Castor or Alpha Geminorum (Alp Gem) is the 2nd brightest naked eye star in the constellation Gemini. With an apparent magnitude of 1.58c, Castor is the 23rd brightest star in the entire sky (see: 50 Brightest Stars ). Its absolute magnitude is 0.59 and its distance is 52 light years. The Equinox J2000 equatorial coordinates are RA = 07h 34m 36.0s, Dec = +31° 53' 18".

Castor has a spectral type of A1V, a surface temperature of 10,300° Kelvin and a luminosity 30 times the Sun. It has a mass of 2.2 solar masses and a diameter 2.3 times the Sun.

The image above shows the uncropped view of Castor (North is up) through the Takahashi E-180 Astrograph.

Castor was discovered to be a visual binary in 1678, with the magnitude of its components being 2.0 and 2.9 (the combined magnitude is 1.58). The separation of the hot white (spectral class A) components is about 6 arc-seconds and the period of revolution is around 467 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 arc-seconds but having the same parallax and proper motion. This companion is an eclipsing binary system with a period slightly less than 1 day, and it is one of only a few known eclipsing binary systems where both companions are class 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 Gemini "twin" stars Castor and Pollux are best seen during northern spring evenings. Unlike real twins, Castor and Pollux have very little in common. While Castor is a white quadruple star with fairly close hot white components, Pollux an orange-colored (spectral type K0IIIb) cool (4770° Kelvin) giant. The close pairing with Castor makies Pollux's color more vivid.

The description above is based on the Castor entry in Wikipedia. For more information about Castor, see Stars (Jim Kaler).

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