Double Star of the Month - March 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.

The constellation of Cancer is a sprawling area of sky with few naked eye stars to betray its presence. The main object of note is the open cluster M44, well seen to the naked eye. Zeta Cnc (8 12 12.7 +17 38 53) was noted as a 6 arc second double by Flamsteed in 1680 and a century later William Herschel divided the brighter of the two stars. It turned out to be a binary with a period of about 60 years and attracted the attention of many 19th century observers because of it's relative ease of measurement (the distance varies from 0.6 to 1.0 arc second - it is currently nearest widest separation). Measurements of the third star with respect to star A indicated a long period of revolution - today this is thought to be over 1000 years but more intriguingly, the motion was not smooth. The apparent path appeared sinusoidal and repeated every 18 years. It was thought, correctly, that this was due to an unseen companion (D) rotating around C. It was not until 2000 that the 4th star was first detected - in the infra-red and at a distance of about 0.2 arc second. The three visible stars are F and G spectral type so appear slightly yellowish, and present a beautiful sight in a 10-cm telescope or bigger.

Vela straddles the southern Milky Way between Puppis and Centaurus and is a rich hunting ground for the deep-sky observer. The brightest star is gamma Velorum (8 09 32.0 -47 20 12). James Dunlop found it was double so it has the catalogue number Dun 65. With magnitudes of 1.8 and 4.3, and a separation of 43 arc seconds, this is one of the most spectacular and easy doubles in the sky. Both stars are very hot and luminous and appear white in the telescope; gamma A or more correctly gamma 2 is the brightest known Wolf-Rayet star. Hipparcos puts the system at a distance of 257 parsecs with an uncertainty of about 15%. This corresponds to a true luminosity of 10,000 sun power (assuming no absorption of the light by interstellar material along the way). In reality gamma 2 is a massive spectroscopic binary, the companion an O7 star with revolves around the common centre of gravity in 78 days. Two further stars of magnitudes 7.3 and 9.5 respectively can be seen at distances of 63 and 94 arc seconds.

Bob Argyle - Double Star Section Director