Double Star of the Month - March 2012
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.
STT 215 (10 16 16.05 +17 44 24.6) is one of Otto Struve's discoveries at Pulkova and is a slow moving binary. In 1844 the stars were just 0".47 apart in position angle 256°. Since then, slow retrograde motion has brought them to 177° and 1".5, sufficiently wide to be well-seen in a small telescope although the relative faintness of the two stars does require a night of good seeing and transparency to get a good view - it is not the easiest of pairs to measure with the Cambridge 8-inch OG. Surprisingly, the star is not in the Dover edition of Webb's `Celestial Objects' but it is described in Hartung and Haas. The current orbit projects a period of 670 years with the separation continuing to increase slowly. Hipparcos puts this star at a distance of 375 light years and the WDS gives the spectral type of the primary as A9IV.
I11 (09 15 14.64 -45 33 19.8) is in Vela, currently prominent from southern latitudes, and which is full of interesting double stars. It is located about 2 degrees south following λ Vel and was one of Robert Innes' first discoveries with a borrowed 6-inch refractor from Sydney in the last decade of the 19th century. Set in a fine field Hartung found both stars to be yellow, although the spectral type of the primary is B8V. This is undoubtedly a binary, albeit of very long period. Having closed slightly since discovery with increasing position angle, it was found at 290° and 0".8 in 1997 when last measured. It is a very distant system, only just giving a significant parallax form the observations made by Hipparcos which place it at a distance of almost 1900 light years.
Bob Argyle - Double Star Section Director