Double Star of the Month - December 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.

STT 95 Tauri (05 05 32.1 +19 48 24) was found by Otto Struve using the 15-inch refractor at Pulkovo. It is a long period binary (760 years) and is currently to be found at 297 degrees and 0".96. The magnitudes are 7.0 and 7.6 so it should be easily resolvable in 15-cm. The apparent orbit is such that the pair will widen to about 1".2 over the next 350 years so this system will gradually become easier for the small telescope user. The primary belongs to the metallic lined A star group, both stars appear white, and the system is at a similar distance from the Sun as theta Ret (below) is. The star lies halfway between iota and 104 Tauri which in turn lie halfway between zeta Tauri and Aldebaran.

Theta Ret (04 17 40.3 -63 15 20) is a beautiful pair of stars (magnitudes 6.4 and 7.7) in the small constellation of Reticulum, more specifically, it is in the same low power field as the stars alpha (mag. 3.4) and eta (mag. 5.4) Ret and 30 arc minutes south of the galaxy NGC 1559. Discovered by Rumker (the pair is RMK 3) the two stars have changed little in position angle and the separation has slowly reduced from 6".4 in 1835 to 4".3 in 2000 when the pair was last measured. Although the primary has a spectral type of B9.5 III both stars appeared deep yellow to E. J. Hartung whilst Ross Gould with 35-cm thought that the primary was a lighter shade of yellow than the secondary. This is almost certainly a binary star with the small proper motion of A not quite accounting for the change in separation. Hipparcos gives the distance to A as 7.02 mas (465 light years) and the primary star is consequently some 50 times brighter than the Sun. In his revision of Smyth's Celestial Cycle, Chambers quotes B. A. Gould as having noted a variation of 0.3 magnitude in the primary. In that volume, the stars are given as 5.5 and 9.0.

Bob Argyle - Double Star Section Director