Double Star of the Month - December 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.

88 Tau (04 35 39.23 +10 09 39.3) is a binocular pair whose visual magnitudes are 4.3 and 7.8, which can be easily found 6 degrees due south of Aldebaran. Smyth referred to it as a `star with comes' and blithely asserted that after the year 2000 the stars would start to approach each other in the southern part of their orbit. He based this on a difference of position angle of 4.5 degrees between 1800 and 1822 when perhaps the most obvious inference is that the two stars are unconnected. In fact, Piazzi's position for 1800 is not very accurate and there has been little change since 1822. The two stars actually have similar proper motions and may have been co-eval. Smyth noted that the colours were bluish-white and cerulean blue whereas T. W. Webb (yellow-white and yellow-red) and, more recently, Sissy Haas (vivid lemon yellow and silvery cherry) seem to differ from this. In fact 88 Tau is a sextuple system. The A star was resolved by speckle into two components separated by 0".12 and mags of 4.4 and 6.6. The period is 18 years. Both of these stars are spectroscopic binaries with periods of 3.57 and 7.59 days. Distant B is also an SB with a period of 1349 days.

BU 391 (04 26 56.93 -24 04 52.8) can be found in a very sparse part of Eridanus, about 8 degrees N of nu3 and nu4 Eri. It is one of Burnhams's earlier discoveries - made with the 6-inch Clark. Burnham was not convinced that there was much change in the system when he compiled his Catalogue in 1900 but subsequently the stars began to close and periastron was passed around 1980. The pair is currently at 151° and 0".4 which makes it a challenge for a medium aperture from the UK. The magnitudes are 6.7 and 7.1 so it should be divided on a good night with 30-cm. The orbital period is 596 years (Scardia 2003) but note that the WDS catalogue notes give a much shorter and incorrect value.

Bob Argyle - Double Star Section Director