Double Star of the Month in Leo

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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 - 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 - 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 - 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