Double Star of the Month Archive 2019

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.

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 Bootis (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