Double Star of the Month in Coma Berenices

• 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 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

• 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

• 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

• 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

• 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 - 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