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Orion IAU.svg
Red circle.svg
Location of γ Orionis (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Orion
Pronunciation /ˈbɛlətrɪks/
Right ascension 05h 25m 07.86325s[1]
Declination +06° 20′ 58.9318″[1]
Apparent magnitude (V) 1.64[2] (1.59 - 1.64[3])
Spectral type B2 III[4]
U−B color index –0.86[2]
B−V color index –0.21[2]
Variable type suspected[3]
Radial velocity (Rv)+18.2[5] km/s
Proper motion (μ) RA: –8.11[1] mas/yr
Dec.: –12.88[1] mas/yr
Parallax (π)12.92 ± 0.52[1] mas
Distance250 ± 10 ly
(77 ± 3 pc)
Absolute magnitude (MV)−2.78[6]
Mass8.6[7] M
Radius5.75[8] R
Diameter5.75[8] D
Luminosity9,211[8] L
Surface gravity (log g)3.60[9] cgs
Temperature22000[9] K
Metallicity [Fe/H]–0.07[8] dex
Rotational velocity (v sin i)46±8[9] km/s
Age25.2[7] Myr
Other designations
Bellatrix, γ Orionis, Amazon Star,[10] 24 Ori, Al Najīd,[10] HR 1790, BD+06°919, HD 35468, SAO 112740, FK5 201, HIP 25336[11]
Database references

Bellatrix, also designated Gamma Orionis (γ Orionis, abbreviated Gamma Ori, γ Ori), is the third-brightest star in the constellation of Orion, 5° west of the red giant Alpha Orionis (Betelgeuse). Just between the first and second magnitude and slightly variable, it is about the 25th-brightest star in the night sky.


From left to right, the stars Bellatrix, the Sun, and Algol B

Bellatrix is a massive star with about 8.6 times the Sun's mass. It has an estimated age of approximately 25 million years—old enough for a star of this mass to consume the hydrogen at its core and begin to evolve away from the main sequence into a giant star.[12] The effective temperature of the outer envelope of this star is 22000 K,[9] which is considerably hotter than the 5,778 K on the Sun. This high temperature gives this star the blue-white hue that occurs with B-type stars.[13] The measured angular diameter of this star, after correction for limb darkening, is 0.72±0.04 mas.[14] At an estimated distance of 250 light-years (77 parsecs),[1] this yields a physical size of about six times the radius of the Sun.[15][12]

Bellatrix was once thought to belong to the Orion OB1 Association of stars that share a common motion through space, along with the "Orion's Belt" stars Zeta Orionis (Alnitak), Epsilon Orionis (Alnilam), and Delta Orionis (Mintaka). However, this is no longer believed to be the case, as Bellatrix is now known to be much closer than the rest of the group.[12] It is not known to have a stellar companion,[16] although researchers Maria-Fernanda Nieva and Norbert Przybilla raised the possibility it might be a spectroscopic binary.[17] A 2011 search for nearby companions failed to conclusively find any objects that share a proper motion with Bellatrix. Three nearby candidates were all found to be background stars.[18]

Standard star

Bellatrix has been used as both a photometric and spectral standard star, but both characteristics have been shown to be unreliable.

In 1963, Bellatrix was included with a set of bright stars used to define the UBV magnitude system. These are used for comparison with other stars to check for variability, and so by definition, the apparent magnitude of Bellatrix was set to 1.64.[19] However, when an all-sky photometry survey was carried out in 1988, this star was itself found to be variable. It ranges in apparent magnitude from 1.59 to 1.64.[20]

The spectral types for O and early B stars were defined more rigorously in 1971 and Bellatrix was used as a standard for the B2 III type.[21] The expected brightness of Bellatrix from this spectral type is about one magnitude brighter than calculated from its apparent magnitude and Hipparcos distance.[22] Analysis of the observed characteristics of the star indicate that it should be a B2 main sequence star, not the giant that it appears from its spectral type.[23] Close analysis of high resolution spectra suggest that it is a spectroscopic binary composed of two similar stars less luminous than a B2 giant.[17]

Etymology and cultural significance

Gamma Orionis is the star's Bayer designation. The traditional name Bellatrix is Latin for "female warrior"; it first appeared in the works of Abu Ma'shar al-Balkhi and Johannes Hispalensis, where it originally referred to Capella, but was transferred to Gamma Orionis by the Vienna school of astronomers in the 15th century, and appeared in contemporary reprints of the Alfonsine tables.[24] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[25] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[26] included a table of the first two batches of names approved by the WGSN; which included Bellatrix for this star. It is now so entered in the IAU Catalog of Star Names.[27]

Bellatrix was also called the Amazon Star, which Richard Hinckley Allen proposed came from a loose translation of the Arabic name Al Najīd, the Conqueror.[10] A c.1275 Arabic celestial globe records the name as المرزم "the lion".[28] Bellatrix is one of the four navigational stars in Orion that are used for celestial navigation.[29]

In Chinese, 參宿 (Shēn Sù), meaning Three Stars (asterism), refers to an asterism consisting of Bellatrix, Alnitak, Alnilam, Mintaka, Betelgeuse, Saiph and Rigel.[30] Consequently, Bellatrix are known as 參宿五 (Shēn Sù wǔ, English: the Fifth Star of Three Stars).[31]

In the 17th century catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Menkib al Jauza al Aisr, which was translated into Latin as Humerus Sinister Gigantis.[32]

The Wardaman people of northern Australia know Bellatrix as Banjan, the sparkling pigment used in ceremonies conducted by Rigel the Red Kangaroo Leader in a songline when Orion is high in the sky. The other stars of Orion are his ceremonial tools and entourage. Betelgeuse is Ya-jungin "Owl Eyes Flicking", watching the ceremonies.[33]

To the Inuit, the appearance of Betelgeuse and Bellatrix high in the southern sky after sunset marked the beginning of spring and lengthening days in late February and early March. The two stars were known as Akuttujuuk "those (two) placed far apart", referring to the distance between them, mainly to people from North Baffin Island and Melville Peninsula.[34]


  1. 1.0 1.1 1.2 1.3 1.4 1.5 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. 2.0 2.1 2.2 Crawford, D. L.; Barnes, J. V.; Golson, J. C. (December 1971), "Four-color, Hbeta, and UBV photometry for bright B-type stars in the northern hemisphere", Astronomical Journal 76: 1058–1071, Bibcode 1971AJ.....76.1058C, doi:10.1086/111220
  3. 3.0 3.1 Samus, N. N.Expression error: Unrecognized word "etal". (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. Bibcode 2009yCat....102025S.
  4. Morgan, W. W.; Keenan, P. C. (1973), "Spectral Classification", Annual Review of Astronomy and Astrophysics 11: 29, Bibcode 1973ARA&A..11...29M, doi:10.1146/annurev.aa.11.090173.000333
  5. Wilson, R. E. (1953), "General Catalogue of Stellar Radial Velocities", Washington (Carnegie Institute of Washington, D.C.), Bibcode 1953GCRV..C......0W
  6. Lamers, H. J. G. L. M.; Harzevoort, J. M. A. G.; Schrijver, H.; Hoogerwerf, R.; Kudritzki, R. P. (1997). "The effect of rotation on the absolute visual magnitudes of OB stars measured with Hipparcos". Astronomy and Astrophysics 325: L25. Bibcode 1997A&A...325L..25L.
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  15. Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library, 1 (3rd ed.), Birkhäuser, ISBN 3-540-29692-1, The radius (R*) is given by:
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  17. 17.0 17.1 Nieva, Maria-Fernanda; Przybilla, Norbert (2012). "Present-day cosmic abundances. A comprehensive study of nearby early B-type stars and implications for stellar and Galactic evolution and interstellar dust models". Astronomy & Astrophysics 539A: 143–63. arXiv:1203.5787. Bibcode 2012A&A...539A.143N. doi:10.1051/0004-6361/201118158.
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  26. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
  27. Lua error in Module:Citation/CS1 at line 379: attempt to call method 'match' (a nil value).
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  31. (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 25 日
  32. 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.
  33. Lua error in Module:Citation/CS1 at line 925: attempt to concatenate local 'str' (a table value).
  34. Lua error in Module:Citation/CS1 at line 886: bad argument #1 to 'sub' (string expected, got table).

External links