Double Star of the Month - November 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.

The two pairs featured this month are both at rather high declinations and hence can be seen for some time either side of this month. Both are unequally bright but easy objects in small telescopes.

Eta Cas = STF 60 (00 49 05.10 +57 48 59.6) Found by William Herschel in August 1779, this beautiful contrasting pair of GO and M0 dwarf stars has been measured on more than one thousand occasions since, and as early as 1906 Thomas Lewis had said that ``in all probability the period does not exceed 233 years". He was wrong - the retrograde motion to date amounts to 250 degrees in 225 years - close on a degree a year - and as the pair is still widening it seems likely that the 480 year period found by Strand in 1969 is much closer to the mark. For 2008.0 the orbit predicts 320.6 degrees and 13.13 arc seconds. The V magnitudes of the stars are 3.52 and 7.36 and at a distance of 5.95 parsecs it is one of the nearest visual binaries to the Sun. The proper motion is 1.22 arc seconds per year and the WDS lists 7 faint field stars within 700 arc seconds, whose distances are changing rapidly as this system speeds past them. Webb and Smyth found the companion purple whilst Sissy Haas notes almond brown.

Zeta Phe - Rmk 2 AC (01 08 23.06 -55 14 45.0) is pair of late B dwarf stars which appear white to observers. Since 1835 the pair has widened from 4 to 6.8 arc seconds whilst the position angle is little changed. The distance would have increased substantially more if the pair were an optical one but at a distance of 85 parsecs this will be a very long period system. For the serious double star observer, there is interest in the faint, close companion to A discovered by Robert Rossiter at Bloemfontein in Dec 1931, and numbered 1205 in the Rst catalogue. This is a binary of about 350 years period as it appears to be moving at about 1 degree a year with the separation fixed at 0.6 arc seconds. The magnitudes of A and B are 4.02 and 6.80 so the pair should be visible in a 30-cm telescope on a good night. It turns out that A is also an Algol system with a period of 1.67 days.

Bob Argyle - Double Star Section Director