Castor (star)

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Gemini constellation map.svg
The position of Castor within the Gemini Constellation.
Observation data
Epoch J2000      Equinox J2000
Constellation Gemini
Right ascension 07h 34m 35.87319s[1]
Declination +31° 53′ 17.8160″[1]
Apparent magnitude (V) 1.93 / 2.97[2]
Spectral type Am + A1V
U−B color index 0.02
B−V color index 0.04
Radial velocity (Rv) +6.0 / –1.2[3] km/s
Proper motion (μ) RA: –191.45[1] mas/yr
Dec.: –145.19[1] mas/yr
Parallax (π) 64.12 ± 3.75[1] mas
Distance 51 ± 3 ly
(15.6 ± 0.9 pc)
α Gem Aa
Mass 2.76[4] M
Radius 2.4[5] R
Surface gravity (log g) 4.2[5] cgs
Temperature 10,286[6] K
Metallicity [Fe/H] 0.98[6] dex
Rotational velocity (v sin i) 18[7] km/s
α Gem Ba
Mass 2.98[4] M
Radius 3.3[5] R
Surface gravity (log g) 4.0[5] cgs
Temperature 8,842[6] K
Rotational velocity (v sin i) 33[7] km/s
Other designations
Castor, α Gem, 66 Gem, YY Gem, BD +32 1581/2, FK5 287, GCTP 1785.00, Gl 278, HD 60178/60179, HIP 36850, HR 2891, LTT 12038, SAO 60198.
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 night sky. Although it has the Bayer designation "alpha", it is actually fainter than Beta Geminorum (Pollux). Castor is 49.8 light years away from Earth.

Physical characteristics[edit]

Castor was discovered to be a visual binary in 1678, with the magnitude of its components being 2.0 and 2.9 (the combined magnitude is 1.58). The separation of the components is currently about 5", 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,[8] with six individual stars gravitationally bound together. Component C has the variable star designation YY Geminorum.

The Castor system[edit]

Parameter Star Component[citation needed]
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[edit]

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. 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.[9]

In Chinese, 北河 (Běi Hé), meaning North River, refers to an asterism consisting of Castor, ρ Geminorum, and Pollux.[10] Consequently, Castor itself is known as 北河二 (Běi Hé èr, English: the Second Star of North River.)[11] The Chinese recognized Castor as Yin, which is, according to the Chinese, one of the two fundamental principles upon which all things depend.[citation needed]


  1. ^ a b c d e van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357 
  2. ^ 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 
  3. ^ Evans, D. S., "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, Academic Press, London, p. 57, Bibcode:1967IAUS...30...57E 
  4. ^ a b Tokovinin, A. (September 2008), "Comparative statistics and origin of triple and quadruple stars", Monthly Notices of the Royal Astronomical Society 389 (2): 925–938, arXiv:0806.3263, Bibcode:2008MNRAS.389..925T, doi:10.1111/j.1365-2966.2008.13613.x 
  5. ^ a b c d Stelzer, B.; Burwitz, V. (May 2003), "Castor A and Castor B resolved in a simultaneous Chandra and XMM-Newton observation", Astronomy and Astrophysics 402: 719–728, arXiv:astro-ph/0302570, Bibcode:2003A&A...402..719S, doi:10.1051/0004-6361:20030286 
  6. ^ 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 
  7. ^ 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, arXiv:astro-ph/0610785, Bibcode:2007A&A...463..671R, doi:10.1051/0004-6361:20065224 
  8. ^ MSC/Tokovinin HD 60178
  9. ^ 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: 429. Bibcode:1895MNRAS..55..429K. doi:10.1093/mnras/55.8.429. 
  10. ^ (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.
  11. ^ (Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, 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″