Castor (star)

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**Castor Aa/Ba**
Observation data Equinox J2000
Constellation	Gemini
Right ascension	07h 34m 36s
Declination	+31° 53' 18"
Apparent magnitude (V)	1.96 / 2.91
Characteristics
Spectral type	A1 V / A2 Vm
U-B color index	0.02
B-V color index	0.04
Variable type
Astrometry
Radial velocity (R_v)	5.2 / −1.2 km/s
Proper motion (μ)	RA: −206.33 mas/yr Dec.: −148.18 mas/yr
Parallax (π)	65.50 ± 1.10 mas
Distance	49.8 ± 0.8 ly (15.3 ± 0.3 pc)
Absolute magnitude (M_V)	1.33 /2.28
Details
Mass	2.15 / 1.7 M_☉
Radius	2.3 / 1.6 R_☉
Luminosity	30 / 14 L_☉
Temperature	10,300 / 8,840 K
Metallicity	950% / 280% Sun
Rotation
Age	200 million years
Other designations
Alpha Geminorum, 66 Geminorum, YY Geminorum, Gl 278, HR 2891, BD +32 1581/2, HD 60178/60179, GCTP 1785.00, LTT 12038, SAO 60198, FK5 287, ADS 6175, Struve 1110, HIP 36850.
Database references
SIMBAD	Castor Aa data
Database references
SIMBAD	Castor Ba 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 nighttime sky. Although it has the Bayer designation "alpha", it is actually fainter than Beta Geminorum (Pollux).

Castor and Pollux are the two "heavenly twin" stars that give the constellation Gemini (meaning "twins" in Latin) its name. The name Castor is traditionally thought by some people to carry the meaning "Beaver", after the generic name of the beaver. Actually the name Castor refers specifically to Castor, one of the twin sons of Zeus and Leda. The star also has the Arabic name Al-Ras al-Taum al-Muqadim, which literally means "The Head of the Foremost Twin". The Chinese recognized Castor as Yin, which is, according to the Chinese, one of the two fundamental principles upon which all things depend.

Astronomically, Castor was discovered to be a visual binary in 1678, with the magnitude of its components being 2.8 and 2.0. The separation of the components is about 6" and the period of revolution is around 350 years. Each of the components of Castor is itself a spectroscopic binary, making Castor a quadruple star system. Castor has a faint companion separated from it by about 72" but having the same parallax and proper motion; this companion is also a spectroscopic binary with a period slightly less than 1 day. Castor can thus be considered to be a sextuple star system, with six individual stars gravitationally bound together. Component C has the variable star designation YY Geminorum.

Data for the six stars that make up Castor:

Parameter	Star Component
Parameter	Aa	Ab	Ba	Bb	Ca	Cb
Spectral type	A1 V	Unknown (probably M5 V)	A2 Vm	M2 V	M0.5 Ve	M0.5 Ve
Mass (M_☉)	2.15	0.4–0.6	1.7	0.4–0.6	0.62	0.57
Radius (R_☉)	2.3	?	1.6	?	0.76	0.68