Gamma Geminorum

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γ Geminorum
Gemini constellation map.svg
Red circle.svg
Location of γ Geminorum (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Gemini
Right ascension  06h 37m 42.71050s[1]
Declination +16° 23′ 57.4095″[1]
Apparent magnitude (V) 1.915[2]
Spectral type A1.5IV+
U−B color index +0.137[2]
B−V color index +0.005[2]
Radial velocity (Rv)-12.5[3] km/s
Proper motion (μ) RA: +13.81[1] mas/yr
Dec.: -54.96[1] mas/yr
Parallax (π)29.84 ± 2.23[1] mas
Distance109 ± 8 ly
(34 ± 3 pc)
Absolute magnitude (MV)−0.68[4]
Period (P)4,614.51 days
Eccentricity (e)0.8933 ± 0.0013
Periastron epoch (T)244399.13 ± 0.77
Argument of periastron (ω)
312.60 ± 0.60°
Semi-amplitude (K1)
11.881 ± 0.068 km/s
Mass2.81[6] M
Radius3.3[7] R
Luminosity123[7] L
Surface gravity (log g)3.60[8] cgs
Temperature9,260[8] K
Metallicity [Fe/H]–0.12[8] dex
Rotational velocity (v sin i)10.7±0.2[9] km/s
Other designations
Alhena, 24 Geminorum, BD+16°1223, FK5 251, GCTP1539.00, HIP 31681, HD 47105, HR 2421, SAO 95912[1][10]
Database references

Gamma Geminorum (γ Geminorum, abbreviated Gamma Gem, γ Gem), formally named Alhena /ælˈhnə/,[11] is the third-brightest object in the constellation of Gemini. It has an apparent visual magnitude of 1.9,[2] making it easily visible to the naked eye even in urban regions. Based upon parallax measurements with the Hipparcos satellite, it is located at a distance of roughly 109 light-years (33 parsecs) from the Sun.[1]


Alhena is an evolving star that is exhausting the supply of hydrogen at its core and has entered the subgiant stage. The spectrum matches a stellar classification of A0 IV.[12] Compared to the Sun it has 2.8[6] times the mass and 3.3[7] times the radius. It is radiating around 123[7] times the luminosity of the Sun from its outer envelope at an effective temperature of 9,260 K.[8] This gives it a white hue typical of an A-class star.[13]

Alhena is a spectroscopic binary system with a period of 12.6 years (4,614.51 days) in a highly eccentric Keplerian orbit.[5]


γ Geminorum (Latinised to Gamma Geminorum) is the star's Bayer designation. The traditional name Alhena is derived from the Arabic الهنعة Al Han'ah, 'the brand' (on the neck of the camel), whilst the alternate name Almeisan is from the Arabic المیسان Al Maisan, 'the shining one.' Al Hanʽah was the name of star association consisting of this star, along with Mu Geminorum (Tejat Posterior), Nu Geminorum, Eta Geminorum (Tejat Prior) and Xi Geminorum (Alzirr). They also were associated in Al Nuḥātai, the dual form of Al Nuḥāt, 'a Camel's Hump'.[14] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[15] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[16] included a table of the first two batches of names approved by the WGSN; which included Alhena for this star.

In the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, this star was designated Nir al Henat, which was translated into Latin as Prima του al Henat, meaning 'the brightest of Al Henat'.[17]

In Chinese, 井宿 (Jǐng Su), meaning Well (asterism), refers to an asterism consisting of γ Geminorum, ε Geminorum, ζ Geminorum, λ Geminorum, μ Geminorum, ν Geminorum, ξ Geminorum and 36 Geminorum.[18] Consequently, γ Geminorum itself is known as 井宿三 (Jǐng Su sān, English: the Third Star of Well.)[19]

In culture[edit]

Alhena was the name of a Dutch ship that rescued many people from an Italian cruise liner, the SS Principessa Mafalda, in October 1927.[20] In addition, the American attack cargo ship USS Alhena (AKA-9) was named after the star.


  1. ^ a b c d e f g 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. ^ a b c d Gutierrez-Moreno, Adelina; et al. (1966), "A System of photometric standards", Publ. Dept. Astron. Univ. Chile, Publicaciones Universidad de Chile, Department de Astronomy, 1: 1–17, Bibcode:1966PDAUC...1....1G
  3. ^ Evans, D. S. (June 20–24, 1966). "The Revision of the General Catalogue of Radial Velocities". In Batten, Alan Henry; Heard, John Frederick (eds.). Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30. Determination of Radial Velocities and their Applications. 30. University of Toronto: International Astronomical Union. p. 57. Bibcode:1967IAUS...30...57E.
  4. ^ Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015.
  5. ^ a b Lehmann, H.; et al. (February 2002), "The spectroscopic binaries 21 Her and gamma Gem", Astronomy and Astrophysics, 383 (2): 558–567, Bibcode:2002A&A...383..558L, doi:10.1051/0004-6361:20011746
  6. ^ a b Shaya, Ed J.; Olling, Rob P. (January 2011), "Very Wide Binaries and Other Comoving Stellar Companions: A Bayesian Analysis of the Hipparcos Catalogue", The Astrophysical Journal Supplement, 192 (1): 2, arXiv:1007.0425, Bibcode:2011ApJS..192....2S, doi:10.1088/0067-0049/192/1/2
  7. ^ a b c d Malagnini, M. L.; Morossi, C. (November 1990), "Accurate absolute luminosities, effective temperatures, radii, masses and surface gravities for a selected sample of field stars", Astronomy and Astrophysics Supplement Series, 85 (3): 1015–1019, Bibcode:1990A&AS...85.1015M
  8. ^ a b c d Adelman, Saul J.; Philip, A. G. Davis (December 1994), "Elemental abundances of the B and A stars. 2: Gamma Geminorum, HD 60825, 7 Sextantis, HR 4817, and HR 5780", Publications of the Astronomical Society of the Pacific, 106 (706): 1239–1247, Bibcode:1994PASP..106.1239A, doi:10.1086/133501
  9. ^ Gray, David F. (April 2014), "Precise Rotation Rates for Five Slowly Rotating a Stars", The Astronomical Journal, 147 (4): 13, Bibcode:2014AJ....147...81G, doi:10.1088/0004-6256/147/4/81, 81
  10. ^ "ALHENA -- Spectroscopic binary", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-02-04
  11. ^ "IAU Catalog of Star Names". Retrieved 28 July 2016.
  12. ^ Cowley, A.; et al. (April 1969), "A study of the bright A stars. I. A catalogue of spectral classifications", Astronomical Journal, 74: 375–406, Bibcode:1969AJ.....74..375C, doi:10.1086/110819
  13. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on 2012-03-10, retrieved 2012-01-16
  14. ^ Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York: Dover Publications Inc. p. 234. ISBN 0-486-21079-0. Retrieved 2010-12-12.
  15. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  16. ^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
  17. ^ 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.
  18. ^ (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.
  19. ^ (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived October 25, 2008, at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
  20. ^ Italian account of the Principessa Malfeda.