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