page_title: Castor (star)
section_title: Stellar system
hierarchical_section_title: Castor (star) / Stellar system
caption_attribution_description: English: Photography of the constellation Gemini, the twins, in morning twilight. Planet Saturn was in Gemini in the year 2003.
context_page_description: Castor is the second-brightest object in the zodiac constellation of Gemini and one of the brightest stars in the night sky. It has the Bayer designation α Geminorum, which is Latinised to Alpha Geminorum and abbreviated Alpha Gem or α Gem. It appears singular to the naked eye, but it is actually a sextuple star system organized into three binary pairs, made up of the stars Castor Aa, Castor Ab, Castor Ba, Castor Bb, Castor Ca, and Castor Cb. Although it is the 'α' member of the constellation, it is fainter than 'β' Geminorum, Pollux.
context_section_description: Castor is a multiple star system made up of six individual stars; there are three visual components, all which are spectroscopic binaries. Appearing to the naked eye as a single star, Castor was first recorded as a double star in 1718 by James Pound, but it may have been resolved into at least two sources of light by Cassini as early as 1678. The separation between Castor A and Castor B has increased from about 2″ (2 arcseconds of angular measurement) in 1970 to about 6″ in 2017. These two binary pairs have magnitudes of 1.9 and 3.0.
  Castor Aa and Ba both have orbits of a few days with a much fainter companion.
  Castor C, or YY Geminorum, was discovered to vary in brightness with a regular period. It is an eclipsing binary with additional variations due to areas of different brightness on the surface of one or both stars, as well as irregular flares. The Castor C components orbit in less than a day. Castor C is believed to be in orbit around Castor AB, but with an extremely long period of several thousand years. It is 73″ distant from the bright components.
  The combined apparent magnitude of all six stars is +1.58.
caption: Castor is one of the two brightest stars in the constellation of Gemini (top left star).
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