Castor (star)

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]
Distance51 ± 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]
Distance49.24 ± 0.04 ly
(15.10 ± 0.01 pc)
Absolute magnitude (MV)+8.950[11]
Details
α Geminorum Aa
Mass2.37[12] M☉
Radius2.089[12] R☉
Surface gravity (log g)4.2[13] cgs
Temperature10,286[14] K
Rotational velocity (v sin i)18[15] km/s
Age290[12] Myr
α Geminorum Ba
Mass1.79[12] M☉
Radius1.648[12] R☉
Surface gravity (log g)4.0[13] cgs
Temperature8,842[14] K
Rotational velocity (v sin i)33[15] km/s
α Geminorum Ca/Cb (components are identical)
Mass0.5992[11] M☉
Radius0.6191[11] R☉
Luminosity0.0733[11] L☉
Surface gravity (log g)4.6317[11] cgs
Temperature3,820[11] K
Metallicity [Fe/H]~0.0[11] dex
Rotational velocity (v sin i)37[11] km/s
Age370[11] Myr
Orbit[12]
Primaryα Geminorum A
Companionα 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]
Primaryα Geminorum Aa
Companionα 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]
Primaryα Geminorum Ba
Companionα 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]
Primaryα Geminorum AB
Companionα Geminorum C
Period (P)14,000 yr
Orbit[11]
Primaryα Geminorum Ca
Companionα 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
SIMBADdata
Castor A
Castor B
Castor C (YY Gem)

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 half a magnitude fainter than 'β' (beta) Geminorum, Pollux.

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