Castor (star): Wikis


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Gemini constellation map.png
The position of Castor within the Gemini Constellation.
Observation data
Epoch J2000      Equinox J2000
Constellation Gemini
Right ascension 07h 34m 36s
Declination +31° 53' 18"
Apparent magnitude (V) 1.96 / 2.91
Spectral type A1 V / A2 Vm
U-B color index 0.02
B-V color index 0.04
Radial velocity (Rv) 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 (MV) 1.33 /2.28
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
Age 200 million years
Other designations
Castor, α Gem, 66 Gem, YY Gem, 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).


Physical characteristics

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 467 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 an eclipsing binary system with a period slightly less than 1 day, and it is one of only a few known eclipsing binary systems where both companions are class M dwarf stars. Castor can thus be considered to be a sextuple star system[1], with six individual stars gravitationally bound together. Component C has the variable star designation YY Geminorum.

The Castor system

Parameter Star Component
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

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 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.


  1. ^ MSC/Tokovinin HD 60178

External links

Coordinates: Sky map 07h 34m 36s, +31° 53′ 18″

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