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| b-v = +0.04〔 | u-b = | variable = }} Castor (α Gem, α Geminorum, Alpha Geminorum) is the second brightest star in the constellation Gemini and one of the brightest stars in the night sky. Although it has the Bayer designation "alpha", it is actually fainter than Beta Geminorum (Pollux). ==The Castor System== Castor was recorded as a visual binary in 1718 by James Pound. It may have been resolved in 1678 by Cassini. The separation of the two stars has increased from 2" in 1907 to 7" in 1997.〔 The two stars form a visual double, with magnitudes of 1.9 and 3.0. A third star is 73" distant from the main components.〔 It was discovered to vary in brightness with a regular period and was thought to be an eclipsing binary, but the variations are now considered to be due to areas of different brightness on the surface of one or both stars. It was given the variable star designation YY Geminorum.〔 All three of the visual components are actually spectroscopic binaries and Castor is a complex multiple star system made up of six individual stars. Castor A and B both have orbits of a few days with a much fainter companion. The Castor C components orbit in less than a day. Castor C is believed to be in orbit around the bright pair, but with an extremely long period of several thousand years. 〔 The combined apparent magnitude of all six stars is +1.58. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Castor (star)」の詳細全文を読む スポンサード リンク
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