Double Star of the Month - November 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.

Three degrees north of alpha Ari is a line of four stars of marginal naked-eye visibility. The most westerly of these stars is 10 Arietis, a close visual binary. The third star is 14 Ari - a coarse triple. 10 Ari (02 03 39.26 +35 56 07.6) was found by Wilhelm Struve at Dorpat and during the remainder of the nineteenth century the star closed slowly, leading Burnham to comment (in 1906) 'Probably orbital motion, although the measures are well represented on the hypothesis of rectilinear motion'. From a distance of 1".98 in 1833, the companion passed by the primary around 1920 at a distance of about 0".3 and has been widening since. The current catalogue period is 325 years and gives a position of for 2014.0 of 346° and 1".48. It is a nice pair in the Cambridge 8-inch but needs a night of good seeing to see the companion of magnitude 7.9 close against the mag 5.8 primary.

omega Fornacis (02 33 50.71 -28 13 56.4) was swept up by John Herschel at Feldhausen in 1834 and is catalogued as HJ 3506. He noted that it was a 'very fine star but ill-defined'. The stars are magnitudes 5.0 and 7.5 and there has been little motion since discovery, the latest position in the WDS gives 10".8 and 245°. Sissy Haas calls this a 'showcase pair' and gives colours of goldish-white and smoke-grey whilst Magda Streicher with a larger aperture notes yellow-white and light grey-blue. Hartung does not pronounce on colours but notes a similar pair about 2' west. It seems likely that the two stars are physically connected. Hipparcos gives a distance of 484 light years and the primary is a subgiant of spectral type B9.5.

Bob Argyle - Double Star Section Director