Double Star of the Month - November 2008

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.

This month's pairs are just visible to the naked eye - each being V=5.6

65 Psc (00 49 53.1 +27 42 37.1) was found by William Herschel in 1783 and is one of the showcase pairs listed by Sissy Haas in her publication Double Stars for Small Telescopes. She finds the two stars to be citrus orange in colour with a 60-mm refractor; Webb found them yellowish and Smyth says both are pale yellow. This would accord with the spectral types which are given variously as gF0 and gF2 (Burnham) and F4III and F5III in Hartung. The system is 273 light-years away and since the first measure the position angle has decreased only 5 degrees with the separation increasing from 4".0 to 4".3. Clearly it is a binary of very long period as the proper motion of almost 0".1 per year for the primary would have separated the stars by 20" today if they were unrelated.

BU 395 in Cetus (00 37 19.79 -24 46 02.0) is one of the most interesting of Burnham's discoveries. Found using the 6-inch Clark in 1875, it has turned out to be a short period system. The period is 25.09 years and the orbital plane is highly inclined so that the separation varies from 0".17 in 2006 to 0".77 in 2015. At present (2009.0) the stars are separated by 0".37 affording those with 30-cm aperture the chance to test the resolving power of their telescope.

By 2010.0 the pair widens to 0".48. The star has also been observed as a double-lined spectroscopic binary with the spectral types given as G8V and G9V. Hipparcos places the system some 50 light-years away and the proper motion carries it about 1".5 annually almost exactly east-west across the line of sight.

Bob Argyle - Double Star Section Director