Double Star of the Month - January 2013
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.
26 Aurigae (STF 753 - 05 38 38.10 +30 29 32.8) is one of William Herschel's class III double stars, and can be found about 3 degrees north-following beta Aurigae. It is a bright and easy pair - the two main stars being visual magnitude 5.4 and 8.4 and the companion 12".2 distant from A. Smyth found the stars to be pale white and violet whilst Sissy Haas notes straw yellow and atlantic blue. It seems likely that this is a physical pair but the same cannot be said of the unassociated star of V = 11.5 which is slowly increasing its distance from A and can be found at 113 degrees and 35". Burnham found it independently in 1872 but noted that it had already been found by Morton using the 7.75-inch refractor at Lord Wrottesley's observatory some 10 km north-west of Wolverhampton. In 1892 Burnham found that the primary was an almost equally bright close pair and indeed it turns out to be a binary with a period of 53 years (BU 1240). At present it is just beginning to close and at 0".2 will require considerable aperture and excellent seeing.
Five degrees following epsilon Col is a wide (13 arc min) pair of stars which make up an easy binocular double and may just be visible to the naked eye. This is gamma1 (V = 4.7) and gamma2 Caeli (V = 6.3). There is much more here for the telescopic observer to note, because both these stars are again double. Gamma1 (05 04 24.40 -35 28 58.7) somehow missed the attention of John Herschel and was swept up by Captain Jacob in 1847. The companion at magnitude 8.2 is currently 3".2 distant in position angle 305 degrees. There is slow retrograde motion and the distance is gradually increasing. The primary is a K3 giant and indeed Hartung notes the colour to be orange, with the companion white. Gamma2 was discovered to be a close unequal pair by the Hipparcos satellite and is now called HDS658. The companion (mag 9.7) is 0".9 away in position angle 195 degrees - there has been about 12 degrees of motion since discovery. This star should be visible in 30-cm on a good night. Both gamma1 and gamma2 are in Hipparcos but gamma1 has a parallax of 17.90 mas whereas that of gamma2 is 10.15 mas.
Bob Argyle - Double Star Section Director