Primary α Gem A Primary α Gem Aa Primary α Gem Ba | Companion α Gem B Companion α Gem Ab Companion α Gem Bb | |
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Castor, also designated Alpha Geminorum (α Geminorum, abbreviated Alpha Gem, α Gem) is the second-brightest star in the constellation of Gemini and one of the brightest stars in the night sky. Although it has the identifier 'alpha', it is fainter than Beta Geminorum (Pollux).
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The Castor System
Castor was recorded as a double star 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 have 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 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.
Physical characteristics
Castor is 51 light-years away from Earth, determined from its large annual parallax. The two brightest stars are both A-class main-sequence stars, more massive and brighter than the Sun. The properties of their red dwarf companions are difficult to determine, but are both thought to have less than half the mass of the Sun. The two red dwarfs of Castor C are almost identical, with masses around a half M☉ and luminosities less than 10% of the Sun.
Castor B is an Am star, with particularly strong spectral lines of certain metals.
Castor C is a variable star, classified as a BY Dra type. BY Draconis variables are cool dwarf stars which vary as they rotate due to star spots or other variations in their photospheres.
All the red dwarfs in the Castor system have emissions lines in their spectra, and all are Flare stars.
Etymology and culture
α Geminorum (Latinised to Alpha Geminorum) is the star's Bayer designation.
Castor and Pollux are the two "heavenly twin" stars that give the constellation Gemini (meaning twins in Latin) its name. The name Castor refers specifically to Castor, one of the twin sons of Zeus and Leda in Greek and Roman mythology. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Castor for this star.
The star was annotated by the Arabic description Al-Ras al-Taum al-Muqadim, which translates as the head of the foremost twin. In the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, this star was designated Aoul al Dzira, which was translated into Latin as Prima Brachii, meaning the first in the paw.
In Chinese, 北河 (Běi Hé), meaning North River, refers to an asterism consisting of Castor, ρ Geminorum, and Pollux. Consequently, Castor itself is known as 北河二 (Běi Hé èr, English: the Second Star of North River.)