Back Castor Afrikaans رأس التوأم المقدم Arabic Cástor (estrella) AST Kastor Azerbaijani Кастар (зорка) Byelorussian Kastor (zvijezda) BS Càstor (estrella) Catalan Castor (hvězda) Czech Castor (Stern) German Κάστορας (αστέρας) Greek

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: −175.88 mas/yr[8]
Dec.: −99.28 mas/yr[8]
Parallax (π)66.356±0.041 mas[8]
Distance49.15±0.027 ly
(15.07±0.01 pc)[8]
Absolute magnitude (MV)+0.986 / +1.886[4]
C
Radial velocity (Rv)+2.5[9] km/s
Proper motion (μ) RA: –201.406 mas/yr[10]
Dec.: –97.000 mas/yr[10]
Parallax (π)66.3110±0.0238 mas[10]
Distance49.19 ± 0.02 ly
(15.080 ± 0.005 pc)
Absolute magnitude (MV)+8.950[11]
Details
α Geminorum Aa
Mass2.371±0.015[8] M☉
Radius2.089±0.005[8] R☉
Surface gravity (log g)4.2[12] cgs
Temperature10,286[13] K
Rotational velocity (v sin i)18[14] km/s
Age290[8] Myr
α Geminorum Ab
Mass0.3859±0.0018[8] M☉
α Geminorum Ba
Mass1.789±0.016[8] M☉
Radius1.648±0.011[8] R☉
Surface gravity (log g)4.0[12] cgs
Temperature8,842[13] K
Rotational velocity (v sin i)33[14] km/s
α Geminorum Bb
Mass0.3865±0.0020[8] M☉
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[8]
Primaryα Geminorum A
Companionα Geminorum B
Period (P)459.1±2.3 yr
Semi-major axis (a)6.722±0.021 "
(101 AU)[a]
Eccentricity (e)0.3382±0.0023
Inclination (i)115.107±0.060°
Longitude of the node (Ω)41.3°
Periastron epoch (T)2,436,785
Argument of periastron (ω)
(secondary)		251.84°
Orbit[8]
Primaryα Geminorum Aa
Companionα Geminorum Ab
Period (P)9.2127496(52) days
Semi-major axis (a)8.002±0.014 mas
(13 R☉)[a]
Eccentricity (e)0.48769(48)
Inclination (i)35.00±0.24°
Longitude of the node (Ω)95.100±0.093°
Periastron epoch (T)HJD−2,400,000 = 55,817.79
Argument of periastron (ω)
(primary)		264.968±0.085°
Semi-amplitude (K1)
(primary)		13.0933±0.0092 km/s
Orbit[8]
Primaryα Geminorum Ba
Companionα Geminorum Bb
Period (P)92.92835083(31) days
Semi-major axis (a)3.4442±0.0093 mas
(5.6 R☉)[a]
Inclination (i)110.50±0.12°
Longitude of the node (Ω)106.47±0.19°
Periastron epoch (T)HJD−2,400,000 = 56,705.49
Semi-amplitude (K1)
(primary)		32.0921±0.0064 km/s
Orbit[15]
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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