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 2017.[17][15] 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.
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.[11] 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.[15]
The combined apparent magnitude of all six stars is +1.58.
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 they are known to have masses 39% that of the Sun.[8]
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 a half M☉ and luminosities less than 10% of the Sun.[11] Since 2018 it is suspected a brown dwarf with a mass at least 49±7 times the mass of Jupiter might be orbiting Castor C with a period of 50 years. If it is confirmed, Castor would turn out to be a seven-star system.[19]
All the red dwarfs in the Castor system have emissions lines in their spectra, and all are flare stars.[12]
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.[20]
In Chinese, 北河 (Běi Hé), meaning North River, refers to an asterism consisting of Castor, Rho Geminorum, and Pollux.[21] Consequently, Castor itself is known as 北河二 (Běi Hé èr, English: the Second Star of North River.)[22]
In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[23] 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.[24]
^ abcThe linear semi-major axis was calculated multiplying the angular semi-major axis (in arcsecs) to the distance (in parsecs). Values in milliarcseconds/mas should be divided by 1000 to convert to arcsec. The retrieved value will be the diameter in astronomical units, and need to be multiplied by 107.5 to convert from AU to R☉.
References
^
Kunitzsch, Paul; Smart, Tim, eds. (2006). A Dictionary of Modern star Names: A short guide to 254 star names and their derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN978-1-931559-44-7.
^ ab
Ducati, J. R. (2002). Ducati, J.R. (ed.). "VizieR Online Data Catalog: Catalogue of stellar photometry in Johnson's 11 color system". Vizier Online Data Catalog. CDS/ADC Collection of Electronic Catalogues. 2237. Bibcode:2002yCat.2237....0D.
^
Samus, N.N.; Durlevich, O.V.; et al. (2009). "VizieR Online Data Catalog: General catalogue of variable stars (Samus+ 2007–2013)". Vizier Online Data Catalog. VizieR On-line Data Catalog: B/GCVS. 1: 02025. Bibcode:2009yCat....102025S. Originally published in: 2009yCat....102025S
^ ab
Smith, M.A. (April 1974). "Metallicism in border regions of the Am domain. III. Analysis of the hot stars Alpha Geminorum A and B and Theta Leonis". Astrophysical Journal. 189: 101–111. Bibcode:1974ApJ...189..101S. doi:10.1086/152776.
^(in Chinese)中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN978-986-7332-25-7.
^"亮星中英對照表" [Chinese-English Comparison Table of Bright Stars]. 研究資源 [Research Resources] (in Chinese). 香港太空館 [Hong Kong Space Museum]. Archived from the original on 29 September 2009. Retrieved 23 November 2010.
