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

<div style="float:left; width:290px; border:solid black 1px; text-align: center; margin: 8px; padding: 4px; font-size: 90%;"> Hierarchy of orbits in the Castor system Castor is a [multiple star system](https://bliptext.com/articles/multiple-star-system) made up of six individual stars; there are three visual components, all of which are [spectroscopic binaries](https://bliptext.com/articles/spectroscopic-binaries). Appearing to the naked eye as a single star, Castor was first recorded as a [double star](https://bliptext.com/articles/double-star) in 1718 by [James Pound](https://bliptext.com/articles/james-pound), but it may have been resolved into at least two sources of light by [Cassini](https://bliptext.com/articles/giovanni-domenico-cassini) as early as 1678. The separation between the [binary systems](https://bliptext.com/articles/binary-star) Castor A and Castor B has increased from about 2″ (2 [arcseconds](https://bliptext.com/articles/arcsecond) of angular measurement) in 1970 to about 6″ in 2017. These pairs have [magnitudes](https://bliptext.com/articles/apparent-magnitude) of 1.9 and 3.0, respectively. 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](https://bliptext.com/articles/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.

Physical properties

Castor is 51 light-years away from Earth, determined from its large annual parallax. The two brightest stars are both A-type 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. Castor B is an Am star, with particularly strong spectral lines of certain metals. Castor C is a variable star, classified as a BY Draconis type. BY Draconis variables are cool dwarf stars which vary as they rotate due to starspots or other variations in their photospheres. The two red dwarfs of Castor C are almost identical, with masses around and luminosities less than 10% of the Sun. 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 system'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. 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, Rho Geminorum, and Pollux. Consequently, Castor itself is known as 北河二 (Běi Hé èr, .) 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 the star α Geminorum Aa. Castor C also has the variable-star designation YY Geminorum.

Candidate brown dwarf

A 2018 study proposed the existence of a possible circumbinary brown dwarf companion to YY Geminorum (Castor C) based on eclipse timing variations. This object would have a minimum mass of 49 Jupiter mass and an orbital period of 54.2 years. Weak evidence of timing variations was suspected as early as 2002.