Double Star of the Month - October 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.

The pairs featured this month are both close but bright visual binaries which afford excellent tests of optics and seeing for the possessors of 20 to 25-cm aperture.

72 Peg (23 33 57.19 +31 19 31.0) is equidistant from the top two stars of the square of Pegasus, alpha and beta, and about 3 degrees above the line joining them. Discovered in 1878 with the 18.5-inch refractor at Dearborn by Burnham, this yellow pair has slowly widened with increasing position angle. At the present the companion can be found at PA 105 degrees and separation 0".53. This is the maximum separation which this system will attain but the period of 260 years means that it will be some years before it appears to be significantly closer. The magnitude of the two stars, 5.7 and 6.1, and the declination of +31 means that in the autumn this pair is high in the mid-northern sky and thus provides an excellent resolution test.

Lambda 1 Scl (00 42 42.89 -38 27 48.6) is a similar system to 72 Peg although somewhat fainter (6.7 and 7.0) and rather wider at the present time (21°, 0".7) having widened somewhat since discovery by Harvard College Observers in Peru in 1901. The primary is A0V and therefore appears white to the observer. The brighter star lambda 2 sits about 18 arc minutes following lambda 1 and provides the binocular observer with a fine sight. The parallaxes of both stars as determined by Hipparcos seem to rule out a physical connection.

Bob Argyle - Double Star Section Director