Double Star of the Month:
Beta Capricorni
AKA: Dabih; STFA 52. Position: 20 hr 21.0 min -14 degrees 46 min 53 sec
Due south at 21:41 (BST) on 15 September 2020.
Image credit: Jeremy Perez (http://www.perezmedia.net/beltofvenus)
Used with permission
I am opening the autumn doubles with another easy double, which is both bright and wide. However Beta Capricorni being lower in the sky is less well known than Beta Cygni. At 205 arc seconds (over three arc minutes) it is very wide indeed. The primary is 3.2 and the secondary is a still respectable 6.1. There is a third member of this system (they are all physically connected) which is much dimmer at 8.8 but is easily seen in a small telescope or large binoculars. If the primary and secondary are at twelve o’clock, the third star is at four o’clock. The colours of the two main stars are another study in contrast. The primary is F8 and hence is yellow-white, and the secondary is A0 which should be white. In practice they are usually seen as yellow and blue but not as strongly as Albireo. The third star looks ashy to me. Beta Capricorni is 23 degrees high when it crosses the meridian and it lies directly underneath Gamma Cygni (Sadr) and below Altair. This September, Jupiter and Saturn more or less point towards Dabih. There is a bonus here. If you look through your telescope’s finderscope or use binoculars, you can see Alpha Capricorni (Giedi; also known as STFA 51) above Beta Capricorni and in the same field. The secondary in this double is even brighter (3.7 and 4.3) and the separation is even wider (381 arc seconds), but it is not a physical double. The stars are both yellow-white.
Beta Capricorni was observed by William Herschel on 26 August 1780, but as it is naked eye double in very dark skies, it must have been known in ancient times as two close stars if not explicitly as a double. Of more interest to us at present is its designation as STFA 52. The reader will have noticed that Albireo was SFTA 43 and that Alpha Capricorni is STFA 51. What does all this STFA stuff mean? In brief STF is the modern shorthand for Struve, specifically the double stars catalogued by Russian-Danish astronomer Wilhelm Struve, and STFA refers to his first supplement (of widely separated doubles) to the main catalogue. He had originally left them out of his catalogue because he thought so widely separated stars could not possible be binary stars (but in fact as in this case we now know that they can be). The old designations were Greek Sigma (Σ) for the main catalogue of 183x and a Sigma with a Roman one (ΣI) for the second catalogue of 183y, but they were dropped as they did not transfer easily to modern computer databases. The STFA doubles are a nice group of easy doubles to observe and as they are relatively few in number (60 compared with 3136 in the main catalogue!).