Double Star of the Month - September 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.

kappa Pegasi (21 44 36.4 +35 47 39.5) is a famous visual binary, having been discovered by Burnham in 1880 using the 18.5-inch refractor at Dearborn. Apart from observations by the Greenwich double star observers around the turn of the century, and later at Herstmonceux and Christopher Taylor with his 12.5-inch it has eluded attempts at resolution from the UK. The separation barely exceeds 0".2 at any point in the 11 year orbit. Of more interest to the smaller aperture user is the Struve companion (STF 2824) which sits about 14" distant at PA 288. The writer has measured it with a 20-cm OG but, it has to be said, with some difficulty, owing to the faintness of B. The WDS gives 10.8 with the primary at V=4.1.

pi Gruis (22 22 44.2 -45 56 53) would be a naked-eye pair if it were a little brighter. There are 2 stars of mags 6.5 (pi 1) and 5.6 (pi 2). Both are double stars in each case with companions about 4 magnitudes fainter and 3 to 5 arc seconds apart and both were discovered by Robert Innes. The easier of the two is pi1 (I 135) which also has the distinction of being a member of the rare S class of stars and varies between visual mag. 5.8 and 6.4. Hartung notes that the primary is bright orange. Whilst the binary nature of pi1 is not yet clear, pi 2 (I 382) seems to be physically connected. For the galaxy observer the edge-on spiral IC 5201 is about 15' SW of pi 2.

Bob Argyle - Double Star Section Director