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