Double Star of the Month - April 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.

The binary stars being highlighted this month have several common traits - they are both bright and somewhat unequal in magnitude and can be easily seen in small telescopes. But whereas xi UMa has been measured almost 1700 times beta Muscae, although no less attractive an object, has but 78 measures in the WDS reflecting the concentration of effort on binaries in the northern hemisphere.

xi UMa (11 18 11.24 +31 31 50.8) is one of the best-known systems in the northern sky. Found by William Herschel in 1780, it became the first pair to submit to the science of orbital analysis by Savary in 1828. Later on, first A and then B were found to be spectroscopic binaries with periods of 1.83 years and 3.98 days respectively and about 20 years ago, speckle observers noted indications of a 5th component, attached to B. This is clearly a very difficult object as it has not been seen since 1994. The multiplicity of xi has clearly caused problems with Hipparcos as the system is missing from the Hipparcos catalogue. To the small telescope the stars appear yellowish orange and the 59.9 year orbit is now currently taking the stars further apart. In 2008, B can be found 1.63 arc seconds distant from A in PA 223°.

beta Muscae = R 207 (12 46 16.87 -68 06 29.1) was discovered by Russell in Sydney in 1880 when the position angle was 317° and the separation 0.54 arc seconds. Since then it has been closing again and in 2008 can be found at 48° and 1.27 arc seconds according to the 383 year orbit calculated by R. R. de Freitas Mourao in 1964. The stars are both white with the primary star being an early B-type dwarf. Hipparcos puts the distance at 340 light years whilst the WDS gives magnitudes of 3.52, 3.98, both some 0.4 magnitudes fainter than the V magnitudes given in Hipparcos.

Bob Argyle - Double Star Section Director