Double Star of the Month in Vulpecula

• August 2017 - Double Star of the Month

  Located about 1.5 degrees south-west of Albireo in the Cambridge Double Star Atlas (2nd edition) is a single point marked magnitude 7 to 7.5 according to the scale. This is the binary star STF 2525 (19 26 33.71 +27 19 21.9) which consists of stars of mag 8.2 and 8.4.

  Found by Struve at Dorpat at a separation of 1".3, over the next 60 years it gradually closed up until it was unresolved between the late 1880's and about 1895 when it began to widen again. It was therefore regarded as an optical pair undergoing a close approach until Thomas Lewis (1906) clarified the situation.

  In the previous decade the stars had made a very close approach and the similarity in magnitude disguised the fact that the companion was heading out back to where it came from.

  The stars were measured in 2016 with the Cambridge 20-cm refractor (291 degrees, 2".38) and are now visible in 15-cm. The uncertain period is 883 years and the maximum separation of 3".3 will be reached around 2320.

  Pi Capricorni (20 27 19.20 -18 12 42.1) is the easterly of three faint naked-eye stars which form a triangle some 4 degrees south and somewhat east of beta Capricorni (V = 3.1). The other two stars in the triangle, omicron Capricorni and rho Capricorni, have already been dealt with in this column in August 2015 and August 2016 respectively.

  Pi Cap. is one of S. W. Burnham's earliest discoveries using the 6-inch Clark, but it transpires that it was first seen by O. M. Mitchel in 1846 and not published until 5 years after Burnham noted it in print.

  From northern latitudes this is not a particularly easy pair. The stars are of magnitude 5.1 and 8.5 and the current position is 3".3 in PA 160 degrees; there has been very little motion since 1846. Hartung notes that the companion is white and the primary is a B8 star.

  The discovery of variable radial velocity in the A star later led to the discovery of a close companion which appears to have a period of the order of 40 years or so.

  For a real test, try the 14.1 mag star found by Burnham with the Washington 26-inch refractor. It is being left behind by AB and is currently 38" away in PA 40 degs.

  Bob Argyle - Double Star Section Director

• 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