Castor (star)

Castor (α Gem / α Geminorum / Alpha Geminorum) is the second brightest star in the constellation Gemini and one of the brightest stars in the nighttime sky. Although it has the Bayer designation "alpha", it is actually fainter than Beta Geminorum (Pollux).

Physical characteristics

Castor was discovered to be a visual binary in 1678, with the magnitude of its components being 2.8 and 2.0. The separation of the components is about 6" and the period of revolution is around 350 years. Each of the components of Castor is itself a spectroscopic binary, making Castor a quadruple star system. Castor has a faint companion separated from it by about 72" but having the same parallax and proper motion; this companion is an eclipsing binary system with a period slightly less than 1 day, it is one of only a few known eclipsing binary systems where both companions are M-dwarf stars. Castor can thus be considered to be a sextuple star system, with six individual stars gravitationally bound together. Component C has the variable star designation YY Geminorum.

The Castor system

Parameter	Star Component
	Aa	Ab	Ba	Bb	Ca	Cb
Spectral type	A1 V	Unknown (probably M5 V)	A2 Vm	M2 V	M0.5 Ve	M0.5 Ve
Mass (M☉)	2.15	0.4–0.6	1.7	0.4–0.6	0.62	0.57
Radius (R☉)	2.3	?	1.6	?	0.76	0.68

Etymology and culture

Castor and Pollux are the two "heavenly twin" stars that give the constellation Gemini (meaning "twins" in Latin) its name. The name Castor is traditionally thought by some people to carry the meaning "Beaver", after the generic name of the beaver. Actually the name Castor refers specifically to Castor, one of the twin sons of Zeus and Leda. The star also has the Arabic name Al-Ras al-Taum al-Muqadim, which literally means "The Head of the Foremost Twin". The Chinese recognized Castor as Yin, which is, according to the Chinese, one of the two fundamental principles upon which all things depend.

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