Double Star of the Month - September 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.
About 2.5 degrees north of the Dumbbell nebula M27 and a little proceeding is 16 Vul = STT 395 (20 02 01.37 +24 56 16.3) , one of Otto Struve's discoveries at Pulkova and when first found in 1843 it was a difficult pair at 89 dgs and 0".5. In the intervening period is had almost doubled in separation and the position angle has increased to 125 degrees so that a 15-cm telescope should resolve this beautiful pair. Hartung notes that both stars are yellow. This is a relatively distant system - the Hipparcos revised parallax is 14.55 mas with an uncertainty of 0.50 mas, putting it at a distance of 69 parsecs. It is certainly a binary system of long period as the significant proper motion testifies.
The spectral types of 21 Sgr (18 25 21.04 -20 32 29.8) are given as A + K2III in the WDS and the magnitude difference is 2.4 in the visual. It is difficult to reconcile the apparent colours seen in this beautiful pair which are orange and greenish according to Hartung with these spectral types. The writer also recently viewed this pair in the 26.5-inch refractor at Johannesburg and the similarity with Antares was immediately apparent - the companion is definitely greenish. The Hipparcos B-V value of +1.3 certainly suggests that the light of the system is dominated by the K giant star and that the spectral types in the catalogue should be reversed. The revised parallax puts the primary at 126 parsecs. There is slow retrograde motion with the discovery position by Jacob from 1846 showing 297 dgs and 1".8 whilst a recent measure put the companion at 280 dgs and 1".7.
Bob Argyle - Double Star Section Director