Double Star of the Month - July 2010
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.
Located close to the head of the Dragon, mu Dra (17 05 20.12 +54 28 12) is a long period binary system, first found by William Herschel in 1781. With a period of 672 years, the apparent separation of the two stars ranges from 2".0, which last occurred about 40 years ago, and 5".7. At the present time it is almost 2".4 apart at position angle 5°. With components of magnitudes 5.66 and 5.69 the star is easily visible to the naked-eye and is an excellent target for small telescopes so it is included in James Mullaney's One Hundred Showpiece Double and Multiple Star list, where he gives the colours as yellowish-white. Sissy Haas also notes the stars as goldish-white, but Smyth sees them only as white. There is evidence for a spectroscopic companion to B which may be bright and wide enough to be resolvable in the optical, and there is a mag 13.7 star at 12" may also be part of the group. The system lies at a distance of 90 light years.
Nu Scorpii (16 11 59.27 -19 06 53) is the southern equivalent of the Double-Double in Lyra, although the brighter pair is more difficult to divide than its northern equivalent, and as a consequence at least 150-mm is needed to see the four components clearly. The wide pair catalogued by Herschel as H V 6 consists of white stars of magnitude 4.2 and 6.0 separated by 41". In 1846 Mitchel, using an 11-inch refractor in Cincinnati, resolved the companion into two stars about 1".3 apart. Burnham then discovered that the primary was also double with his 6-inch Clark in 1873 when the separation was around 0".6. Since that time both pairs have slowly widened and the current values are 1°, 1".3 for A and 55°, 2".3 for B. This is a physical system of high multiplicity since A is double again at the sub-0".1 level and there is also a spectroscopic component of 5.5 day period. The writer has measured both bright pairs from the UK with 20-cm but it needs a night with very steady air to do this.
Bob Argyle - Double Star Section Director