Double Star of the Month - September 2009

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.

Two high declination systems are the subject of this month's column.

Cepheus is a rich hunting ground for the northern double star enthusiast and Webb lists about 80 pairs in this constellation, many of which are suitable for the small aperture. xi Cep (22 03 47.2 +64 37 40) did not attract much interest from Smyth as the binary nature of the system was not then apparent, the change in angle having amounted to only 3 degrees from the observation of Herschel some 80 years previously. Since then the curvature of the apparent orbit has tempted the production of an orbit of period 3800 years and in 2010 the companion can be found at 274 degrees and 8".34. The system is relatively close by (30 parsecs) and the main interest for the small telescope observer are the colours of the two components. Webb called them white and tawny or ruddy whilst Smyth thought them both bluish. Sissy Haas considers them lemon white and royal blue. The spectral types are A3 and F8. For the large telescope observer, the A component is a close interferometric and spectroscopic binary of period 2.254 years and the separation never exceeds 0".06.

In the far south Octans straddles the celestial pole. Lambda Oct (21 50 54.5 -82 43 08) precedes beta Oct by a few degrees in a rather sparse area of the sky but the effort of finding it is certainly worthwhile. It is one of John Herschel's discoveries from South Africa (HJ 5278) and is one of the more attractive ones. Hartung calls it a `bright elegant close pair, deep yellow and white.' It is clearly a physical pair and the position for 2002, when it was last measured according to the WDS, is 63 degrees and 3".5. This is a distant pair, more than 400 light years away according to Hipparcos, and the spectra are G8 and KOIII which makes Hartung's comments about colour all the more interesting. The magnitudes are 5.6 and 7.3. For more observations of Octans pairs, see the article by Magda Streicher in Deep-Sky Observer 145, 2008.

Bob Argyle - Double Star Section Director