Castor kăs´tər [key], bright star in the constellation Gemini; Bayer designation α Geminorum; 1992 position R.A. 7h34.1m, Dec. +31°54′. Slightly dimmer than Pollux, with which it forms the Twins, Castor has an apparent magnitude of 1.58, which still makes it one of the 25 brightest stars in the sky. Castor is actually a six-star system, being a visual triple each component of which is a binary star; the three components are an eclipsing binary and a pair of spectroscopic binaries. The two brightest components are white, main-sequence stars of spectral classes A1 and A5. The system is about 50 light-years distant.

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