Castor kăs´tər [key], bright star in the constellation Gemini ; Bayer designation α Geminorum; 1992 position R.A. 7 h34.1 m, 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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