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
