Double Star of the Month - January 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.

Rigel = beta Orionis (05 14 32.27 -08 12 05.9) is perhaps the most intrinsically luminous star in a double star system which can be seen in a small telescope. Whilst the measured luminosity of other bright stars such as Canopus have varied wildly over many years, the advent of the Hipparcos mission has been able to pin down the geometric distance to these objects much more accurately. The current trigonometric parallax for Rigel from Hipparcos is 3.78 milli-arcseconds with an uncertainty of about 10%. This translates to a luminosity of 48,000 times that of the Sun. The faint companion star whose estimated magnitude has also fluctuated over the years might be dominated by the light of the dazzling primary star 9 arc seconds distant but it is an equally interesting object in its own right. In 1871, Burnham, using his 6-inch Clark refractor, suspected an elongation of this star, and having then examined it with the 18.5-inch OG at Dearborn was convinced that there was a `real and measurable' elongation. Around 1900, Aitken, Hussey and Barnard using the 36-inch refractor at Lick all recorded separations in the 0.09 to 0.16 arc second range for BC. Occasional sightings have been reported since then, including as late as 2005 but no orbit exists and in fact the reality of the companion to B must still regard as unproven. Rigel B is known to be a spectroscopic binary but this cannot be BC. It needs a good night to see Rigel B clearly in the glare of the primary. The magnitude is near to 6.8 rather than the 10.4 first assigned to it by Burnham.

19 Cam (05 22 33.53 +79 13 52.1) is a mag 5.1 late F dwarf star some 68 light years distant. A companion star of mag. 9.2 was first noted, apparently by Piazzi, in the compilation of his Palermo catalogue and later catalogued by Struve as STF 634.This is a good example of an optical double. In 1825, South found star B at 346° and 37", by late 1926 it was at 62°, 9" and by 2008 it had reached 133°, 27". Both stars have significant proper motions (0.18 and 0.16 arc seconds per annum) but almost in opposite directions on the sky. This is an attractive pair for small telescopes although rather difficult to find being in a sparse area near the north celestial pole - the colours were given as light yellow and pale blue by Smyth.

Bob Argyle - Double Star Section Director