Double Star of the Month - October 2011

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.

Located in northern Cepheus and about 15 degrees from the NCP, pi Cep (230753.8 +752315) is an awkward target for the user of the classical refractor and the writer has only measured this binary once, in 1994, when the position angle was 347° and the separation 1".1. Since then direct motion of 10 degrees has occurred with virtually no change in separation. It has now almost reached the point where Otto Struve first measured it in 1846 and the current period is given as 163 years. Around 1920, a number of observers recorded the star as single but the current orbit predicts separation in the 0.5 to 0.6 arc second range so it is not clear why the star should have been unresolved. Is the companion perhaps a variable? The primary is an early G star and both Webb and Smyth thought it very yellow or deep yellow, respectively, with both men agreeing that the secondary appeared purple. It is also a spectroscopic binary with a period of 556 days and it is speculated that the invisible companion is itself a short period binary. John Herschel found a 12.2 mag. comes at 58". Pi Cep is an attractive object but the stars are more than 2 magnitudes apart and it needs good optics and a steady air to resolve it cleanly. It will remain in range of 15-cm or so for a few decades to come.

Sculptor contains a few nice pairs and one of these, kappa1 Scl (000921.0 -275917) is a good test for a 10-cm as the stars are magnitudes 6.1 and 6.2 and currently 1".3 apart. It was found low in the Chicago sky by Burnham using his 6-inch refractor when it was considerably closer than it is today and a preliminary orbit gives a period of over 600 years. Willem Luyten, in his proper motion survey work, found an 18th magnitude companion at a distance of 73" moving through space with the bright pair, making this a physical triple. Hipparcos puts the bright pair at a distance of 256 light years, some 10 light years more distant than pi Cep. The low power field also contains kappa2 Scl, a 5.4 mag. K giant, and most curiously, this too has a very faint (mag. 21) cpm companion found by Luyten which appears in the WDS as LDS2099.

Bob Argyle - Double Star Section Director