 in Ursa Major. It was missed by the elder Herschel and found by Struve at Dorpat in 1827. The eccentric nature of its apparent orbit(e = 0.80) was such that it disappeared from view to all observers between 1859 and 1876. The current orbit by Soderhjelm gives a period of 228 years and predicts a separation of 0.2 arc seconds for 1864. The star is currently just starting another run into periastron so it is well placed for observers with small telescopes. The magnitudes are 5.0 and 7.0 and the position for 2009 is 97° 1".7. The revised Hipparcos parallax is 16.45 mas putting it at a distance of 61 parsecs. The primary is an A7 subgiant and there are few colour estimates in the literature. Webb makes them white and blue, as does Chambers is his revision of Smyth. The original Bedford catalogue does not contain an entry for this star.

  beta Hya = HJ 4478 (11 52 54.56 -33 54 29.3) was found by the younger Herschel during one of his sweeps at the Cape in 1834. The star is given as both beta Crateris and beta Hydrae in Herschel's 1847 volume and beta Crateris in Chambers version of Smyths Bedford catalogue from 1881. This pair is now a challenge for the 20-cm telescope and it likely that it will be many years before it opens up again so it is worth making an effort to see it if possible whilst it is well-placed in the sky for the southern observer. Hartung gives the colours of both stars as pale yellow but the WDS lists the spectrum of the primary as B9III. The separation has decreased from 1".7 at discovery to about 0".7 now and the position angle is increasing. magnitudes are 4.7 and 5.5 and the distance is 95 parsecs.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - March 2009

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Gamma Leo = STF1424 (10 19 58.1 +19 50 30.7) is one of the finest double stars in the sky in any telescope, with its components of visual brightness 2.37 and 3.64. It was found on Feb 11, 1782 by William Herschel using a 7-foot reflector with magnifications ranging from 227 to 6652 when, not surprisingly, he says `I had but a single glimpse of the star quite disfigured'. Herschel thought the brighter star white whilst the smaller was `..white inclining a little to pale red'. The WDS catalogue gives the spectral types as KOIII and G5III so many modern observers see yellow in both stars (viz. Hartung). Smyth found bright orange and greenish yellow whilst Webb noted gold and greenish-red. The early micrometer measures seemed to indicate relative rectilinear motion but Burnham pointed out that the considerable annual proper motion of about 0".4 per year was shared by both stars and therefore the motion was definitely orbital. Since Herschel's first measure of 84° 3".0 in 1782 the position angle has increased by some 42 degrees and the separation is now near 4".6. Orbits are bound to be preliminary and the one currently in the catalogue gives a period of 510 years, predicting closest separation of 1".1 in 1724, too close for any instruments of the time to resolve, and increasing to 4".6 in 2030. The revised Hipparcos parallax is 25 mas, which translates to 130 light years with an uncertainty of about 2.5 light years. In the same low-power field, some 6' distant, is AD Leo, a flare star which is a close binary. The companion was first noted in 1943 thanks to the astrometric perturbation on the primary star and has been detected at a wavelength of 750 nm. The period is about 27 years.

  Near the south pole, delta1 Cha (10 45 16.38 -80 28 10.3) is a slow-moving binary, which is a good test for 12-cm aperture and was discovered by Robert Innes in 1898. Motion is direct with the 1898 position of 60° 0".6 increasing to 85° 0".8 when it was last measured in 1996. According to the WDS, the stars are of mags 6.15 and 6.49. There is interest here too for the binocular observer with the presence of the mag 4.5 delta¹ some 6 arc min distant. Interestingly, Hipparcos gives the same parallax within the errors for both delta¹ and delta² (about 9 mas or 360 light years) but they have significantly different proper motions. Hartung makes delta¹ and delta² white and deep yellow whilst Sissy Haas notes that Ross Gould makes them pale yellow and deep yellow respectively.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - February 2009

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  STF1338 (09 20 59.4 +38 11 17.9) is a beautiful binary about 1.5 degrees north of the 4th magnitude star 38 Lyncis which is also a pretty pair. Its almost equal components are given as mags 6.72 and 7.08 in the WDS which also notes a third faint star (mag 11.4) some 144" away in PA 166 degs. Hartung finds the colours both bright yellow whilst Smyth and Chambers in the Cycle of Celestial Objects second edition of 1881 give both stars to be white. The WDS perhaps favours the latter colours listing the spectral types as F2V and F4V. Orbital motion is slow and from the discovery position of 121 degrees, 1".76 the pair has advanced to 303 degrees, 1".01 in 2009 according to the 303 year period given in the USNO 6th Orbit Catalogue. This pair is 42 parsecs distant according to Hipparcos and is an easy object in a small telescope at all times. It reached a maximum distance of 1".65 in 1865 and closest approach will be about 0".96 in 2044.

  Gamma Volantis (or Piscis Volantis) (07 08 44.82 -70 29 57.1) is a showcase pair according to Sissy Haas in her book and an observation by Ross Gould with 35-cm records the colours as deep yellow and dull yellow. With magnitudes 3.86 and 5.43 this is clearly one of the sky's most spectacular pairs but Smyth and Chambers merely note: 'A double star'. Hartung gives bright golden and pale yellow for the stars whose spectral types are KOIII and F2V. The discoverer was Dunlop and it is number 42 in his catalogue. Since 1826 there has been little motion in either PA or separation - possibly a slight closing but at 14".4 this is a pair for the small telescope or stabilized binoculars. The bright star is some 53 parsecs away.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - January 2009

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  This months column features two stars which are, nominally at least, the 7^th brightest in their respective constellations. Theta Aurigae = STT 545 (05 59 43.24 +37 12 46.0) is one of Otto Struve's discoveries at Pulkovo using the 15-inch refractor in 1852 but there are no measures on record until 1871. Orbital motion is slow but retrograde and covers some 60 degrees over the last 130 years. The separation has very slowly increased from 2".1 to the present 3".8. Whilst this may appear to present no problems to the small telescope user, the difference of magnitude certainly does and this system is one of the classic tests for the small aperture. The WDS gives the magnitudes as 2.6 and 7.2 so its not just the brightness difference but the glare from the primary star that has to be dealt with. However, it is the case that small apertures tend to work better on this star than larger ones as the quality of the atmosphere is also an important consideration when observing it. The writer finds it difficult to measure in an 8-inch refractor when the red field illumination of the micrometer is switched on. Theta Aurigae is about 165 light years from the Sun.

  Theta Pictoris = DUN 20 (05 24 46.29 -52 18 58.2) was observed by John Herschel in 1835 and noted to be a `fine' pair. It had been discovered by Dunlop in 1826 and whilst Hartung refers to the two components (visual magnitudes 6.24 and 6.74) as pale yellow, the WDS catalogue lists the spectral types as A0V and A2V. In the last 180 years there has been virtually no relative motion between the two stars and the system is ideal for use as a calibrator for filar or eyepiece micrometers (2010.0: 287.6, 38".14). In 1901 Robert Innes found that A was itself a close pair and the system is a difficult one for any but the largest visual telescopes with the current separation not exceeding 0.2 arc seconds until 2010. It then widens to about 0".46 in 2079, the period being 191 years. There are reasons to believe that theta Pic is in fact a quintuple system. One of the stars in the close pair is a spectroscopic binary and the distant companion is also suspected of variable radial velocity. The group is remote with the revised Hipparcos parallax giving the distance as 512 light years with an uncertainty of 25 light years.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - December 2008

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  zeta Per = STF 464 (03 54 07.92 +31 53 01.2) is a rare animal - a supergiant in a visual binary system. The star was thought to be a member of the Perseus OB2 association but the recently revised Hipparcos parallax puts it around 230 parsecs away, significantly closer than the association. The WDS notes 3 other companions two of which appear by dint of differing proper motions, to be optical but Kaler thinks that both B and E are physical stars. The early type of the primary (B1Ib) and star B (B8V) result in the system appearing white to the small telescope user but Smyth says `flushed white and small blue' whilst Webb says `green white and ash' and Haas has `banana-yellow and blue'. The position of B has changed little since it was measured by Struve in 1824 and is currently near 209 degrees and 12".9. The magnitudes of A and B are 2.85 and 9.16 making it not particularly easy for a small aperture. The WDS notes that A is a spectroscopic binary but no orbit appears to have been calculated for it and a paper in 2003 shows no variation in radial velocity. Both stars appear white to the writer.

  f Eri = Dun 16 (03 48 35.82 -37 37 12.5) `Superb double star but ill defined' wrote John Herschel in 1847 commenting on his observations made 11 years earlier. Hartung gives both stars as pale yellow whilst the WDS spectral types of B9V and A1V seem at odds with this assessment. The stars are magnitudes 4.72 and 5.25 and the separation has increased from 7".0 in 1826 to 8".4 in 2002 with the angle increasing from 202 to 218 degrees in the same period. This system is some 185 light years distant and the proper motion is sufficiently large that over 200 years the stars would have separated by about 15 arc seconds if they were not binary in nature. `Showcase pair' says Sissie Haas in her book.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - November 2008

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  This month's pairs are just visible to the naked eye - each being V=5.6

  65 Psc (00 49 53.1 +27 42 37.1) was found by William Herschel in 1783 and is one of the showcase pairs listed by Sissy Haas in her publication Double Stars for Small Telescopes. She finds the two stars to be citrus orange in colour with a 60-mm refractor; Webb found them yellowish and Smyth says both are pale yellow. This would accord with the spectral types which are given variously as gF0 and gF2 (Burnham) and F4III and F5III in Hartung. The system is 273 light-years away and since the first measure the position angle has decreased only 5 degrees with the separation increasing from 4".0 to 4".3. Clearly it is a binary of very long period as the proper motion of almost 0".1 per year for the primary would have separated the stars by 20" today if they were unrelated.

  BU 395 in Cetus (00 37 19.79 -24 46 02.0) is one of the most interesting of Burnham's discoveries. Found using the 6-inch Clark in 1875, it has turned out to be a short period system. The period is 25.09 years and the orbital plane is highly inclined so that the separation varies from 0".17 in 2006 to 0".77 in 2015. At present (2009.0) the stars are separated by 0".37 affording those with 30-cm aperture the chance to test the resolving power of their telescope.

  By 2010.0 the pair widens to 0".48. The star has also been observed as a double-lined spectroscopic binary with the spectral types given as G8V and G9V. Hipparcos places the system some 50 light-years away and the proper motion carries it about 1".5 annually almost exactly east-west across the line of sight.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - October 2008

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  This month's pairs ar both bright, well-observed binaries and well seen in small to medium apertures.

  36 And (00 54 58.02 +23 37 42.4) is a beautiful pair following the Square of Pegasus, some 3 degrees south following zeta Andromedae. The two stars are strong yellow (Webb) or golden (Smyth) and form a binary system whose period is 167.7 years so it has passed the position it occupied when discovered by F G W Struve in 1827. The current PA and separation are 322 degrees and 1".1 making it a fine sight in a 15-cm telescope. It is strangely absent from Hartung's book, which includes more northerly objects of less distinction such as AC 1. This system is almost 38 pc distant and the primary is a K1 subgiant. The star itself is just visible to the naked-eye with the components being magnitudes 6.1 and 6.5.

  p Eri (01 39 47.24 -56 11 47.2) is one of Dunlop's discoveries (Dun 5) and is probably the nearest equivalent to 61 Cygni in the southern hemisphere. It is close (5.3 parsecs according to Hipparcos), it contains two K dwarfs (in this case K0 and K5) which have visual magnitudes 5.8 and 5.9 respectively, has a similarly long period (483 years) and is also well-separated - reaching a maximum distance of 11.8 arc seconds in 2040. The Chambers edition of Smyth's Celestial Objects which contains a southern extension, also identifies the star as 6 Eri but does not mention colours, neither does John Herschel in his Cape observations. Hartung records both as deep yellow.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - September 2008

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  About 2.5 degrees north of the Dumbbell nebula M27 and a little proceeding is 16 Vul = STT 395 (20 02 01.37 +24 56 16.3) , one of Otto Struve's discoveries at Pulkova and when first found in 1843 it was a difficult pair at 89 dgs and 0".5. In the intervening period is had almost doubled in separation and the position angle has increased to 125 degrees so that a 15-cm telescope should resolve this beautiful pair. Hartung notes that both stars are yellow. This is a relatively distant system - the Hipparcos revised parallax is 14.55 mas with an uncertainty of 0.50 mas, putting it at a distance of 69 parsecs. It is certainly a binary system of long period as the significant proper motion testifies.

  The spectral types of 21 Sgr (18 25 21.04 -20 32 29.8) are given as A + K2III in the WDS and the magnitude difference is 2.4 in the visual. It is difficult to reconcile the apparent colours seen in this beautiful pair which are orange and greenish according to Hartung with these spectral types. The writer also recently viewed this pair in the 26.5-inch refractor at Johannesburg and the similarity with Antares was immediately apparent - the companion is definitely greenish. The Hipparcos B-V value of +1.3 certainly suggests that the light of the system is dominated by the K giant star and that the spectral types in the catalogue should be reversed. The revised parallax puts the primary at 126 parsecs. There is slow retrograde motion with the discovery position by Jacob from 1846 showing 297 dgs and 1".8 whilst a recent measure put the companion at 280 dgs and 1".7.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - August 2008

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  95 Herculis (18 01 30.40 +21 35 44.5) is a double star much beloved of the Victorian observers, due to the suspicion that the colours of the components changed over a period of years. Hartung records pale and deep yellow, a conclusion agreed with by Frew and Malin in their revision of his book. Chambers, in his revised version of Smyth's `Celestial Cycle' notes that a friend of Smyth, a Mr. Higgens of Bedford, claimed that the intensity of the green and red colours of A and B varied from time to time and that the green star recovered its hue first. `On this statement being submitted to Sir G. B. Airy he did not view it with favour'.

  It seems likely that this pair is binary - the significant proper motion in dec of A (0.039 arc seconds per year) would have carried it 9 arc seconds away from B over 230 years, whilst the separation has reduced from 9".0 in 1777 to 6".3 in 2007 with a small decrease in position angle. The revised Hipparcos parallax puts the A5 giant primary at 123 parsecs. This is one of the finest pairs in the northern sky for small telescopes.

  The Australian amateur Walter Gale has his name on three stars in the WDS catalogue. Gle 3 was described in August 2007 notes and Gle 2 is xi Pavonis, (18 23 13.62 -61 29 38.1) a bright yellow giant K4 star. The revised Hipparcos parallax puts the star at 143 parsecs and the relative position of the companion has changed from 140°, 4".0 in 1894 to 156°, 3".4 in 1988. Hartung gives the colour of the companion, some 3.7 magnitudes fainter than the V=4.4 primary, as white.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - July 2008

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  At this time of year the constellations of Hercules and Ophiuchus straddle the northern hemisphere meridian in the evening. Ophiuchus contains one of the finest binaries in the sky in 70 Oph (18 05 27.21 +02 30 08.8) - a pair with a period of 88 years and a long history of observation. Its proximity to the Sun means that the apparent orbit is large and the two components can be seen in small telescopes at any point in the orbital cycle. The last periastron occurred in 1991 and the telescopic distance between the stars has more than

  tripled since then. In mid-2008 the apprent position of B is 132.9 and 5".50. The writer has been following this pair every year since 1990 since when the position angle has decreased from 217 to 134 degrees. Early attempts to compute the orbit led to suggestions that there was a third body in the system. Later and more accurate measurements, along with substantial radial velocity investigations, have not shown any evidence for this idea.

  Herschel found the pair in 1779 and called it H II 4. It is number 2272 in Struve's Dorpat catalogue. Hipparcos gives the distance as 5.1 pc and the magnitudes are 4.22 and 6.17. The colours are particularly splendid, the spectral types being K0V and K4V.

  Triangulum Australe crosses the southern meridian about an hour after alpha Centauri on July nights. The brightest stars are magnitudes 1.9, 2.8 and 2.9. About 3 degrees north of beta is Dunlop 194 (15 54 52.64 -60 44 37.1), two stars of magnitude 6.4 and 10.0 separated by about 44 arc seconds and having changed little since John Herschel's measures from Feldhausen. R. P. Sellors using an 11-inch refractor at Sydney found that A was a closer double, the 8.1 mag companion being located about 0.6 arc seconds E of the primary. In this system too there has been little change since discovery. A is a luminous B star which Hipparcos places 500 parsecs away with an uncertainty of perhaps half this distance.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - June 2008

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  The two systems this month are both bright and relatively easy objects to see in small telescopes now but both have highly eccentric orbits resulting in large ranges of apparent separation during the orbital cycle.

  The apparent orbit of 44 Boötis (STF 1909 - 15 03 47.68 +47 39 14.5) is a very elongated ellipse with the primary star somewhat nearer the eastern end than the western end. At

  present the companion is perched at the eastern end of the ellipse having been virtually stationary for the last few years. It will soon be closing noticeably and accelerating to pass by the primary at a distance of 0.20 arc seconds in 2019. At this point the angular motion will be very pronounced - more than 1 degree per week. The pair was discovered by Herschel in 1781 at PA 60.1 degrees but he made no note of the separation. A measure made a few weeks ago (i.e. early July 2008) showed the companion to be at precisely 60.0 degrees.

  The current orbit by Soderhjelm gives a period of 206 years. The stars are F7 and K4 dwarfs and the visual magnitudes 5.20 and 6.10. The secondary is one of the brightest W UMa eclipsing systems known with a period of 6 hours.

  xi Sco (16 04 21.63 -11 22 24.8) was found to be triple by Herschel the year after he found 44 Boo. The wide pair was separated by about 6.7 arc seconds and since then the angle has decreased 50 degrees and the separation has increased somewhat. Herschel gave no separation for AB - although his measure for PA put the pair at 188 degrees. In Lewis' book (1906) several orbits were listed, all of them with a period near 100 years and very low eccentricity. However, the equality in brightness of the two stars led to differences of 180 degrees in some position angles and it was left to Aitken to show that the period was nearer 45 years and was highly eccentric. The currently accepted period is 45.9 years and the is pair is now approaching maximum distance (1.13 arc seconds in 2021) so this is a good chance to see it with 15-cm aperture. xi Sco is missing from the Hipparcos catalogue but it is still possible to estimate its distance because it shares a common proper motion and radial velocity with the 12 arc second pair STF1999 some 5 arc minutes SE so that the whole system is quintuple. STF1999 has a parallax of 33 mas but with a significant uncertainty. The WDS gives magnitudes of 5.16, 4.87, and 7.3 for the three stars of xi Sco.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - May 2008

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Both binaries in this months notes had orbits calculated for them about 20 years ago by Wulff Heintz, a great visual observer who died on June 10, 2006 after more than 50 years of observational activity. The orbits remain in the catalogue.

  Located in the western part of Boötes, STF1785 (13 49 0.28 +26 58 48.5) is a rather faint (mags 7.4 and 8.1) but attractive pair of orange K-type dwarf stars, discovered by Sir James South in 1823. The system is located fairly near the Sun at a distance of about 45 light years. The predicted position for mid-2008 is 179o.9, 3".13 but recent measures by the writer seem to indicate that the position angle is about 3 degrees smaller than this possibly suggesting that the 155.75 year period is a little short.

  Lupus is a bright constellation located halfway between the head of Scorpio and alpha and beta Centauri and it contains some beautiful pairs for telescopes of all apertures. Gamma Lupi, (15 35 08.46 -41 10 00.1) a brilliant white binary whose primary is a distant B subgiant has a highly inclined orbit of 190 years period and at times of closest approach the two

  components are only 0.07 arc seconds apart as happened in 1930. The stars of apparent magnitude 2.95 and 4.45 are currently almost at maximum separation (0".83 in 2013) so see them whilst you can. Fortunately this pair was near widest separation when found by John Herschel from the Cape in 1835.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - April 2008

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  The binary stars being highlighted this month have several common traits - they are both bright and somewhat unequal in magnitude and can be easily seen in small telescopes. But whereas xi UMa has been measured almost 1700 times beta Muscae, although no less attractive an object, has but 78 measures in the WDS reflecting the concentration of effort on binaries in the northern hemisphere.

  xi UMa (11 18 11.24 +31 31 50.8) is one of the best-known systems in the northern sky. Found by William Herschel in 1780, it became the first pair to submit to the science of orbital analysis by Savary in 1828. Later on, first A and then B were found to be spectroscopic binaries with periods of 1.83 years and 3.98 days respectively and about 20 years ago, speckle observers noted indications of a 5th component, attached to B. This is clearly a very difficult object as it has not been seen since 1994. The multiplicity of xi has clearly caused problems with Hipparcos as the system is missing from the Hipparcos catalogue. To the small telescope the stars appear yellowish orange and the 59.9 year orbit is now currently taking the stars further apart. In 2008, B can be found 1.63 arc seconds distant from A in PA 223°.

  beta Muscae = R 207 (12 46 16.87 -68 06 29.1) was discovered by Russell in Sydney in 1880 when the position angle was 317° and the separation 0.54 arc seconds. Since then it has been closing again and in 2008 can be found at 48° and 1.27 arc seconds according to the 383 year orbit calculated by R. R. de Freitas Mourao in 1964. The stars are both white with the primary star being an early B-type dwarf. Hipparcos puts the distance at 340 light years whilst the WDS gives magnitudes of 3.52, 3.98, both some 0.4 magnitudes fainter than the V magnitudes given in Hipparcos.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - March 2008

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  The two stars this month are both relatively difficult pairs to see. They have periods of over 100 years but the apparent separations vary quite widely throughout the complete orbit.

  omega Leo (09 28 27.4 +09 03 24) William Herschel found this pair in 1782 and catalogued it as number 26 in his first class of double stars. He gave it a position angle of 110.9 degrees and estimated the separation at 1". By the time that F. G. W. Struve observed the star in 1825 the star had advanced 43 degrees in angle with unchanged distance. By 1838 Struve could only elongate it and the modern orbit of 118.227 years by van Dessel predicts a separation of 0".71. In 2008 the star is almost back to its discovery position so here is a chance to see it as Herschel would have done. The magnitudes are 5.69 and 7.28 which add considerably to the difficulty of measuring it, and the revised Hipparcos parallax is 30.12 mas with an uncertainly of 0.71 mas.

  delta Vel (08 44 44.2 -54 42 31) When Robert Innes lived in Sydney at the end of the 19th century, he used a small refractor in a search for new double stars. One of the first was delta Velorum, a mag 1.9 star which turned out to have a 5th magnitude companion at a distance of about 2" and a PA at about 170 degrees. In fact, delta Argus (as it was then) was first found by Solon Bailey in Arequipa, Peru in 1894 using the 13-inch Harvard refractor but Innes was first into print and thus gained priority. The star closed slowly until the early 1950's after which there were no observations until Hipparcos in 1991 with exception of one observation in 1978 which it is now believed is of the Innes companion but at first was thought to be a 3rd component. The Hipparcos observations showed the pair at 0".7 and widening, having been close in the 1980's. An orbit by Andreas Alzner with the benefit of a speckle measure made in 1999 showed the companion swinging around the end of its long apparent ellipse and heading back for its discovery position. The period is 142 years and in 2008 it will be at 319 degrees and 0".66, a difficult object for a 30-cm telescope. It is also now known to be the brightest eclipsing binary in the sky, a 45.16 day period with a primary dip of about 0.4 mags having been found by Sebastian Otero in 1997. A faint John Herschel pair at a distance of 69 arc seconds shares the proper motion of delta so this is a quintuple system some 25 parsecs from us.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - February 2008

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  This month the two binary systems in question both have late-type giant stars as primaries.

  In 1882, using the 12-inch refractor on Mount Hamilton, Burnham found that eta Gem (06 15 52.70 +22 30 24.6) was double and it eventually became number 1008 in his catalogue. In several cases where he discovered very faint and close companions, Burnham tended to underestimate the brightness of the companion. When, for instance, he found alpha UMa to be double (BU 1077) he gave the magnitudes as 2.0 and 11.1 whilst the modern values for A and B, as found by Hipparcos are 1.95 and 4.87. In the case of eta Gem, he estimated A and B to be 3.0 and 8.8 - again Hipparcos notes that B is considerably less faint than Burnham's estimate and gives V equivalent magnitudes of 3.3 and 6.0 with the companion at a distance of 1".1. Since then the position angle has reduced by about 40 degrees and the separation has increased to 1".5. Even in 1961, Hartung was able to see it with 10.5-cm aperture and these days it is somewhat wider than that. The primary, spectral type M3.5I-II is orange but no colour estimate of B is noted.

  In the 1880s, astronomers at Sydney Observatory were busy looking at the double stars, including re-observation of those of John Herschel using an 11.4-inch refractor. Under the directorship of H. C. Russell, R. P. Sellors was one of the observers. The WDS shows 24 pairs under his name, the first and brightest of which is beta Phoenicis - a bright, close binary. The second brightest pair is SLR 8, located in Vela at (08 32 04.97 -53 12 43.1). Consisting of stars of magnitudes 6.13 and 7.08 this pair was separated by 0".4 in 1892, widened to about 1" in 1925 and is now closing again. An aperture of 15-cm should show it but there have been no measures since 1991 - an indication of how the southern pairs continue to be neglected. The colours are orange-yellow and whitish, reflecting the spectral types of KOIII and A3. This system is 227 parsces distant according to the revised Hipparcos parallax.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - January 2008

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  The two binary stars being highlighted this month have in common long orbital periods but differ in other respects. Whilst 14 Ori has a relatively circular, face-on orbit, the orbit of HJ 3683 is both highly inclined and highly eccentric.

  14 Ori = STT98 (05 07 52.87 +08 29 54.9) This is one of Otto Struve's discoveries with the 15-inch refractor at Pulkova. The stars are magnitude 5.76 and 6.67 and both appear white to the writer but Hartung saw them as pale and deep yellow. The primary is of spectral type Am. The orbital period of 198 years sees the stars range in separation from 0.7 to 1.1 arc seconds so they can be seen with small to moderate apertures on most occasions, although the quoted magnitude difference of 0.9 always seems a little optimistic and good seeing is essential to see them well from the latitude of the UK. The writer found the pair at 305°.9, 0".90 in late 2006. Hartung points out that the fainter pair STF643 some 6 arc minutes south has the same proper motion.

  HJ 3683 (04 40 17.72 -58 56 39.6) was picked up by John Herschel in sweep 518 with his 20-foot reflector and he noted the pair as `very fine' and noted them as equally bright on two occasions whilst the WDS gives the magnitudes as 7.33 and 7.45. At the time of discovery the separation was about 3.5 arc seconds but the pair began to close and when Innes observed it in 1922, the star was single. At the last measurement recorded in the WDS for 2002 the pair appeared close to its discovery position. This is a system very similar to gamma Virginis but with an even more eccentric orbit (e = 0.95) and longer period (326 years). At periastron in 1918 the angular separation was 0.03 arc seconds and the angular velocity 1 degree per day. This pair of G dwarfs is 31.2 parsecs distant according to the revised parallax calculated by Floor van Leeuwen in his book `Hipparcos, the New Reduction of the Raw Data' issued by Springer (2007).

  Bob Argyle - Double Star Section Director

• Double Star of the Month - December 2007

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Epsilon Per (03 57 51.2 +40 00 36) With the newly invigorated Comet Holmes causing a stir in observational circles, this month's northern double star is in Perseus where the comet has spent much of the last few months. Epsilon is located in the east of the constellation about 15 degrees following Algol. Coincidentally, its primary star - a hot B dwarf is also an eclipsing system in the Algol class. Almost due north, about 9 arc seconds distant is the companion, an A2 dwarf and so it might be expected that both stars would appear white in a small telescope. Hartung, however, notes star B to be slate gray whilst Smyth in 'Celestial Cycle' records that it is lilac. The latter also informs the reader that John Herschel suggested putting a piece of paper at the centre of the objective, (Smyth used a 2-inch stop on his 5.9-inch refractor), in order better to see the faint companion which is 6 magnitudes fainter than A. This is a relatively distant system - the revised Hipparcos catalogue, produced recently by Dr. Floor van Leeuwen gives a parallax of 5.12 +/- 0.22 mas corresponding to 630 light years.

  Jc 8 (03 12 25.7 -44 25 11) In 1835 John Herschel found a close double star at this position and gave it his catalogue number 3556. He had trouble in making out that it was wedge-shaped and estimated the position as 230 degrees+/- and 1.5 arc seconds. In March 1856 Captain W. S. Jacob in Madras, whilst making measures of southern pairs with his 6.3-inch refractor made by Lerebours and Secretan, suspected the A component of being double in the S direction. This has turned out to be a rapid visual binary with a period of 45.2 years with the separation varying from 0.04 to 0.75 arc seconds. Currently the separation of AB is 0".70 and the pair is beginning to close again, whilst that of AB-C has widened to 3".7 making the whole group an attractive sight in a 20-cm telescope or bigger.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - November 2007

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  The two pairs featured this month are both at rather high declinations and hence can be seen for some time either side of this month. Both are unequally bright but easy objects in small telescopes.

  Eta Cas = STF 60 (00 49 05.10 +57 48 59.6) Found by William Herschel in August 1779, this beautiful contrasting pair of GO and M0 dwarf stars has been measured on more than one thousand occasions since, and as early as 1906 Thomas Lewis had said that ``in all probability the period does not exceed 233 years". He was wrong - the retrograde motion to date amounts to 250 degrees in 225 years - close on a degree a year - and as the pair is still widening it seems likely that the 480 year period found by Strand in 1969 is much closer to the mark. For 2008.0 the orbit predicts 320.6 degrees and 13.13 arc seconds. The V magnitudes of the stars are 3.52 and 7.36 and at a distance of 5.95 parsecs it is one of the nearest visual binaries to the Sun. The proper motion is 1.22 arc seconds per year and the WDS lists 7 faint field stars within 700 arc seconds, whose distances are changing rapidly as this system speeds past them. Webb and Smyth found the companion purple whilst Sissy Haas notes almond brown.

  Zeta Phe - Rmk 2 AC (01 08 23.06 -55 14 45.0) is pair of late B dwarf stars which appear white to observers. Since 1835 the pair has widened from 4 to 6.8 arc seconds whilst the position angle is little changed. The distance would have increased substantially more if the pair were an optical one but at a distance of 85 parsecs this will be a very long period system. For the serious double star observer, there is interest in the faint, close companion to A discovered by Robert Rossiter at Bloemfontein in Dec 1931, and numbered 1205 in the Rst catalogue. This is a binary of about 350 years period as it appears to be moving at about 1 degree a year with the separation fixed at 0.6 arc seconds. The magnitudes of A and B are 4.02 and 6.80 so the pair should be visible in a 30-cm telescope on a good night. It turns out that A is also an Algol system with a period of 1.67 days.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - October 2007

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  STF 3062 (0 06 15.54 +58 26 12.1) is a star on the limit of naked-eye visibility, the south-easternmost one of a pair some 10 arc mins apart in a low power field to the SE of, and including, beta Cas. Its duplicity was noted by the elder Herschel on 1782 May 25 and it is catalogued as H I 39. The parallax is 49.30 +/- 1.05 mas and the proper motion approaches 0.25 arc seconds per year. The Millennium Star Atlas indicates that the star is also V640 Cas but the explanation for variability in the Hipparcos catalogue is `duplicity possibly causing spurious variability'. In fact, Griffin shows that the Hipparcos satellite does not confirm the period or amplitude found in 1983 when the star was claimed to be an eclipsing binary and, in consequence, that there should be no variable star designation at all. The fainter visual component is however a spectroscopic binary of 47 day period. The visual pair have almost completed 2 revolutions since F G W Struve measured the pair in the 1820's. The separation at present is just over 1.5 arc seconds with apastron being reached in a few years time and the angle is increasing by about 2 degrees a year, so a 20-cm telescope will show the two white stars very clearly.

  beta Tucanae (0 31 32.56 -62 57 29.1) The two brightest components of beta Tucanae are currently about 27 arc seconds apart and in the WDS catalogue this pair has the designation LCL 119. At magnitudes 4.33 and 4.53 they form one of the most splendid double stars visible to binoculars or telescopes in the sky. Although the corresponding Hipparcos parallaxes are 23.95 and 18.35 mas the formal error on the latter star is 3.34 mas so it might be argued that the stars are a physical system. Certainly the proper motions are similar and large enough that taken in conjunction with the small change in relative position since 1826 the two stars are moving through space together. This proper motion is shared by a third star of mag 5.1 (beta³) some 5 arc minutes away so that to the small telescope user, this is a beautiful triple system. Bring a powerful telescope, such as the 26.5-inch refractor at the Union Observatory in Johannesburg to bear on the group, as W. H. van den Bos did in 1925, and further stars appear. Robert Innes, using the same telescope which now bears his name, had already found that beta² was a very close and unequal double star which turns out to be a binary of period 44.7 years. The current separation is 0.40 arc second and closing. van den Bos added companions to both beta¹ and beta³, now B 7 and B 8 in the WDS. The companion of B 7, some 10 magnitudes fainter than the primary at a distance of only 2 arc seconds must be a formidably difficult star to see. B 8 is pair of 6th magnitude stars separated by little more than 0.1 arc second. Little is known about this latter pair - it has not been measured since 1964.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - September 2007

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  61 Cygni (21 06 50.84 +38 44 29.4) Although observed by Flamsteed in 1690, the duplicity of this star seems to have been first noted by Bradley in 1753. A recent orbit by Kiseleva indicates that the separation of the two stars would have been around 10 arc seconds for 1690 - certainly within Flamsteed's resolving power. He did, for instance, see the third star in the zeta Cancri system in1680 when it was 6 arc seconds from AB.

  Piazzi discovered the large common proper motion in 1806, and Bessel then made his pioneering measurement of the parallax of 61 Cyg A indicating that it was nearby. Even so, as late as 1891, Burnham was not convinced that the pair was a binary one, being discouraged by the large separation which amounted to 21 arc secs at that time. At present this has widened to almost 31 seconds and, according to Kiseleva, reaches 34.3 secs in 2106 before closing to a minimum of 9 arc seconds around 2350. Long series of astrometric plates taken during the last century led to speculation that star A was accompanied by a planetary mass companion in a short period but this has not yet been confirmed.

  The pair is a very attractive one for the small telescope user. The stars are both late K dwarf stars and hence orange in hue, although Hartung gives colours of orange and red. Hipparcos finds parallaxes 287.1 and 285.4 milliarcsecond for A and B with errors of 1.5 and 0.7 mas respectively.

  theta Indi (HJ 5258) (21 19 51.9 -53 26 57.4) John Herschel remarked `Beautiful' as he recorded this pair found with the 20-foot reflector at Feldhausen on sweep 468 dated 1834 Jul 8. He gives the magnitudes as 6 and 10 and notes that it is number 7003 in the Brisbane catalogue. He later made four measures of position angle and two of separation to give a weighted mean measure for 1834.51 of 307°.0, 3".67. Modern catalogues such as the WDS list the magnitude as 4.5 and 6.9, a significant difference in the brightness of star B, but the distance has now almost doubled and it may be this is why the companion looks brighter in recent years.

  There is no doubt that this is a physical system as the proper motion, common to both stars is 0.13 arc seconds per year and the parallax is 33.58+/- 0.76 mas equivalent to a distance of 97 light years. Hartung found the stars were pale yellow and distinctly reddish.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - August 2007

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  beta Cygni (19 30 40.29 +27 57 34.9) One of the most famous and beautiful pairs in the sky has been a favourite for small telescopes for many years but the use of modern imaging techniques has confirmed that this is a multiple system, possibly quintuple. The bright pair was certainly noted by Flamsteed in June 1691. Spectroscopy showed the primary to have a composite spectrum, a K giant combined with a late B star, whilst the wide visual companion is a B8 dwarf. This difference in spectral type explains the marked contrast in colours between the stars. Exact shades depend upon the individual but the Victorians called them topaz and sapphire whilst modern descriptions tend towards yellow and blue. Whichever applies, the pair is a magnificent sight in binoculars. The primary component was resolved in 1976 by Harold McAlister using speckle interferometry, and subsequently seen visually by Charles Worley with the 26-inch refractor at Washington. Aa is a difficult pair, since the visual magnitude difference is about 2. A recent orbit by Marco Scardia and colleagues gives a period of 213 years and the current separation is 0".37. This pair has been resolved from the UK by Christopher Taylor with a 12.5-inch Calver reflector.

  In 1980 another component, closer in than a was reported by Bonneau and Foy and confirmed about a decade later but has not been seen since. There is little doubt that A and B form a very long period binary system. The Hipparcos parallaxes agree within the errors of both, placing the stars about 118 parsecs away, whilst the proper motions are similar. In 2007 February, a paper published by Roberts et al reports the presence of a faint companion to B some 4 magnitudes fainter in the I band which may be a G dwarf.

  Gale 3 (19 17 12.22 - 61 39 39.7) is a bright, relatively close naked-eye star in the constellation of Pavo. It consists of two white stars of spectral types A5 and A8. The period of the pair is 156.7 years and in mid-2007 the position angle is 340 degrees and the separation 0".51 making it a good test for a 10-inch telescope. The separation increases slowly to 0".56 over the next 50 years and then closes down to 0".15 one hundred years from now.

  This is one of five pairs that Walter F. Gale (1865-1945) found with an 8.5-inch With reflector from New South Wales. Gale noted the pairs in 1894 and this list appears in Astronomische Nachrichten (AN 143, 293, 1897). However, R. T. A. Innes also found two of the stars independently but somewhat later in 1894, and acknowledges Gale's contribution in his paper - `detected by Mr. Gale on his 8.5-inch, previously to my seeing them '. Since Innes published his list first (in MN 55, 312, 1895) the pairs are given the catalogue letter I rather than GLE. Ironically, Innes was using a 6.25-inch Cooke refractor of 1851 which he had borrowed from Gale!

  Bob Argyle - Double Star Section Director

• Double Star of the Month - July 2007

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  The double stars selected for this month have a number of common features. Both are relatively close-by, both have short periods, and both are visible in small to medium-sized telescopes at the present time.

  zeta Herculis (16 41 17.48 +31 36 06.8) is a famous binary found by the elder Herschel on 1782 Jul 18. Although the separations at the closest and widest points in the apparent orbit are respectively 0".6 and 1".6, the large difference in magnitude, and the glare of the 3rd magnitude primary makes the system a difficult one to measure accurately.

  In his 1906 volume, Thomas Lewis devotes five pages to the orbit of this pair which he found could only be explained if the orbital period was changing with time. This seemed to imply the presence of a third component but no such star has been found and there is no evidence from the extensive radial velocity history of this star. Recent direct measurements of the diameter of star A, a G7 giant, show that it is some 2.5 times the diameter of the Sun. The distance from Hipparcos measurements is 35.2 light years and the V magnitudes are 2.9 and 5.6. The pair, whose orbit has a period of 34.45 years, is now widening and in 2007.5 the position will be 200°.2, 1".08.

  MLO 4 = BU 416 = R 298 (17 18 56.36 -34 59 22.5) is just visible to the naked-eye on a clear night and can be found sitting some 3 degrees north-west of lambda Sco in the tail of the Scorpion. It was found to be double by Burnham with the 6-inch Clark in 1876 and it appears as BU 416 in his 1906 General Catalogue. His estimates of the magnitudes were 6.0 and 8.5. The modern measures of the magnitude difference by Hipparcos are nearer 0.9. It was also found independently by Russell using the 11-inch refractor at Sydney. According to R. T. A. Innes however, the pair was `first noted at the Melbourne Observatory in 1867', hence its catalogue name.

  The distance to MLO 4 is 22.7 light-years, although, strangely, the quoted Hipparcos parallax error is almost 12%, with the annual proper motion exceeding 1 arc second. The period of the binary is 42.15 years, according to Soderhjelm in 1999 and for 2007.5 the position is 206°.0, 1".47.

  The orange hues of each component betray the late spectral types; they are dwarf stars of class K3 and K5. There is some evidence that a more distant member of the system (31") is a M-type star. It has similar proper motion to AB so is physically connected.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - June 2007

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  sigma CrB (16 14 41.04 +33 51 31.8) was discovered by William Herschel on 1780 August 7 and appears as the third entry in his class I pairs (separation 0 - 4"). The components passed within about 1".2 seconds of each other in 1830 and have been widening ever since. The maximum distance according to the 889 year orbit of Scardia occurs in 2245 at 9". Victorian observers suspected changes of colour and magnitude. Webb, in `The Intellectual Observer' for 1863 (page 134) offers the following summary: `Creamy-white and smalt-blue. There is much discrepancy about the smaller star. Struve calls it ``certainly not blue'', and differing very little from the other, 1825; Struve, white 1836.69; Dembowski, yellow, ashy, and doubtful blue, 1854 to 1857; Secchi, sometimes blue, sometimes yellow, 1855 to 1857. I fancied it, with a 3.7-inch object-glass, at one time ruddy, at another bluish, from 1850 to 1855, apparently changing even while being looked at; a versatility of hue which I have remarked on other stars similarly circumstanced, and which may possibly depend upon equal sensitiveness to colour in different conditions of the retina; during a short glimpse with 5.5-inches, 1862.57, the companion seemed bluish; at the same time I thought, as I had done in former years, that there was more than 0.5 mag of difference. Struve gave more than 1 mag. Secchi's discordances are considerable, ranging between 0.5 and 2 mags, from 1855 to 1857; but the honesty of that excellent observer, in recording every temporary impression, must be allowed for'. Small telescopes will show a fainter third star which does not belong to the system. In 1829 it was 44" distant from the main pair and last year measures put the distance at 90", due to the motion of nearby sigma (the Hipparcos parallax yields a distance of 21.68 parcsecs).

  mu Lupi (15 18 32.05 -47 52 30.7) This beautiful triple system can be well-seen in apertures of 10-cm and above. Strangely John Herschel does not mention the third component is any of his observations made with the 7-foot and 20-foot telescopes from Feldhausen, yet star C is mag 7.1 compared with the 5.1 and 5.2 of A and B. The close pair is clearly binary but the period is several hundred years since the motion in the last 180 years is 40 degrees retrograde whilst separation has halved to its current value of about 1".0. Hartung thought the distant star reddish. This star is also a physical member of the system since Hipparcos assigns it the same parallax as AB - 11.22 mas - corresponding to a distance of 90 parcsecs.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - May 2007

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  xi Boo (14 51 23.3 +19 06 02) was discovered by Herschel the elder on 1780, Apr 9 with the note "Double L. (large star) pale r. or nearly r. S. (small star) garnet, or deeper r. than the other" (my brackets). With a parallax of 0".149 it is a nearby system and so the large apparent orbit means that for most of the 151 year orbit the pair is well within range of the small telescope. It is also a beautiful pair - the colours are yellow and orange, but unequally bright, the apparent V magnitudes being 4.76 and 6.95. The stars are presently closing slowly and in mid-2007 there are to be found at PA 311 degrees and separation 6".2. Closest approach occurs in 2066 when the stars are barely 2" apart. The WDS lists 1351 measurements of this pair and only 432 for alpha Centauri - a measure of how much the southern hemisphere is missing its double star observers.

  alpha Centauri (14 39 40.9 -60 50 07) is the nearest binary star to the Sun and the most spectacular visual system in the sky. Discovered by Father Richaud from Pondicherry in 1689 whilst observing a comet ("je remarquai que le pied le plus oriental & le plus brillant etoit un double etoile aussi bien que le pied de la Croissade - I noted that the brightest star at the easternmost foot (of Centaurus) was a double star as good as that at the foot of the Cross" i.e. alpha Crucis.

  In 1838 Henderson first measured the parallax at about 1 arc second but it was left to Bessel to publish the parallax of 61 Cygni first. A modern value obtained by the Hipparcos satellite (0".74212) is equivalent to 4.395 light years with a formal error of 0.008 light years. The stars are yellowish and orange-yellow reflecting the spectral types of G2V and K1V and the orbital period is 80 years. Because the system is so close the apparent separation of the orbit varies from 1.7 to 21.7 arc seconds so that they can be seen by small telescopes throughout the orbital cycle. At present the stars are closing (235 degrees and 8".67 for 2007.5). In 1915, Robert Innes found a third star of V=11 some 150 arc minutes away which shares the proper motion of alpha but which has a slightly larger parallax. The star became known as Proxima Centauri and is our nearest stellar neighbour. In 2006, a search was made in the infra-red around alpha Cen B using adaptive optics on one of the VLT telescopes. The modelled mass for this star is about 0.027 solar mass less than that derived from the orbit and it was thought that this might be explained by the presence of a sub-stellar object circling B. In all 252 faint background stars were found within 15" of alpha Cen B but nothing co-moving and therefore nothing physically connected to B itself.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - April 2007

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Although not strictly a northern pair, gamma Virginis (12 41 40.0 -01 26 58) is one of the most spectacular pairs in the sky and the recent close approach, the first for almost 170 years, excited some interest in double star aficionados. The pair was certainly noted by Bradley in 1718 when the separation was about 6 arc seconds, the stars closed slowly until the 1830s when the motion accelerated considerably. It attracted the attention of Sir John Herschel who applied the new science of orbital analysis to the pair but his first attempt did not represent the observed motion. By 1835 the separation was down to 1 arc second, and in the UK Dawes and Smyth made measurements of the pair, whilst F.G.W.Struve, using the 9.6-inch Fraunhofer refractor at Dorpat also followed events. In early 1836, Herschel found the star single with the 20-foot reflector at the Cape and in the spring Struve found an elongation, giving a separation of 0.25 arc seconds. The pair rapidly widened from then on reaching 2 arc seconds by 1843. Since the recent closest approach of 0.37 arc second in mid-2005 the separation has now increased to 0.74 arc second at PA 53 degrees (2007.3) with the angular motion about 2 degrees per month at present. Both stars are F0 dwarfs and appear yellow to the telescope user. A 6-inch telescope should almost resolve them, while an 8-inch will definitely do so.

  The small and bright constellation of Crux bestrides the meridian in southern latitudes during the late evening in April. The brightest star, alpha or Acrux, (12 36 35.9 -63 05 57) is one of the most spectacular pairs in the sky and is accessible to small telescopes. Discovered by Jesuit priest-astronomers in Siam in 1685, it was measured by Dunlop in 1826 who found a separation of 5.4 arc seconds and a PA of 86 degrees. There has been a small decrease in distance to 4.0 arc seconds and an increase in PA to 114 degrees at the time of writing. The two stars form a binary pair and the bright 5th magnitude star HR 4729 some 90 arc seconds away also shares in the proper motion. According to Andrei Tokovinin, all 3 stars are spectroscopic binaries and coronagraphic imaging of the distant companion shows three very faint stars close to C of which one, some 2 arc seconds distant may be physical. If so, then Acrux is a septuple system. The three visual components of Acrux are luminous B stars and therefore appear blue-white in the telescope. The Hipparcos satellite determined a parallax for alpha¹ equivalent to a distance of 321 light years with an uncertainty of about 21 light years.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - March 2007

  In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  The constellation of Cancer is a sprawling area of sky with few naked eye stars to betray its presence. The main object of note is the open cluster M44, well seen to the naked eye. Zeta Cnc (8 12 12.7 +17 38 53) was noted as a 6 arc second double by Flamsteed in 1680 and a century later William Herschel divided the brighter of the two stars. It turned out to be a binary with a period of about 60 years and attracted the attention of many 19th century observers because of it's relative ease of measurement (the distance varies from 0.6 to 1.0 arc second - it is currently nearest widest separation). Measurements of the third star with respect to star A indicated a long period of revolution - today this is thought to be over 1000 years but more intriguingly, the motion was not smooth. The apparent path appeared sinusoidal and repeated every 18 years. It was thought, correctly, that this was due to an unseen companion (D) rotating around C. It was not until 2000 that the 4th star was first detected - in the infra-red and at a distance of about 0.2 arc second. The three visible stars are F and G spectral type so appear slightly yellowish, and present a beautiful sight in a 10-cm telescope or bigger.

  Vela straddles the southern Milky Way between Puppis and Centaurus and is a rich hunting ground for the deep-sky observer. The brightest star is gamma Velorum (8 09 32.0 -47 20 12). James Dunlop found it was double so it has the catalogue number Dun 65. With magnitudes of 1.8 and 4.3, and a separation of 43 arc seconds, this is one of the most spectacular and easy doubles in the sky. Both stars are very hot and luminous and appear white in the telescope; gamma A or more correctly gamma ² is the brightest known Wolf-Rayet star. Hipparcos puts the system at a distance of 257 parsecs with an uncertainty of about 15%. This corresponds to a true luminosity of 10,000 sun power (assuming no absorption of the light by interstellar material along the way). In reality gamma ² is a massive spectroscopic binary, the companion an O7 star with revolves around the common centre of gravity in 78 days. Two further stars of magnitudes 7.3 and 9.5 respectively can be seen at distances of 63 and 94 arc seconds.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - February 2007

  In this new series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  The constellation of Gemini is well up in the northern sky during the middle of the month with the famous pair of stars Castor and Pollux at the eastern end of the group. The two stars are well contrasted with Castor showing pure white, as befits its membership of the class of A stars. Pollux, on the other hand, is orange, in reality a cool giant star and is actually the brighter of the two visually, prompting suggestions that one of the stars has changed in output in recent times.

  For the small telescope user, Castor (7 34 35.9 +31 53 18) is of real interest. The star is a brilliant binary with two white components of magnitudes 2.0 and 2.9 currently separated by 4.4 arc seconds in position angle 59 degrees, making it easily visible in a 60-cm telescope. It may have been first resolved by Cassini in 1678 but it was certainly noted by Bradley in 1718. Since then the position angle has decreased by almost 300 degrees, with closest approach around 1965 when the separation was 1.8 arc seconds. Some 72 arc seconds to the south-east is a star of magnitude 9, known as Castor C. This star revolves around Castor AB in a period of many thousands of years. The remarkable fact about the Castor system is that all three visible stars are spectroscopic binaries, making the Castor system a rare example of a sextuple star.

  Sirius (06 45 08.9 -16 42 58, mags -1.5, 8.5). The brightest star in the sky is also one of the nearest, located 8.7 light years away. The details of the discovery of the white dwarf companion are well-established. Bessel first noted that the proper motion of Sirius was not linear but the predicted companion was not seen until January 1862 when Alvan Clark was testing the 18.5-inch objective for Dearborn Observatory. Uniquely, Peters calculated an orbit for the Sirius system 11 years before the star was first seen. His value for the period, 50.01 years is very close to the currently accepted value.

  There is much speculation about the smallest aperture required to see the Pup. It depends crucially on several factors - the separation of B from A, the quality of the atmosphere and the quality of the telescope optics. When B is near periastron it cannot be seen in any telescope. Between 1890 and 1897 when the separation was less than 4 arc seconds, there were no sightings recorded.

  A recent observation of Sirius B was reported by Ralph Aguirre of the Sacramento Valleys Active Astronomers in March 2006. At a separation of 7.3 arc seconds B was seen with a 130-mm Takahashi refractor at x140 but he found it was better seen at x220, a point which earlier observers seem to agree about. This year the writer plans to use a hexagonal diaphragm on the 8-inch refractor at Cambridge in an attempt to get his first glance of this elusive object.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - January 2007

  In this new series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  With Orion as well-placed as it gets in northern latitudes, it is worth taking a look at lambda Ori (05 35 08.9 +09 56 03) in the head of the Hunter. In binoculars, lambda forms a coarse triangle with phi 1 and phi 2 Ori. In larger telescopes and those operating in the infra-red, there appears to be a cluster of about a dozen B stars and a number of low-mass stars. Lambda Ori makes its own H II region by ionising the surrounding cool gas which appears to be in the form of an expanding ring. It may be that this is the site of an ancient supernova, some 300,000 years ago because the small proper motion of lambda itself does not project to the centre of the ring and that it may have been given a kick by a putative binary partner after that star went supernova. The young neutron star Geminga is also though to have been in that area at the time of the explosion. For the small telescope user, lambda is a fine pair with the 3.5 and 5.5 mag stars both brilliant white, reflecting their spectral types of 09.5II and B0.5V. Never less several observers have seen colour including Webb who thought they were yellowish and purple, whilst Olcott considered them yellow and red. The separation of 4".4 is virtually unchanged since records began.

  In the sprawling southern constellation of Puppis, there are many fine pairs for the small and medium telescope. A particularly noteworthy quadruple system can be found in Dunlop (Delta) 30 (06 29 49.1 -50 14 20). First listed by Dunlop in 1826 this unequally bright pair of stars is given by Hartung as yellow and reddish. The WDS lists the magnitudes as 5.97 and 7.98 and the separation is currently 12 arc seconds, making it an easy object. In later surveys, first Russell, and then the Harvard College expeditions to Arequipa in Peru found that both components were close visual doubles. The brighter component is known as R65 and has a period of 52.9 years. It is currently 0".7 apart and closing. The fainter component of the wide pair is known as HDO 195 and has a period of 101 years - it is also closing and in 2007 its separation is just below 0".4. Both systems should be resolvable in a 30-cm telescope.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - December 2006

  In this new series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  gamma And (02 03 53.9 +42 19 48) is one of the most impressive double stars in the northern hemisphere. It first appears in the list of pairs found by Christian Mayer and published in the Astronomische Jahrbuch in 1784. William Herschel first measured it in the late 1770's when he found 70.4 degs and 9".25 `a mean of two years observation'. The colours are striking - yellow and blue green, the latter a contrast effect - reflecting the spectral types of K3 and B8. The relative positions of the two bright stars has changed little since then - the modern values being 63 and 9.5 In 1842, Otto Struve, using the new 15-inch refractor at Pulkova found the companion to be a close pair. Subsequent observations show that BC is a 61 year binary with a highly inclined and eccentric orbit, closing from 0".4 at discovery to become single in all telescopes by 1892. It is currently closing again (the latest orbit showing 0".33 for 2007.0); between 2014 and 2016 it will swing through 280 degrees of position angle and then start to widen again, having reached a minimum distance of just over 0.01 arc seconds in late 2015. This may be the last opportunity for some time for owners of 25-cm telescopes to see the pair elongated.

  theta Eri (Acamar - 2 58 15.7 -40 18 17) is a brilliant white A star some 20 degrees north following Achernar (alpha Eridani). The parallax as determined by Hipparcos is 20.72 mas yielding a distance of 157 light years. The small proper motion of about 0.06 arc seconds per year of star A would have changed the separation of the pair by some 10 arc seconds over the last 200 years. No such change appears in the relative measures so the pair is clearly a long period binary. The WDS shows that the PA has changed by 8 degrees between 1835 and now whilst the separation has edged closer from 8".7 to 8".4. The notes to that catalogue also indicate that the primary star is a spectroscopic binary but it does not appear in the 9th Spectroscopic Binary Catalogue. The WDS has it under the catalogue name PZ 2 and in Piazzi's Praecipuarum Stellarum Inerrantium Positiones Mediae of 1814 the stars are given magnitudes 4.5 and 5.6. The modern magnitudes are 2.9 and 4.0. The fainter star of this beautiful pair is also an A star so both components appear white. It is visible in small telescopes and possibly larger stabilized binoculars. It reminds the writer of the pair theta Aql.

  Bob Argyle - Double Star Section Director

• Double Star of the Month - November 2006

  In this new series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

  Well placed in mid-November in the north is the constellation of Aries, its three brightest stars nestling midway betweeen the Square of Pegasus and the Pleiades.

  The brightest is Alpha Ari, V=2.0 also known as Hamal. It is a K2 giant at a distance of 20.2 parcsecs.

  Beta Ari (Sheratan, V=2.65) is a close pair - it was first found as a spectroscopic binary over a century ago by Vogel at Potsdam. More recently, it has been resolved by long base-line interferometers. The apparent orbit is very eccentric and the separation varies between 4 and 60 milli-arcseconds.

  The primary is an A5 dwarf and the difference in magnitude is 2.6. The system is 18.9 parsecs distant.

  But for the small telescope user, the best double star in Aries is gamma, first found by Robert Hooke in 1664 whilst following a comet. Also known as Mesarthim the proper motion of star A is about 0".1 per year but as the relative positions have changed by less than 2 arc seconds over the past 170 years, it can be assumed that this is a long period binary. Both appear to be pure white and can be separated in a 2-inch telescope or possibly in stabilized binoculars. The separation has slowly diminished since the first measures (8".6 in 1830) and is now 7".6 with the position angle almost exactly 0 degrees.

  Fornax occupies the area of sky from about 2 to 4 hours RA and between declinations -23 and -39 approximately. It is more notable for extragalactic objects - hosting as it does the Fornax cluster of galaxies, the Fornax dwarf gakaxy and NGC 1365, a spectacular barred spiral galaxy. The brightest star alpha For, a yellow F6 subgiant at 03 12.1 -28 59 (2000) is of considerable interest to the double star observer. Found by John Herschel in 1835 (HJ 3555) at PA 310 and separation 5".30, the pair continued to close and was single to Robert Innes at Johannesburg using a 9-inch refractor in the first years of the last century. It was not until 1925 that the companion appeared on the other side of the primary at a distance of 0".8 when van den Bos found it to be an easy object, but the following year, as it widened, it seemed to have faded by as much as 1 magnitude. Hipparcos notes a range of 0.04 magnitude in the visual brightness of the system, which, if it occurs only in star B implies that B varies about 0.13 mag. This is small compared to observed variations which perhaps support a period of variation of decades. The Hipparcos mission lasted only 3 years. The apparent orbit is a very narrow elongated ellipse and the companion is currently now near maximum separation which will occur in 2042 so should be easily visible in a 20-cm telescope.

  The period is about 290 years and the pair is 14 parsecs distant. Burnham identifies the star as 12 Eridani in his General Catalogue. The colours have been noted as yellow and greenish.

  Bob Argyle - Double Star Section Director