Double Star of the Month - March 2011

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.

STF1333 (09 18 25.97 +35 21 51.3) appears with a brief description in Chambers revision of Smyth's Celestial Cycle but not in the original publication. Both stars appear `very white' in this tome but a more recent observation by Sissy Haas calls them `pure lemon yellow' but she also notes that they are in a very low power field with the orange star alpha Lyncis and also close to the bright, unequal pair 38 Lyn. The primary is spectral type A8V and the secondary probably similar although the WDS gives no spectral type. Discovered by William Herschel in 1782, Struve measured it in 1827 and found 40° and 1".5. The pair has slowly widened to 1".9 with little change of angle, so it is likely to be a long period binary but the relatively slow movement makes it a fine test for a small telescope with the components of magnitude 6.6 and 6.7. The distance according to Hipparcos is 308 light years.

Mu Velorum (10 46 50.36 -49 25 12.8) is a magnitude 2.8 yellow giant located in a relatively blank area of sky to the naked eye some 11° north of the eta Carina nebula. In 1880, Russell, in Sydney, found it to be double with the companion some 2.7 magnitudes fainter located at PA 55° and a separation of 2".8. The pair turned out to be binary and by 1949 it had closed to 0".2. The currently accepted period is 138 years so the system is now almost at the point in its orbit where it was discovered. The relatively large difference in magnitude makes this star a southern equivalent of zeta Herculis although the separation range is much greater thanks to an eccentricity of 0.84. It will be near widest separation for many years and thus easily accessible to small telescopes on nights of steady seeing. Hartung notes that both stars are yellow, the companion being a G2 dwarf and therefore almost identical to the Sun. A is also a spectroscopic binary.

Bob Argyle - Double Star Section Director