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 fainter than 'β' (beta) Geminorum, Pollux.

Quick Facts Observation data Epoch J2000 Equinox J2000, Pronunciation ...

Castor

Location of Castor (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Gemini
Pronunciation	/ˈkæstər/[1]
A
Right ascension	07h 34m 35.863s[2]
Declination	+31° 53′ 17.79″[2]
Apparent magnitude (V)	1.93[2]
B
Right ascension	07h 34m 36.100s[2]
Declination	+31° 53′ 18.57″[2]
Apparent magnitude (V)	2.97[2]
C
Right ascension	07h 34m 37.584s[2]
Declination	+31° 53′ 17.8160″[2]
Apparent magnitude (V)	9.83[2]
Characteristics
A
Spectral type	A1V + dM1e[3]
B−V color index	+0.03[4]
B
Spectral type	Am + dM1e[3]
B−V color index	+0.04[4]
C
Spectral type	dM1e + dM1e[3]
U−B color index	+1.04[5]
B−V color index	+1.49[5]
Variable type	BY Dra[6]
Astrometry
AB
Radial velocity (Rv)	5.40[7] km/s
Proper motion (μ)	RA: –191.45[8] mas/yr Dec.: –145.19[8] mas/yr
Parallax (π)	64.12 ± 3.75 mas[8]
Distance	51 ± 3 ly (15.6 ± 0.9 pc)
Absolute magnitude (MV)	+0.986 / +1.886[4]
C
Radial velocity (Rv)	+2.5[9] km/s
Proper motion (μ)	RA: –201.490[10] mas/yr Dec.: –97.104[10] mas/yr
Parallax (π)	66.2323 ± 0.0511 mas[10]
Distance	49.24 ± 0.04 ly (15.10 ± 0.01 pc)
Absolute magnitude (MV)	+8.950[11]
Details
α Geminorum Aa
Mass	2.37[12] M☉
Radius	2.089[12] R☉
Surface gravity (log g)	4.2[13] cgs
Temperature	10,286[14] K
Rotational velocity (v sin i)	18[15] km/s
Age	290[12] Myr
α Geminorum Ba
Mass	1.79[12] M☉
Radius	1.648[12] R☉
Surface gravity (log g)	4.0[13] cgs
Temperature	8,842[14] K
Rotational velocity (v sin i)	33[15] km/s
α Geminorum Ca/Cb (components are identical)
Mass	0.5992[11] M☉
Radius	0.6191[11] R☉
Luminosity	0.0733[11] L☉
Surface gravity (log g)	4.6317[11] cgs
Temperature	3,820[11] K
Metallicity [Fe/H]	~0.0[11] dex
Rotational velocity (v sin i)	37[11] km/s
Age	370[11] Myr
Orbit[12]
Primary	α Geminorum A
Companion	α Geminorum B
Period (P)	459 yr
Semi-major axis (a)	6.722″
Eccentricity (e)	0.338
Inclination (i)	115.1°
Longitude of the node (Ω)	41.3°
Periastron epoch (T)	2436785
Argument of periastron (ω) (secondary)	251.84°
Orbit[16]
Primary	α Geminorum Aa
Companion	α Geminorum Ab
Period (P)	9.2128 days
Eccentricity (e)	0.5
Periastron epoch (T)	2427543.938
Argument of periastron (ω) (primary)	266.4°
Semi-amplitude (K1) (primary)	12.9 km/s
Orbit[16]
Primary	α Geminorum Ba
Companion	α Geminorum Bb
Period (P)	2.9283 days
Periastron epoch (T)	2427501.703
Argument of periastron (ω) (primary)	94.7°
Semi-amplitude (K1) (primary)	31.9 km/s
Orbit[16]
Primary	α Geminorum AB
Companion	α Geminorum C
Period (P)	14,000 yr
Orbit[11]
Primary	α Geminorum Ca
Companion	α Geminorum Cb
Period (P)	0.814 days
Eccentricity (e)	0
Inclination (i)	86.29 ± 0.10°
Longitude of the node (Ω)	7.315°
Semi-amplitude (K1) (primary)	121.0 km/s
Semi-amplitude (K2) (secondary)	119.0 km/s
Other designations
Castor, α Gem, 66 Geminorum, FK5 287, GJ 278, HIP 36850, SAO 60198
A: BD+32°1581A, HD 60179, HR 2891, PLX 1785.00
B: BD+32°1581B, HD 60178, HR 2890
C: YY Geminorum, BD+32°1582
Database references
SIMBAD	data
	Castor A
	Castor B
	Castor C (YY Gem)

Close

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.^[18][16] 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.^[16]

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-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.^[16]

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]

All the red dwarfs in the Castor system have emissions lines in their spectra, and all are flare stars.^[13]

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.^[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]

Castor C also has the variable-star designation YY Geminorum.

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• List of nearest bright stars
• Historical brightest stars