Castor (star)

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Gemini constellation map.svg
Castor within the constellation Gemini
Observation data
Epoch J2000      Equinox J2000
Constellation Gemini
Right ascension 07h 34m 35.863s[1]
Declination +31° 53′ 17.79″[1]
Apparent magnitude (V) 1.93[1]
Right ascension 07h 34m 36.100s[1]
Declination +31° 53′ 18.57″[1]
Apparent magnitude (V) 2.97[1]
Right ascension 07h 34m 37.584s[1]
Declination +31° 53′ 17.8160″[1]
Apparent magnitude (V) 9.83[1]
Spectral type A1V + dM1e[2]
B−V color index +0.03[3]
Spectral type Am + dM1e[2]
B−V color index +0.04[3]
Spectral type dM1e + dM1e[2]
U−B color index +1.04[4]
B−V color index +1.49[4]
Variable type BY Dra[5]
Proper motion (μ) RA: –191.45[6] mas/yr
Dec.: –145.19[6] mas/yr
Parallax (π) 64.12 ± 3.75[6] mas
Distance 51 ± 3 ly
(15.6 ± 0.9 pc)
Radial velocity (Rv) +6.0[7] km/s
Absolute magnitude (MV) +0.986[3]
Radial velocity (Rv) –1.2[7] km/s
Absolute magnitude (MV) +1.886[3]
Radial velocity (Rv) +2.5[8] km/s
Absolute magnitude (MV) +8.950[9]
α Gem Aa
Mass 2.76[10] M
Radius 2.4[11] R
Surface gravity (log g) 4.2[11] cgs
Temperature 10,286[12] K
Metallicity [Fe/H] 0.98[12] dex
Rotational velocity (v sin i) 18[13] km/s
α Gem Ba
Mass 2.98[10] M
Radius 3.3[11] R
Surface gravity (log g) 4.0[11] cgs
Temperature 8,842[12] K
Rotational velocity (v sin i) 33[13] km/s
Mass 0.5992 / 0.5971[9] M
Radius 0.6191[9] R
Luminosity 0.0733[9] L
Surface gravity (log g) 4.6317[9] cgs
Temperature 3,820[9] K
Metallicity [Fe/H] ~0.0[9] dex
Rotational velocity (v sin i) 37[9] km/s
Age 370[9] Myr
Primary α Gem A
Companion α Gem B
Period (P) 445[10] yr
Semi-major axis (a) 7.369″
Eccentricity (e) 0.360
Inclination (i) 112.9°
Longitude of the node (Ω) 41.7°
Periastron epoch (T) 2401950.650
Argument of periastron (ω)
Primary α Gem Aa
Companion α Gem Ab
Period (P) 9.2128 days
Eccentricity (e) 0.5
Periastron epoch (T) 2427543.938
Argument of periastron (ω)
Semi-amplitude (K1)
12.9 km/s
Primary α Gem Ba
Companion α Gem Bb
Period (P) 2.9283 days
Periastron epoch (T) 2427501.703
Argument of periastron (ω)
Semi-amplitude (K1)
31.9 km/s
Primary α Gem AB
Companion α Gem C
Period (P) 14,000 yr
Semi-amplitude (K1)
121.0 km/s
Semi-amplitude (K2)
119.0 km/s
Primary α Gem Ca
Companion α Gem Cb
Period (P) 0.814 days
Eccentricity (e) 0
Inclination (i) 86.29 ± 0.10°
Longitude of the node (Ω) 7.315°
Other designations
Castor, α Gem, 66 Gem, FK5 287, Gliese 278, HIP 36850, SAO 60198
A: BD+32°1581A, PLX 1785.00, HR 2891, HD 60179
B: BD+32°1581B, HR 2890, HD 60178
C: YY Gem, BD +32° 1582
Database references
Castor B
Castor C (YY Gem)

Castor, also designated Alpha Geminorum (α Geminorum, abbreviated Alpha Gem, α Gem) is the second-brightest star in the constellation of Gemini and one of the brightest stars in the night sky. Although it has the identifier 'alpha', it is fainter than Beta Geminorum (Pollux).

The Castor System[edit]

Castor A and B

Castor was recorded as a double star in 1718 by James Pound. It may have been resolved in 1678 by Cassini. The separation of the two stars has increased from 2" in 1970[15] to about 6" in 2017.[14] The two stars have magnitudes of 1.9 and 3.0.

A third star is 73" distant from the main components.[14] It was discovered to vary in brightness with a regular period and was thought to be an eclipsing binary, but the variations are now considered to be due to areas of different brightness on the surface of one or both stars. It was given the variable star designation YY Geminorum.[9]

All three of the visual components are spectroscopic binaries and Castor is a complex multiple star system made up of six individual stars. Castor A and B both have orbits of a few days with a much fainter companion. The Castor C components orbit in less than a day. Castor C is believed to be in orbit around the bright pair, but with an extremely long period of several thousand years.[14]

The combined apparent magnitude of all six stars is +1.58.

Physical characteristics[edit]

Castor is 51 light-years away from Earth, determined from its large annual parallax. The two brightest stars are both A-class 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.[14] The two red dwarfs of Castor C are almost identical, with masses around a half M and luminosities less than 10% of the Sun.[9]

Castor B is an Am star, with particularly strong spectral lines of certain metals.

Castor C is a variable star, classified as a BY Dra type. BY Draconis variables are cool dwarf stars which vary as they rotate due to star spots or other variations in their photospheres.

All the red dwarfs in the Castor system have emissions lines in their spectra, and all are Flare stars.[11]

Etymology and culture[edit]

α Geminorum (Latinised to Alpha Geminorum) is the star'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. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[16] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[17] included a table of the first two batches of names approved by the WGSN; which included Castor for this star.

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.[18]

In Chinese, 北河 (Běi Hé), meaning North River, refers to an asterism consisting of Castor, ρ Geminorum, and Pollux.[19] Consequently, Castor itself is known as 北河二 (Běi Hé èr, English: the Second Star of North River.)[20]


  1. ^ a b c d e f g h i Fabricius, C.; et al. (March 2002), "The Tycho double star catalogue", Astronomy and Astrophysics, 384: 180–189, Bibcode:2002A&A...384..180F, doi:10.1051/0004-6361:20011822 
  2. ^ a b c Pourbaix, D.; Tokovinin, A. A.; Batten, A. H.; Fekel, F. C.; Hartkopf, W. I.; Levato, H.; Morrell, N. I.; Torres, G.; Udry, S. (2004). "SB9: The ninth catalogue of spectroscopic binary orbits". Astronomy and Astrophysics. 424 (2): 727–732. Bibcode:2004A&A...424..727P. ISSN 0004-6361. arXiv:astro-ph/0406573Freely accessible. doi:10.1051/0004-6361:20041213. 
  3. ^ a b c d Barrado y Navascues, D. (1998). "The Castor moving group. The age of Fomalhaut and VEGA". Astronomy and Astrophysics. 339: 831. Bibcode:1998A&A...339..831B. arXiv:astro-ph/9905243Freely accessible. 
  4. ^ a b Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0. Bibcode:2002yCat.2237....0D. 
  5. ^ Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007–2013)". VizieR On-line Data Catalog: B/gcvs. Originally published in: 2009yCat....102025S. 1: 02025. Bibcode:2009yCat....102025S. 
  6. ^ a b c van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, Bibcode:2007A&A...474..653V, arXiv:0708.1752Freely accessible, doi:10.1051/0004-6361:20078357 
  7. ^ a b Evans, D. S. (1967). "The Revision of the General Catalogue of Radial Velocities". In Batten, Alan Henry; Heard, John Frederick. Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30 held at the University of Toronto 20-24 June, 1966. Determination of Radial Velocities and their Applications. 30. Academic Press, London. p. 57. Bibcode:1967IAUS...30...57E. 
  8. ^ Wilson, Ralph Elmer (1953), "General Catalogue of Stellar Radial Velocities", Washington, Washington: Carnegie Institution of Washington: 0, Bibcode:1953GCRV..C......0W. 
  9. ^ a b c d e f g h i j k l Torres, Guillermo; Ribas, Ignasi (2002). "Absolute Dimensions of the M‐Type Eclipsing Binary YY Geminorum (Castor C): A Challenge to Evolutionary Models in the Lower Main Sequence". The Astrophysical Journal. 567 (2): 1140–1165. Bibcode:2002ApJ...567.1140T. ISSN 0004-637X. arXiv:astro-ph/0111167Freely accessible. doi:10.1086/338587. 
  10. ^ a b c Tokovinin, A. (September 2008), "Comparative statistics and origin of triple and quadruple stars", Monthly Notices of the Royal Astronomical Society, 389 (2): 925–938, Bibcode:2008MNRAS.389..925T, arXiv:0806.3263Freely accessible, doi:10.1111/j.1365-2966.2008.13613.x 
  11. ^ a b c d e Stelzer, B.; Burwitz, V. (May 2003), "Castor A and Castor B resolved in a simultaneous Chandra and XMM-Newton observation", Astronomy and Astrophysics, 402 (2): 719–728, Bibcode:2003A&A...402..719S, arXiv:astro-ph/0302570Freely accessible, doi:10.1051/0004-6361:20030286 
  12. ^ a b c Smith, M. A. (April 1974), "Metallicism in border regions of the Am domain. III. Analysis of the hot stars Alpha Geminorum A and B and Theta Leonis", Astrophysical Journal, 189: 101–111, Bibcode:1974ApJ...189..101S, doi:10.1086/152776 
  13. ^ a b Royer, F.; Zorec, J.; Gómez, A. E. (February 2007), "Rotational velocities of A-type stars. III. Velocity distributions", Astronomy and Astrophysics, 463 (2): 671–682, Bibcode:2007A&A...463..671R, arXiv:astro-ph/0610785Freely accessible, doi:10.1051/0004-6361:20065224 
  14. ^ a b c d e f g h Tokovinin, A. A. (1997). "MSC - a catalogue of physical multiple stars". Astronomy and Astrophysics Supplement Series. 124 (1): 75–84. Bibcode:1997A&AS..124...75T. ISSN 0365-0138. doi:10.1051/aas:1997181. 
  15. ^ Heintz, W. D. (1980). "Micrometer Observations of Double Stars and New Pairs - Part Ten". The Astrophysical Journal Supplement Series. 44: 111. Bibcode:1980ApJS...44..111H. ISSN 0067-0049. doi:10.1086/190686. 
  16. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016. 
  17. ^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016. 
  18. ^ Knobel, E. B. (June 1895). "Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of Mohammad Al Achsasi Al Mouakket". Monthly Notices of the Royal Astronomical Society. 55 (8): 429. Bibcode:1895MNRAS..55..429K. doi:10.1093/mnras/55.8.429. 
  19. ^ (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  20. ^ (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived September 29, 2009, at the Wayback Machine., Hong Kong Space Museum. Accessed on line November 23, 2010.

External links[edit]

  • "Castor 6". SolStation. Retrieved December 5, 2005. 

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