Double Star of the Month Archive 2020

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.

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