Double Star of the Month - October 2007

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.

STF 3062 (0 06 15.54 +58 26 12.1) is a star on the limit of naked-eye visibility, the south-easternmost one of a pair some 10 arc mins apart in a low power field to the SE of, and including, beta Cas. Its duplicity was noted by the elder Herschel on 1782 May 25 and it is catalogued as H I 39. The parallax is 49.30 +/- 1.05 mas and the proper motion approaches 0.25 arc seconds per year. The Millennium Star Atlas indicates that the star is also V640 Cas but the explanation for variability in the Hipparcos catalogue is `duplicity possibly causing spurious variability'. In fact, Griffin shows that the Hipparcos satellite does not confirm the period or amplitude found in 1983 when the star was claimed to be an eclipsing binary and, in consequence, that there should be no variable star designation at all. The fainter visual component is however a spectroscopic binary of 47 day period. The visual pair have almost completed 2 revolutions since F G W Struve measured the pair in the 1820's. The separation at present is just over 1.5 arc seconds with apastron being reached in a few years time and the angle is increasing by about 2 degrees a year, so a 20-cm telescope will show the two white stars very clearly.

beta Tucanae (0 31 32.56 -62 57 29.1) The two brightest components of beta Tucanae are currently about 27 arc seconds apart and in the WDS catalogue this pair has the designation LCL 119. At magnitudes 4.33 and 4.53 they form one of the most splendid double stars visible to binoculars or telescopes in the sky. Although the corresponding Hipparcos parallaxes are 23.95 and 18.35 mas the formal error on the latter star is 3.34 mas so it might be argued that the stars are a physical system. Certainly the proper motions are similar and large enough that taken in conjunction with the small change in relative position since 1826 the two stars are moving through space together. This proper motion is shared by a third star of mag 5.1 (beta³) some 5 arc minutes away so that to the small telescope user, this is a beautiful triple system. Bring a powerful telescope, such as the 26.5-inch refractor at the Union Observatory in Johannesburg to bear on the group, as W. H. van den Bos did in 1925, and further stars appear. Robert Innes, using the same telescope which now bears his name, had already found that beta² was a very close and unequal double star which turns out to be a binary of period 44.7 years. The current separation is 0.40 arc second and closing. van den Bos added companions to both beta¹ and beta³, now B 7 and B 8 in the WDS. The companion of B 7, some 10 magnitudes fainter than the primary at a distance of only 2 arc seconds must be a formidably difficult star to see. B 8 is pair of 6th magnitude stars separated by little more than 0.1 arc second. Little is known about this latter pair - it has not been measured since 1964.

Bob Argyle - Double Star Section Director