November 2018 - Double Star of the Month

As a by-product of his survey at Dorpat for new double stars F. G. W. Struve came across hundreds of pairs which were very wide, and obviously of not much significance. He consequently placed them in two appendix catalogues - now denoted by STFA and STFB in modern WDS parlance.

His son, Otto also compiled a catalogue of wide pairs which he happened across during his searches and which appear in the Washington Double Star Catalog (WDS) as STTA. Most of these pairs can be seen in binoculars but some of them are quite attractive and worth seeking out.

One such pair is STTA19, also known as S 398 (01 28 22.92 +07 57 40.9). It is 3.5 degrees East of the attractive bright pair zeta Psc. The WDS gives magnitudes of 6.3 and 8.0 and the K1 giant primary appears orange, although T. W. Webb calls it rosy, whilst the companion is 'bluish'. W.H. Smyth found yellow and pale blue.

Modern observations have shown that there is more to this system than meets the eye. Gaia DR2 indicates that the stars are at the same distance (391 light years) from us and moving through space with the same considerable proper motion of more than 0".1 per year. In addition the B star is a close pair which has moved about 30 degrees in Position Angle (PA) since discovery in 1999. The separation is currently about 0".4 whilst the components are mags 8.1 and 11.9.

Some of the more interesting and difficult visual binaries were found by the Clark brothers, Alvan and Alvan G., during the course of testing some of their objectives on stars.

In 1853 Alvan was assessing the performance of a 7.5-inch objective when he alighted on 95 Cet = AC2 (03 18 22.43 -00 55 49.0) and noted it had a faint and close companion. When William Rutter Dawes heard about this his interest was aroused. The following year Dawes met Clark during the latter's visit to England and bought the objective and telescope.

Dawes soon looked at 95 Cet and was able to measure the new companion which was at 73 degrees and 0".7. What made it hard to measure was the significant difference in magnitude.

After Dawes' observations, made on three nights, there were no further measures for 30 years according to Burnham. The American master relates how he spent many nights with various apertures only to find no trace of the companion and he succeeded only once, in 1888, in seeing the B star.

By 1900 even Aitken with the Lick 36-inch could not see the companion. It has long been suspected that the B star is variable, which would explain some of the negative results.

Modern observations show that the visual magnitude difference is between 2 and 3, but currently the stars are close to maximum separation (2019.0, 260 degs, 1".18) and this represents a good opportunity to divide the pair; it probably requires 20-cm and a night of fine seeing.

Bob Argyle - Double Star Section Director