Double Star of the Month - November 2010

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.

Psi Cas (01 25 55.90 +68 07 48.8) is a pretty triple star found about 8 degrees due north of delta Cas the second left-hand-most star in the 'W'. AC (mags 4.7, 9.2) was found by William Herschel in 1783 and has closed up considerably since then. In 2007 the position was 128° and 20".3, a change which is due entirely to the proper motion of A. Both stars are double again and with a small telescope CD is quite difficult since the components are magnitude 9.4 and 10.0. Burnham found a 14th magnitude companion (B) to A which was last measured in 1970 at separation 2".4 and must be considered beyond the range of most amateur instruments. Chambers gives colours of orange tint, blue and reddish for A, C and D. In 1850 Webb found A orange and C blue.

Beta Phe (01 06 05.11 -46 43 06.6) is a bright visual binary which has been under-observed since it was discovered by R. P. Sellors in 1891 with an 11-inch refractor. It widened to about 1".4 in the mid C20 but then started to close again and was measured occasionally as it closed up with a measure in 1999 giving 258.8°, 0".29. This was the last observation until 2008 and during that time the star passed unobserved through 140 degrees of position angle and more crucially, periastron passage. In 2002 Andreas Alzner calculated the first orbit but more recent measures by Rainer Anton (see JDSO) indicate that a further revision is necessary. This is now in progress. The star is now widening and at the time of writing is around 0".40 so that it should be seen as double in 25-cm on a good night. Steady air is essential since the stars are very bright - magnitudes 4.0 and 4.2. When looking for this pair, take time to look for Slr 2 some 30 arc minutes to the east. It is somewhat wider and fainter but it is doubtless also a long period binary

Bob Argyle - Double Star Section Director