Heavenly twins

The two stars Castor and Pollux are affectionately referred to as the Heavenly Twins.

Just at present Saturn is so placed in the sky with reference to Regulus as to resemble the heavenly twins. (Dia. 1)

Dia. 1. The sky looking east at 23h 30m UT as seen from Orkney on Christmas night, 25 December 2006.

However, Saturn is slowly retrograding (appearing to move against the star background in a westerly direction (L to R) and so is drawing away from Regulus. By the time Saturn appears in the east in early autumn next year it will have undergone another retrograde loop and will appear east (to the left) of Regulus.

Another less convincing twin is that formed by Procyon and the much fainter Gomeisa.

In star catalogue nomenclature Castor is alpha Geminorum and Pollux is beta. But Pollux is clearly brighter than Castor to the unaided eye.

This anomaly of sequence in which a fainter star is labeled above a brighter one according to the Greek alphabet is not uncommon. Various explanations for it are to be found including the idea that the stars have altered in magnitude since they were designated in Bayer's Celestial Atlas (Uranometria) of 1603 using this system. This is not all together convincing and breaks down in extreme cases of rampant disorder such as appears in the constellation Sagittarius.

Another example occurs in the case of Leo where gamma is a little brighter than beta in that constellation. Matters are not helped when catalogues of a stars and computer programmes appear to conflict. Thus, in the case of the two stars just mentioned it is not unusual to find Algieba considerably demoted in the brightness league tables because the database draws on information  which treat this stars a double, which it is of course. Therefore, because the magnitude of the primary is 2.2 (the secondary is 3.5) this star alone would appear fainter than the combined light of the two. But, and it is a valid BUT, the unaided eye perceives gamma as a single star of magnitude 1.99 (Denebola is 2.14).

Another upset occurs with Castor since it too is double (like Algieba, a true binary) the components of which are magnitude 1.9 and 2.9. Their combined magnitude is 1.57 which makes Castor appear significantly brighter than Algieba to the eye. Yet in the table of BRIGHT STARS of the Handbook of the British Astronomical Association, Castor is rated virtually the same as Algieba!

JV 11/12/06