Thanks to James B. Kaler. These contents are the property of the author and are reproduced from original without the author's express consent because of fair use and valid educational purposes.
In classical mythology, Castor is the mortal twin of Pollux, the twin warriors whose stars dominate the bright zodiacal constellation of Gemini. Though Castor is the fainter of the two, it still received the Alpha designation from Bayer, who made Pollux the Beta star. Castor's intimate mythological and celestial association with Pollux commonly lofts it into the "first magnitude category, though in fact it is the brightest of the second magnitude stars, coming in just behind Adhara in Canis Major. Castor and Pollux make a most attractive sight at the northern end of Gemini, Pollux an orange giant star, Castor a contrasting white. To the naked eye, Castor shines down to us as a seemingly ordinary hydrogen-fusing "class A" star that appears much like Vega, the "A" stars fairly hot, with temperatures between about 7000 and 10,000 degrees Kelvin. Castor has no physical relation with Pollux, and at a distance of 52 light years is half again as far away as its mythological companion. The telescope reveals Castor's real claim to fame as a remarkable multiple star. Even a modest amateur instrument shows bright Castor to consist of a pair of similar stars only a couple seconds of arc apart. The brighter is mid- second magnitude, the fainter mid-third, both of them class A, the fainter one the cooler. About a minute of arc away to the south lies a ninth magnitude third companion. The bright pair are in elliptical orbit about each other with a 400 year period, and are now about as close in the sky as they can get, making them something of a challenge to separate. The dim companion is so far away from the bright pair, about 1000 times the distance between the Earth and the Sun, that its orbital motion is too slow to have been seen. Now look deeper, with the spectrograph, which separates light into its component colors. With this instrument, we find each of the two bright components, Castor A and Castor B, is yet again double! Castor A consists of almost identical stars, each with about two solar masses, in a 9.2 day orbit about each other, the stars about a tenth of Mercury's distance from the Sun apart. Castor B's twin stars orbit even faster, making their circuit in a mere 2.9 days. Moreover, the faint distant member, Castor C, is ALSO double! It too consists of nearly identical stars, though much cooler low-mass "class M" stars with temperatures of only about 4000 Kelvin. The two are only about two solar diameters apart and orbit each other every 20 hours. One or both are "flare stars," stars that will suddenly change their brightnesses as a result of erratic surface magnetism. How ironic that one of the "twins" should in fact be made of three sets of twins, Castor certainly the sky's ranking sextuple, double- double-double, star. Such stars are thought to be made when their contracting birth-clouds divide, then divide and each divide again as a result of rapid rotation. Castor may be part of a hugely extended, physically related, group of stars called the "Castor Moving Group" that includes Vega, Fomalhaut, Zubenelgenubi, and Alderamin in Cepheus. The properties of the whole set suggest an age, and an age for its member stars, of about 200 million years.