Double Star of the Month in Cassiopeia

• 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

• 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 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

• 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

• 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

• 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 - 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 - 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