Double Star of the Month - October 2008

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.

This month's pairs ar both bright, well-observed binaries and well seen in small to medium apertures.

36 And (00 54 58.02 +23 37 42.4) is a beautiful pair following the Square of Pegasus, some 3 degrees south following zeta Andromedae. The two stars are strong yellow (Webb) or golden (Smyth) and form a binary system whose period is 167.7 years so it has passed the position it occupied when discovered by F G W Struve in 1827. The current PA and separation are 322 degrees and 1".1 making it a fine sight in a 15-cm telescope. It is strangely absent from Hartung's book, which includes more northerly objects of less distinction such as AC 1. This system is almost 38 pc distant and the primary is a K1 subgiant. The star itself is just visible to the naked-eye with the components being magnitudes 6.1 and 6.5.

p Eri (01 39 47.24 -56 11 47.2) is one of Dunlop's discoveries (Dun 5) and is probably the nearest equivalent to 61 Cygni in the southern hemisphere. It is close (5.3 parsecs according to Hipparcos), it contains two K dwarfs (in this case K0 and K5) which have visual magnitudes 5.8 and 5.9 respectively, has a similarly long period (483 years) and is also well-separated - reaching a maximum distance of 11.8 arc seconds in 2040. The Chambers edition of Smyth's Celestial Objects which contains a southern extension, also identifies the star as 6 Eri but does not mention colours, neither does John Herschel in his Cape observations. Hartung records both as deep yellow.

Bob Argyle - Double Star Section Director