May 2015 - Double Star of the Month
STF 1694 (12 49 13.80 +83 24 46.3) is one of the most northerly of the bright double stars and lies in a rather sparse area of the sky. It is worth making the effort to find it as the planetary nebula IC 3568 lies 1.5 degrees south-west. Both stars are in the Hipparcos Catalogue and though the parallaxes seem significantly different (8.14 mas for A and 5.57 mas for B), the large error on that of B means that they are the same within the quoted uncertainties. However, B is a spectroscopic binary and this may well account for the Hipparcos satellite's problems with defining its distance accurately. The primary star (V = 5.3) is an A1 giant whilst the companion (V = 5.7) consists of a pair of A0 and A2 dwarfs. The current position angle and separation is 324° and 20".9, little changed from the epoch of discovery. The proper motion of RA of both stars is significant and similar so it would appear that this is a common-proper-motion pair. In 1944 Wallenquist found a third star of magnitude 11.5 at 223° and 73" but both these values are decreasing quite quickly. Sissy Haas calls the primary 32 Cam and notes that both stars are lucid-white.
4 Centauri (H N 51) is located at 13 53 12.54 -31 55 39.4. Also known as h Cen, this is one of the pairs found by William Herschel in his last concerted campaign of double star observation and was observed on 1787, Mar 15. It is in an arc of faint naked-eye stars some 5 degrees north-west of theta Centauri and is a splendid sight in a small aperture. The stars, magnitudes 4.7 and 8.5, were measured by the writer in 2013 and were found to be at 185 degrees and 14".8. Hartung notes that the colours are pale yellow and ashy, and that both stars are spectroscopic binaries. The primary star is a subgiant of spectral class B4 which Hipparcos puts at a distance of 637 light years.
Bob Argyle - Double Star Section Director