Double Star of the Month - August 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.
beta Cygni (19 30 40.29 +27 57 34.9) One of the most famous and beautiful pairs in the sky has been a favourite for small telescopes for many years but the use of modern imaging techniques has confirmed that this is a multiple system, possibly quintuple. The bright pair was certainly noted by Flamsteed in June 1691. Spectroscopy showed the primary to have a composite spectrum, a K giant combined with a late B star, whilst the wide visual companion is a B8 dwarf. This difference in spectral type explains the marked contrast in colours between the stars. Exact shades depend upon the individual but the Victorians called them topaz and sapphire whilst modern descriptions tend towards yellow and blue. Whichever applies, the pair is a magnificent sight in binoculars. The primary component was resolved in 1976 by Harold McAlister using speckle interferometry, and subsequently seen visually by Charles Worley with the 26-inch refractor at Washington. Aa is a difficult pair, since the visual magnitude difference is about 2. A recent orbit by Marco Scardia and colleagues gives a period of 213 years and the current separation is 0".37. This pair has been resolved from the UK by Christopher Taylor with a 12.5-inch Calver reflector.
In 1980 another component, closer in than a was reported by Bonneau and Foy and confirmed about a decade later but has not been seen since. There is little doubt that A and B form a very long period binary system. The Hipparcos parallaxes agree within the errors of both, placing the stars about 118 parsecs away, whilst the proper motions are similar. In 2007 February, a paper published by Roberts et al reports the presence of a faint companion to B some 4 magnitudes fainter in the I band which may be a G dwarf.
Gale 3 (19 17 12.22 - 61 39 39.7) is a bright, relatively close naked-eye star in the constellation of Pavo. It consists of two white stars of spectral types A5 and A8. The period of the pair is 156.7 years and in mid-2007 the position angle is 340 degrees and the separation 0".51 making it a good test for a 10-inch telescope. The separation increases slowly to 0".56 over the next 50 years and then closes down to 0".15 one hundred years from now.
This is one of five pairs that Walter F. Gale (1865-1945) found with an 8.5-inch With reflector from New South Wales. Gale noted the pairs in 1894 and this list appears in Astronomische Nachrichten (AN 143, 293, 1897). However, R. T. A. Innes also found two of the stars independently but somewhat later in 1894, and acknowledges Gale's contribution in his paper - `detected by Mr. Gale on his 8.5-inch, previously to my seeing them '. Since Innes published his list first (in MN 55, 312, 1895) the pairs are given the catalogue letter I rather than GLE. Ironically, Innes was using a 6.25-inch Cooke refractor of 1851 which he had borrowed from Gale!
Bob Argyle - Double Star Section Director