Double Star of the Month:
Castor
AKA: Alpha Geminorum; STF 1110
Position: 07 hr 34 min 36 sec +31 degrees 53 min 18 sec
Due south at 20:03 (GMT) on 15 March 2022
Image credit: Jeremy Perez (http://www.perezmedia.net/beltofvenus)
Used with permission
There are relatively few double stars where both components are bright. If one takes double stars with both stars being brighter than magnitude four and with a separation of at least 2 arcseconds (a feasible split in a small telescope), there are only six pairs which are visible from Havering. I have already covered two of them (Algieba in March 2020 and Mizar in May 2020), and I will cover two more (Porrima and Alnitak) in the future. Here I will discuss Castor or Alpha Geminorum. The sixth pair, Theta-2 Tauri, is simply two stars in the Hyades cluster. Castor was first measured by James Pound and his nephew James Bradley – who later became Astronomer Royal in succession to Edmond Halley – at Wanstead in 1719, although it had probably been seen as a double star earlier by other astronomers, notably by Giovanni Cassini. William Herschel first observed Castor on 8 April 1778, and it was the second double star he observed; Omicron Ceti was the first in October 1777. More importantly Castor was the system he used to show in 1803 that stars could be bound together by gravity (i.e. be binary stars) and not just a chance alignment of two unrelated stars.
The main pair is relatively nearby being about 51 light years distant. The orbit of the two stars is 460 years and hence the separation of the stars varies quite considerably. In the late 1960s the separation was barely 2 arcseconds, so it was a difficult split, but since then it has been widening and has now reached 5.5 arcseconds, and will be almost 6 arcseconds in 2030. They are both hot large A spectral type stars. Their magnitudes are 1.9 and 3.0. There is a third star which is 72 arcseconds below the AB pair, but it is only magnitude 9.8. It is a member of the same physical system but it is far away from the main pair. All three stars are spectroscopic binaries so there are actually six stars in the system. There is no difficulty in finding Castor. Just look for a pair of bright stars above Procyon; the left-hand or south-eastern star, Pollux, is yellowish and the right-hand or north-western star is Castor which is white. The pair looks white or perhaps slightly greenish white. I agree with Herschel that they look more equal in brightness than their magnitude difference would suggest. It is no longer a difficult split but you should use a high magnification: at least 80x and 150x would be even better.