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Castor (star)

Coordinates: Sky map 07h 34m 36s, +31° 53′ 18″
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Location of Castor (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Gemini
Pronunciation /ˈkæstər/[1]
Right ascension 07h 34m 35.863s[2]
Declination +31° 53′ 17.79″[2]
Apparent magnitude (V) 1.93[2]
Right ascension 07h 34m 36.100s[2]
Declination +31° 53′ 18.57″[2]
Apparent magnitude (V) 2.97[2]
Right ascension 07h 34m 37.584s[2]
Declination +31° 53′ 17.8160″[2]
Apparent magnitude (V) 9.83[2]
Spectral type A1V + dM1e[3]
B−V color index +0.03[4]
Spectral type Am + dM1e[3]
B−V color index +0.04[4]
Spectral type dM1e + dM1e[3]
U−B color index +1.04[5]
B−V color index +1.49[5]
Variable type BY Dra[6]
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]
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]
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
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)
Primaryα Geminorum Aa
Companionα Geminorum 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
Primaryα Geminorum Ba
Companionα Geminorum Bb
Period (P)2.9283 days
Periastron epoch (T)2427501.703
Argument of periastron (ω)
Semi-amplitude (K1)
31.9 km/s
Primaryα Geminorum AB
Companionα Geminorum C
Period (P)14,000 yr
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)
121.0 km/s
Semi-amplitude (K2)
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
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 fainter than 'β' (beta) Geminorum, Pollux.

Stellar system

Period = 9.2128 d
Separation = 3.9″
Period = 2.9283 d
Separation = 71″
Period = 0.814 d

Hierarchy of orbits in the Castor system[17]

Castor is a multiple star system made up of six individual stars; there are three visual components, all of which are spectroscopic binaries. Appearing to the naked eye as a single star, Castor was first recorded as a double star in 1718 by James Pound, but it may have been resolved into at least two sources of light by Cassini as early as 1678. The separation between the binary systems Castor A and Castor B has increased from about 2″ (2 arcseconds of angular measurement) in 1970 to about 6″ in 2017.[18][16] These pairs have magnitudes of 1.9 and 3.0, respectively.

Castor Aa and Ba both have orbits of a few days with a much fainter companion.

Castor C, or YY Geminorum, was discovered to vary in brightness with a regular period. It is an eclipsing binary with additional variations due to areas of different brightness on the surface of one or both stars, as well as irregular flares.[11] The Castor C components orbit in less than a day. Castor C is believed to be in orbit around Castor AB, but with an extremely long period of several thousand years. It is 73″ distant from the bright components.[16]

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

Physical properties


Castor is 51 light-years away from Earth, determined from its large annual parallax.

The two brightest stars are both A-type 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.[16]

A visual band light curve for YY Geminorum (Castor C), adapted from Butler et al. (2015)[19]

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

Castor C is a variable star, classified as a BY Draconis type. BY Draconis variables are cool dwarf stars which vary as they rotate due to starspots or other variations in their photospheres. The two red dwarfs of Castor C are almost identical, with masses around a half M and luminosities less than 10% of the Sun.[11]

All the red dwarfs in the Castor system have emissions lines in their spectra, and all are flare stars.[13]

Etymology and culture


α Geminorum (Latinised to Alpha Geminorum) is the star system'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.

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.[20]

In Chinese, 北河 (Běi Hé), meaning North River, refers to an asterism consisting of Castor, Rho Geminorum, and Pollux.[21] Consequently, Castor itself is known as 北河二 (Běi Hé èr, English: the Second Star of North River.)[22]

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[23] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Castor for the star α Geminorum Aa.[24]

Castor C also has the variable-star designation YY Geminorum.

See also



  1. ^ Kunitzsch, Paul; Smart, Tim, eds. (2006). A Dictionary of Modern star Names: A short guide to 254 star names and their derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
  2. ^ a b c d e f g h i Fabricius, C.; Høg, E.; Makarov, V.V.; Mason, B.D.; Wycoff, G.L.; Urban, S.E. (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. ^ a b c Pourbaix, D.; Tokovinin, A.A.; Batten, A.H.; Fekel, F.C.; Hartkopf, W.I.; Levato, H.; et al. (2004). "SB9: The ninth catalogue of spectroscopic binary orbits". Astronomy and Astrophysics. 424 (2): 727–732. arXiv:astro-ph/0406573. Bibcode:2004A&A...424..727P. doi:10.1051/0004-6361:20041213. ISSN 0004-6361. S2CID 119387088.
  4. ^ a b c Barrado y Navascues, D. (1998). "The Castor moving group. The age of Fomalhaut and Vega". Astronomy and Astrophysics. 339: 831–839. arXiv:astro-ph/9905243. Bibcode:1998A&A...339..831B.
  5. ^ a b Ducati, J. R. (2002). Ducati, J.R. (ed.). "VizieR Online Data Catalog: Catalogue of stellar photometry in Johnson's 11 color system". Vizier Online Data Catalog. CDS/ADC Collection of Electronic Catalogues. 2237. Bibcode:2002yCat.2237....0D.
  6. ^ Samus, N.N.; Durlevich, O.V.; et al. (2009). "VizieR Online Data Catalog: General catalogue of variable stars (Samus+ 2007–2013)". Vizier Online Data Catalog. VizieR On-line Data Catalog: B/GCVS. 1: 02025. Bibcode:2009yCat....102025S. Originally published in: 2009yCat....102025S
  7. ^ Gontcharov, G.A. (November 2006). "Pulkovo compilation of radial velocities for 35,495 Hipparcos stars in a common system". Astronomy Letters. 32 (11): 759–771. arXiv:1606.08053. Bibcode:2006AstL...32..759G. doi:10.1134/S1063773706110065. S2CID 119231169.
  8. ^ a b c 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. S2CID 18759600.
  9. ^ Wilson, Ralph Elmer (1953). "General Catalogue of Stellar Radial Velocities". Carnegie Institute Washington D.C. Publication. Washington, DC: Carnegie Institution of Washington. Bibcode:1953GCRV..C......0W.
  10. ^ a b c Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  11. ^ 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. arXiv:astro-ph/0111167. Bibcode:2002ApJ...567.1140T. doi:10.1086/338587. ISSN 0004-637X. S2CID 16780943.
  12. ^ a b c d e f Torres, Guillermo; et al. (2022). "The Orbits and Dynamical Masses of the Castor System". The Astrophysical Journal. 941 (8): 8. arXiv:2210.16322. Bibcode:2022ApJ...941....8T. doi:10.3847/1538-4357/ac9d8d. hdl:10871/132423.
  13. ^ a b c 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. arXiv:astro-ph/0302570. Bibcode:2003A&A...402..719S. doi:10.1051/0004-6361:20030286. S2CID 15268418.
  14. ^ a b 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.
  15. ^ 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. S2CID 18475298.
  16. ^ a b c d e f 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. doi:10.1051/aas:1997181. ISSN 0365-0138.
  17. ^ Hussain, G.A.J.; Brickhouse, N.S.; Dupree, A.K.; Reale, F.; Favata, F.; Jardine, M.M. (June 2012). "Chandra study of the eclipsing M dwarf binary, YY Gem". Monthly Notices of the Royal Astronomical Society. 423 (1): 493−504. Bibcode:2012MNRAS.423..493H. doi:10.1111/j.1365-2966.2012.20894.x. hdl:10447/73026.
  18. ^ 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. doi:10.1086/190686. ISSN 0067-0049.
  19. ^ Butler, C.J.; Erkan, N.; Budding, E.; Doyle, J.G.; Foing, B.; Bromage, G.E.; et al. (February 2015). "A multiwavelength study of the M dwarf binary YY Geminorum". Monthly Notices of the Royal Astronomical Society. 446 (4): 4205–4219. arXiv:1501.03930. Bibcode:2015MNRAS.446.4205B. doi:10.1093/mnras/stu2398.
  20. ^ 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.
  21. ^ (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.
  22. ^ "亮星中英對照表" [Chinese-English Comparison Table of Bright Stars]. 研究資源 [Research Resources] (in Chinese). 香港太空館 [Hong Kong Space Museum]. Archived from the original on 29 September 2009. Retrieved 23 November 2010.
  23. ^ "IAU Working Group on Star Names (WGSN)". IAU Working Groups. International Astronomical Union. Retrieved 22 May 2016.
  24. ^ "Bulletin of the IAU Working Group on Star Names" (PDF). International Astronomical Union. Retrieved 28 July 2016.