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The star Alpha Geminorum, which appears to the naked eye of magnitude 1.6 but which is actually a complex multiple star. A small telescope divides Castor into two stars of magnitudes 1.9 and 2.9, both A-type dwarfs, which orbit each other with a period of about 450 years. Each is a spectroscopic binary. There is a third star, known as Castor C or YY Geminorum, an eclipsing pair of M1 dwarfs that ranges from magnitude 9.2 to 9.6 in a period of 0.8 days. The six-star system lies 51 l.y. away.

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