Double Star of the Month - April 2007

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.

Although not strictly a northern pair, gamma Virginis (12 41 40.0 -01 26 58) is one of the most spectacular pairs in the sky and the recent close approach, the first for almost 170 years, excited some interest in double star aficionados. The pair was certainly noted by Bradley in 1718 when the separation was about 6 arc seconds, the stars closed slowly until the 1830s when the motion accelerated considerably. It attracted the attention of Sir John Herschel who applied the new science of orbital analysis to the pair but his first attempt did not represent the observed motion. By 1835 the separation was down to 1 arc second, and in the UK Dawes and Smyth made measurements of the pair, whilst F.G.W.Struve, using the 9.6-inch Fraunhofer refractor at Dorpat also followed events. In early 1836, Herschel found the star single with the 20-foot reflector at the Cape and in the spring Struve found an elongation, giving a separation of 0.25 arc seconds. The pair rapidly widened from then on reaching 2 arc seconds by 1843. Since the recent closest approach of 0.37 arc second in mid-2005 the separation has now increased to 0.74 arc second at PA 53 degrees (2007.3) with the angular motion about 2 degrees per month at present. Both stars are F0 dwarfs and appear yellow to the telescope user. A 6-inch telescope should almost resolve them, while an 8-inch will definitely do so.

The small and bright constellation of Crux bestrides the meridian in southern latitudes during the late evening in April. The brightest star, alpha or Acrux, (12 36 35.9 -63 05 57) is one of the most spectacular pairs in the sky and is accessible to small telescopes. Discovered by Jesuit priest-astronomers in Siam in 1685, it was measured by Dunlop in 1826 who found a separation of 5.4 arc seconds and a PA of 86 degrees. There has been a small decrease in distance to 4.0 arc seconds and an increase in PA to 114 degrees at the time of writing. The two stars form a binary pair and the bright 5th magnitude star HR 4729 some 90 arc seconds away also shares in the proper motion. According to Andrei Tokovinin, all 3 stars are spectroscopic binaries and coronagraphic imaging of the distant companion shows three very faint stars close to C of which one, some 2 arc seconds distant may be physical. If so, then Acrux is a septuple system. The three visual components of Acrux are luminous B stars and therefore appear blue-white in the telescope. The Hipparcos satellite determined a parallax for alpha¹ equivalent to a distance of 321 light years with an uncertainty of about 21 light years.

Bob Argyle - Double Star Section Director