Double Star of the Month:
Eta Cassiopeiae
AKA: Achird; STF 60. Position: 00 hr 49.1 min 57 degrees 48 min 55 sec
Due south at 00:11 (BST) on 16 October 2020. It is practically overhead.
Image credit: Jeremy Perez (http://www.perezmedia.net/beltofvenus)
Used with permission
Up to now, I have tended to avoid doubles with a relatively small separation or a marked difference in the magnitudes of the two stars. This time I am making an exception, because Eta Cassiopeiae (Achird) is both beautiful and very interesting. The magnitudes of the two stars are 3.5 and 7.4, and their separation is 13.4 arc seconds. The two stars are a binary with an orbit of 480 years round their mutual centre of gravity. Eta Cassiopeiae is also very nearby being only 19.4 light years away. The main interest of this star to observers is that is one of those pairs which confuse the human eye. The main star is G0, which should be yellow-white, and the secondary is more yellow being K7 and should be orange. However as I have remarked before when the eye sees a secondary which is more yellow than the primary, it often refuses to accept the fact. So the observers often see the main star as yellow (rather than yellow-white) and cannot agree on the colour of the secondary. The colour has been described as garnet (deep red), red, almond brown and most often purple, but very rarely as orange. William Herschel first observed this double on 17 August 1779 and described the colours as “fine white” and “fine garnet” saying that they were beautiful colours. Achird is often called the Easter egg double because of this yellow and purple combination. Altogether there are ten visible stars in this system, but only two of the others are below the tenth magnitude. H is magnitude 8.4; it lies more or less in the same direction as AB, but it is 701 arc seconds away from A. G is magnitude 9.5 and lies at roughly two o’clock to AB’s twelve o’clock; the separation is 420 arc seconds. Eta Cassiopeiae lies just below the line between Gamma and Alpha Cassiopeiae (Shedir), roughly half-way between them and directly beneath Beta Cassiopeiae (Caph). There is a fairly bright pair of stars (Nu1 and Nu2 Cassiopeiae) between Gamma Cassiopeiae and Eta Cassiopeiae.