Double Star of the Month - April 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.
35 Com (12 53 17.77 +21 14 42.1) is a visual binary with a period of 359 years meaning that since the first measures were made in 1829 we have observed exactly half its orbit. For 2010.5 the companion can be found at 196 degs and 1".03 but as the difference in magnitudes is almost 2 (the stars are 5.15 and 7.08), it is not an easy pair for the small telescope. A third star of magnitude 9.76 is some 27" away and is physically connected to AB. 35 Com lies to the north of the main Coma group of galaxies but is only 1 degree south preceding the galaxy M64. It is about 280 light years away and the primary is a giant star of spectral type G7. Gould, with 20-cm, estimates A and B are both orange-yellow with the distant C "possibly blue".
The appearance of theta Muscae = Rmk 16 (13 08 07.16 -65 18 21.7) as a wide pair of bright stars belies its true nature. There are four hot young stars in this system which is so far away that Hipparcos was unable to get a handle on the distance. Estimates from other sources put the group 2,000 to 2,500 light years away. A is a spectroscopic binary of period 19 days consisting of a WC6 star with an O6 companion whilst some 0".04 distant is a B0 supergiant. The companion as discerned by the small telescope user is about 5".4 distance and is probably another Wolf-Rayet star. Deep-exposures of this star show very faint concave nebulae facing the stars which are probably the result of old shell ejection phases. This is a splendid object for the small telescope; Hartung gives the colours as yellow and white, the magnitudes are 5.63 and 7.55 and the pair sits in a rich field.
Bob Argyle - Double Star Section Director