Double Star of the Month - December 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.
With the Pleiades now high in the sky there is the opportunity to get a good look at this young star cluster some 420 light years distant.
James Mullaney and Wil Tirion in their Atlas note nine double stars within a radius of 3 degrees of the cluster centre. Most western of these is 7 Tau (033426.62 +242752.1) and although it lies at the same distance as the cluster its proper motion indicates that it does not belong. William Herschel noted the wide companion of V = 9.9 at a distance of 22" and it resides in the WDS as H 4 88. Thomas Lewis considered it was an optical companion but the WDS says that it is physically connected. It was Freidrich Struve who resolved the primary into a close pair of stars of mags 6.6 and 6.9 at a distance of 0".6. The pair closed to 0".2 about 90 years ago and has now widened again to 0".7 and PA 353 (2012). In a 20-cm OG it is usually a difficult pair and requires a steady air for a good view. The period appears to be around 520 years and both stars appear white to the writer; Smyth made them white and yellowish.
Some 20 degrees below the tip of the `V' of Taurus can be found the naked-eye pair omicron 1 and omicron 2 Eri. Omicron 2 is the nearby triple star STF518 which contains two dwarf stars but this month the spotlight falls on 39 Eri (041423.69 -101521.2) which is 3 degrees south of omicron 2. Another discovery by William Herschel this beautiful pair is a slow binary having moved some 10 degrees retrograde since discovery and is now somewhat closer than then also (2009, 6".7). The significant proper motion in declination of 0".16 per year would have separated the stars by more than 30" since discovery if the pair were not connected. Modern catalogue values for the magnitudes are 5.03 and 8.53 although Smyth noted star B as mag 11 in the `Celestial Cycle'. The primary is a K3 giant so both Hartung and Gould, observing from the southern hemisphere where the star is high in the sky, noted an orange hue whilst the faint secondary appeared white in both cases. Webb, observing from the Welsh border made them yellow and blue.
Bob Argyle - Double Star Section Director