Epoch J2000 Equinox J2000
|Right ascension||07h 34m 35.863s|
|Declination||+31° 53′ 17.79″|
|Apparent magnitude (V)||1.93|
|Right ascension||07h 34m 36.100s|
|Declination||+31° 53′ 18.57″|
|Apparent magnitude (V)||2.97|
|Right ascension||07h 34m 37.584s|
|Declination||+31° 53′ 17.8160″|
|Apparent magnitude (V)||9.83|
|Spectral type||A1V + dM1e|
|B−V color index||+0.03|
|Spectral type||Am + dM1e|
|B−V color index||+0.04|
|Spectral type||dM1e + dM1e|
|U−B color index||+1.04|
|B−V color index||+1.49|
|Variable type||BY Dra|
|Proper motion (μ)||RA: –191.45 mas/yr
Dec.: –145.19 mas/yr
|Parallax (π)||64.12 ± 3.75 mas|
|Distance||51 ± 3 ly
(15.6 ± 0.9 pc)
|Radial velocity (Rv)||+6.0 km/s|
|Absolute magnitude (MV)||+0.986|
|Radial velocity (Rv)||–1.2 km/s|
|Absolute magnitude (MV)||+1.886|
|Radial velocity (Rv)||+2.5 km/s|
|Absolute magnitude (MV)||+8.950|
|α Gem Aa|
|Surface gravity (log g)||4.2 cgs|
|Metallicity [Fe/H]||0.98 dex|
|Rotational velocity (v sin i)||18 km/s|
|α Gem Ba|
|Surface gravity (log g)||4.0 cgs|
|Metallicity [Fe/H]||0.45 dex|
|Rotational velocity (v sin i)||33 km/s|
|Surface gravity (log g)||4.6317 cgs|
|Metallicity [Fe/H]||~0.0 dex|
|Rotational velocity (v sin i)||37 km/s|
|Primary||α Gem A|
|Companion||α Gem B|
|Period (P)||445 yr|
|Semi-major axis (a)||7.369"|
|Longitude of the node (Ω)||41.7°|
|Periastron epoch (T)||2401950.650|
|Argument of periastron (ω)
|Primary||α Gem A1|
|Companion||α Gem A2|
|Period (P)||9.2128 days|
|Periastron epoch (T)||2427543.938|
|Argument of periastron (ω)
|Primary||α Gem B1|
|Companion||α Gem B2|
|Period (P)||2.9283 days|
|Periastron epoch (T)||2427501.703|
|Argument of periastron (ω)
|Primary||α Gem AB|
|Companion||α Gem C|
|Period (P)||14,000 yr|
|Primary||α Gem C1|
|Companion||α Gem C2|
|Period (P)||0.814 days|
|Inclination (i)||86.29 ± 0.10°|
|Longitude of the node (Ω)||7.315°|
|A: BD+32°1581A, PLX 1785.00, HR 2891, HD 60179|
|B: BD+32°1581B, HR 2890, HD 60178|
|C: YY Gem, BD +32° 1582|
|SIMBAD||Castor A data|
|SIMBAD||Castor B data|
|SIMBAD||Castor C data|
Castor (α Gem, α Geminorum, Alpha Geminorum) is the second brightest star in the constellation Gemini and one of the brightest stars in the night sky. Although it has the Bayer designation "alpha", it is actually fainter than Beta Geminorum (Pollux).
The Castor System
Castor was recorded as a visual binary 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 form a visual double, with 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 actually 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.
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.
Etymology and culture
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. 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.) The Chinese recognized Castor as Yin, which is, according to the Chinese, one of the two fundamental principles upon which all things depend.
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