Castor (star)
star in the constellation Gemini
Castor is the second-brightest object in the zodiac constellation of Gemini. It has the Bayer designation α Geminorum, which is Latinised to Alpha Geminorum and abbreviated Alpha Gem or α Gem. With an apparent visual magnitude of 1.58, it is one of the brightest stars in the night sky. Castor appears singular to the naked eye, but it is actually a sextuple star system organized into three binary pairs. Although it is the 'α' (alpha) member of the constellation, it is half a magnitude fainter than 'β' (beta) Geminorum, Pollux.
Stellar system
Castor is a multiple star system made up of six individual stars; there are three visual components, all of 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 the binary systems Castor A and Castor B has increased from about 2″ (2 arcseconds of angular measurement) in 1970 to about 6″ in 2026. These pairs have magnitudes of 1.9 and 3.0, respectively.
Castor Aa and Ba both have orbits of a few days with a much fainter companion.
This brief starts where responsible research should: with the source description of “Castor (star)” as star in the constellation Gemini. Everything that follows is an evidence route, not borrowed authority.
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The subject matters to the general reference register because the source frames it as star in the constellation Gemini. Its deeper value depends on whether names, dates, institutions and citations support that framing.
Named sources, stable identifiers and responsible institutions provide the strongest route from overview to verifiable evidence. The source revision retrieved here is dated Sep 8, 2026. The linked authority identifier is Q13029. VIAF identifies the subject as 7831163464619405680006. None of the 1 selected statements returned an explicit reference. The first chronological checks are 1718, 1678, 1970 and 2026.
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This entry incorporates text from “Castor (star)” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.