October 2016 - Double Star of the Month

STF3050 (23 59 29.33 +33 43 26.9) is a beautiful pair of white stars forming a long period binary system to the north of the Square of Pegasus. More specifically, it is 5 degrees north of and 2 degrees preceding Alpheratz (alpha And).

The components are mags 6.4 and 6.8 and the spectral type of star A is F8V, whilst that of B is likely to be similar. The first observation in the WDS is for 1777 representing the discovery of the system by Christian Mayer. It is the 80th and last entry in his pioneering double star catalogue. William Herschel later recovered it on December 13, 1787 and called it H N 58.

Thomas Lewis in his book on the Struve stars published in 1906 said it was evidently a binary, whilst Burnham in his General Catalogue of the same year notes apparently rectilinear motion. Subsequent observations have proved Lewis right and the pair is now significantly wider than it was at the beginning of the last century when it closed to less than 2". An orbit produced in 2011 by W. Hartkopf predicts a period of 717 years, with a position in late 2016 of 340 degs and 2".4. This makes it an easy target for 75-mm apertures and above.

Lewis noted that both stars were yellowish.

Beta PsA, (22 31 30.33 -32 20 45.9) bemoans Jim Kaler on his Stars website, is a neglected object most likely all alone, apparently unloved by a companion, or, for that matter, by astronomers and he compares it unfavourably with the much more referenced star alpha PsA.

Beta is certainly a fine double star and worth a visit, and the question of whether it is binary or not can be settled by considering the proper motions of the two stars.

The Hipparcos satellite reveals than A (mag. 4.3) is 143 light years away and moves across the sky, mostly to the east, at about 60 milliarcseconds per year. Star B has an almost identical proper motion and radial velocity and so is physically connected to A. Over the last 200 years the separation has reduced from 35".3 to 30".6 and the PA is almost fixed at 172 degrees.

The SIMBAD spectral types of A1 and G1 might suggest stars of white and pale yellow when in fact the observed colours (by Hartung) are given as pale yellow and white, although he notes that B sometimes appears reddish.

Bob Argyle - Double Star Section Director