May 2016 - Double Star of the Month
Virgo straddles the celestial equator and this month two pairs are included from this constellation - one north and the other south of the zero line of declination.
The primary of STF 1764 (13 37 44.01 +02 22 56.5) is a K2 giant and the note in Sissy Haas' book says that the stars are yellow and blue and that the colours are
vivid. This system has been left out of Hartung and the Rev. Webb dismisses it with the comment yellow and ash.
The pair can be found about 3 degrees north and slightly following zeta Virginis. It forms an equilateral triangle with 84 and 78 Vir. There has been very little change since discovery. The stars are mags 6.8 and 8.6 and last year the writer measured the pair and found PA 32° and separation 16".2. The distance of A has been measured by Hipparcos but the resulting large value (2038 light years) is very uncertain. There are two further and fainter companions 10.4 at 139° and 172" (C) and D 10.7 at 143° and 207" (D), which together form the pair STF 1765.
SHJ 162 (13 14 55.85 -11 22 07.3) misses the cut in both Webb and Hartung but Haas was obviously impressed by the colours of the two wide stars A and B - white and pale red. John Nanson, however, using a 6-inch f/10 lens and x84 in 2011, noted that they were yellow and white.
It was measured by James South on 1823 May 7 using his 5-foot equatorial when the distance of AB was 45". The writer has not observed this pair but apart from AB there is another star in the system, close to A which was discovered by Richard Rossiter from Bloemfontein in 1937 using the 27-inch refractor. This has turned out to be a binary of period 122 years and in mid-2016 the companion (a) is at 161° and 0".55. It is about 1.5 magnitudes fainter than A but might be seen in 30-cm on a good night. The rapid proper motion of Aa (0".37 per year - distance 128 light years) is leaving behind star B which is now 112" away. There is a star of mag 13.3 at 67". The system is 3 degrees directly preceding Spica.
Bob Argyle - Double Star Section Director