Double Star of the Month - May 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.
78 UMa (13 00 43.59 +56 21 58.8) was found to be double by S. W. Burnham in 1889 using the then new 36-inch refractor at Lick Observatory. The pair can be found just following epsilon UMa. Although the stars are very unequal in brightness (the WDS gives 5.02 and 7.88 and Baize and Petit list the system in their catalogue of double stars with variable components), it was apparent from the proper motion of A and the relative motion of B that the pair formed a binary system. A recent orbit gives a period of 106.4 years but at no point in its apparent orbit can the pair be called `easy'. On good nights with the 8-inch OG at Cambridge the companion was measured several times in the last two decades but since then the separation has reduced and in mid-2010 the companion can be found at 99 degs, 1".18. The motion is relatively slow however with minimum separation of 0".48 not occurring until 2026. Sissy Haas notes an observation with 275-mm which gives colours of gold-white and bluish-turquoise.
Another double star with a variable component is the wide southern system alpha Circini (14 42 30.69 -64 58 28.5). Catalogued by Dunlop (Delta 166) this beautiful pair is one of the finest systems in the southern hemisphere. The primary is a member of the rapidly oscillating peculiar A stars which has a basic period of 6.8 minutes and an amplitude of a few millimagnitudes. Other frequencies of oscillation have also been found. The visual magnitude of A is 3.18. B (mag. 8.47) is a K0 dwarf now separated some 15 arc seconds from A and the two stars form a very long period binary system since the proper motions of both are substantial and identical. Hipparcos places the stars at a distance of 54 light years. Since 1826, the companion has widened from a distance of 10" and the position angle has decreased by 38 degrees to about 225 degrees now. Several observers make the colours yellow and red including Hartung some 50 years ago and more recently Richard Jaworski, both from Australia.
Bob Argyle - Double Star Section Director