, bright star in the constellation Gemini
; Bayer designation α Geminorum; 1992 position R.A. 7h
, 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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