CASTOR (Alpha Geminorum). 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 50 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 (1.9), the fainter mid-third
(2.9), both of them class A (A1 and A7), 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 (M1) dwarfs with temperatures of only about 3280 Kelvin and
(from their orbit) masses of 0.60 and 0.62 solar. 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 also be part of a hugely
extended, physically related, group of stars called the "Castor
Moving Group" that includes Vega, Fomalhaut, Zubenelgenubi, and Alderamin. The properties of the whole
set suggest an age, and an age for its member stars, of about 200
million years. Yet there are still questions. The luminosities
and temperatures of Castor A and B show them to be 370 million
years old, while those of Castor C are only 30 to 85 million years old.
Either something is wrong the observation or with theory, or "C"
was not originally part of the system and was somehow added later.