Epsilon Canis Majoris

Epsilon Canis Majoris is a binary star system and the second-brightest object in the constellation of Canis Major. Its name is a Bayer designation that is Latinised from ε Canis Majoris, and abbreviated Epsilon CMa or ε CMa. This is the 22nd-brightest star in the night sky with an apparent magnitude of 1.50. About 4.7 million years ago, it was the brightest star in the night sky, with an apparent magnitude of −3.99.[9] Based upon parallax measurements obtained during the Hipparcos mission, it is about 430 light-years distant from the Sun.[4]

ε Canis Majoris
Canis Major constellation map.svg
Red circle.svg
Location of ε Canis Majoris (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Canis Major
Right ascension 06h 58m 37.6s
Declination –28° 58′ 19″
Apparent magnitude (V) 1.50[1]
Spectral type B2 II[2]
U−B color index –0.93[1]
B−V color index –0.21[1]
Radial velocity (Rv)+27.3 km/s
Proper motion (μ) RA: +2.63 mas/yr[3][4]
Dec.: +2.29 mas/yr[3][4]
Parallax (π)7.57 ± 0.57 mas[3][4]
Distance430 ± 30 ly
(132 ± 10 pc)
Absolute magnitude (MV)–4.8[5]
Mass12.6±1.0[2] M
Radius13.9[6] R
Luminosity38,700[7] L
Surface gravity (log g)3.39[7] cgs
Temperature22,900[7] K
Rotational velocity (v sin i)25[8] km/s
Age22.5±2.6[2] Myr
Other designations
Adhara, Adharaz, Undara, ε CMa, 21 CMa, CD−28°3666, FK5 268, HD 52089, HIP 33579, HR 2618, SAO 172676, ADS 5654
Database references

The two components are designated ε Canis Majoris A, officially named Adhara /əˈdɛərə/ – the traditional name of the system,[10][11] and B.


ε Canis Majoris (Latinised to Epsilon Canis Majoris) is the binary system's Bayer designation. The designations of the two components as ε Canis Majoris A and B derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[12]

ε Canis Majoris bore the traditional name Adhara (sometimes spelled Adara, Adard, Udara or Udra), derived from the Arabic word عذارى 'aðāra', "virgins".[13] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[14] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire star systems.[15] It approved the name Adhara for the star ε Canis Majoris A on 21 August 2016 and it is now so included in the List of IAU-approved Star Names.[11]

In the 17th-century catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Aoul al Adzari (أول العذاري awwal al-adhara), which was translated into Latin as Prima Virginum, meaning First of the Virgins.[16] Along with δ Canis Majoris (Wezen), η Canis Majoris (Aludra) and ο2 Canis Majoris (Thanih al Adzari), these stars were Al ʽAdhārā (العذاري), 'the Virgins'.[13][17]

In Chinese, 弧矢 (Hú Shǐ), meaning Bow and Arrow,[18] refers to an asterism consisting of ε Canis Majoris, δ Canis Majoris, η Canis Majoris, κ Canis Majoris, ο Puppis, π Puppis, χ Puppis, c Puppis and k Puppis. Consequently, ε Canis Majoris itself is known as 弧矢七 (Hú Shǐ qī, English: the Seventh Star of Bow and Arrow).[19]

Physical propertiesEdit

ε Canis Majoris is a binary star.[3][20] The primary, ε Canis Majoris A, has an apparent magnitude of +1.5 and belongs to the spectral classification B2. Its color is blue or blueish-white, due to the surface temperature of 22,200K. It emits a total radiation equal to 38,700 times that of the Sun. This star is the brightest source of extreme ultraviolet in the night sky.[21] It is the strongest source of photons capable of ionizing hydrogen atoms in interstellar gas near the Sun, and is very important in determining the ionization state of the Local Interstellar Cloud.[22]

The +7.5-magnitude (the absolute magnitude amounts to +1.9) companion star, ε Canis Majoris B, is 7.5" away with a position angle of 161° of the main star. Despite the relatively large angular distance the components can only be resolved in large telescopes, since the primary is approximately 250 times brighter than its companion.

A few million years ago, ε Canis Majoris was much closer to the Sun than it is at present, causing it to be a much brighter star in the night sky. About 4.7 million years ago, Adhara was 34 light-years from the Sun, and was the brightest star in the sky with a magnitude of –3.99. No other star has attained this brightness since, nor will any other star attain this brightness for at least five million years.[9]

In cultureEdit

USS Adhara (AK-71) was a U.S. Navy Crater-class cargo ship named after the star.

ε Canis Majoris appears on the national flag of Brazil, symbolising the state of Tocantins.[23]


  1. ^ a b c 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.
  2. ^ a b c Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society, 410 (1): 190–200, arXiv:1007.4883, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x, S2CID 118629873
  3. ^ a b c Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J.; Hoeg, E.; Bastian, U.; Bernacca, P. L.; Crézé, M.; Donati, F.; Grenon, M.; Grewing, M.; Van Leeuwen, F.; Van Der Marel, H.; Mignard, F.; Murray, C. A.; Le Poole, R. S.; Schrijver, H.; Turon, C.; Arenou, F.; Froeschlé, M.; Petersen, C. S.; et al. (1997). "The HIPPARCOS Catalogue". Astronomy and Astrophysics. 323: L49–L52. Bibcode:1997A&A...323L..49P.
  4. ^ 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
  5. ^ Snow, Theodore P.; Lamers, Henny J. G. L. M.; Lindholm, Douglas M.; Odell, Andrew P. (1994). "An atlas of ultraviolet P Cygni profiles". The Astrophysical Journal Supplement Series. 95: 163. Bibcode:1994ApJS...95..163S. doi:10.1086/192099.
  6. ^ Underhill, A. B.; et al. (November 1979), "Effective temperatures, angular diameters, distances and linear radii for 160 O and B stars", Monthly Notices of the Royal Astronomical Society, 189 (3): 601–605, Bibcode:1979MNRAS.189..601U, doi:10.1093/mnras/189.3.601
  7. ^ a b c Lyubimkov, L. S.; Rostopchin, S. I.; Lambert, D. L. (June 2004), "Surface abundances of light elements for a large sample of early B-type stars - III. An analysis of helium lines in spectra of 102 stars", Monthly Notices of the Royal Astronomical Society, 351 (2): 745–767, Bibcode:2004MNRAS.351..745L, doi:10.1111/j.1365-2966.2004.07825.x
  8. ^ Abt, Helmut A.; Levato, Hugo; Grosso, Monica (July 2002), "Rotational Velocities of B Stars", The Astrophysical Journal, 573 (1): 359–365, Bibcode:2002ApJ...573..359A, doi:10.1086/340590.
  9. ^ a b Tomkin, Jocelyn (April 1998). "Once and Future Celestial Kings" (PDF). Sky and Telescope. 95 (4): 59–63. Bibcode:1998S&T....95d..59T.[permanent dead link]
  10. ^ Kunitzsch, Paul; Smart, Tim (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.
  11. ^ a b "Naming Stars (including "List of IAU-approved Star Names as of January 1st, 2021")". IAU.org. Retrieved 28 February 2021.
  12. ^ Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
  13. ^ a b Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York: Dover Publications. p. 130. ISBN 0-486-21079-0. Retrieved 2018-07-05.
  14. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  15. ^ "WG Triennial Report (2015-2018) - Star Names" (PDF). IAU.org. 2018. p. 5. Retrieved 2018-07-14.
  16. ^ 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): 429–438. Bibcode:1895MNRAS..55..429K. doi:10.1093/mnras/55.8.429.
  17. ^ ε CMa as Aoul al Adzari or Prima Virginum (First of the Virgins), Omicron2 Canis Majoris as Thanih al Adzari or Secunda Virginum (Second of the Virgins) and Delta Canis Majoris as Thalath al Adzari or Tertia Virginum (Third of the Virgins). Eta Canis Majoris should be Rabah al Adzari or Quarta Virginum (Fourth of the Virgins) consistently, but it was given by the name Aludra, meaning Virgin (same meaning with Adhara or Al ʽAdhārā)
  18. ^ 弧矢 (Hú Shǐ) is westernized into Koo She. R.H. Allen opined that Koo She refers to the asterism including Delta Velorum and Omega Velorum. AEEA opinion is, Delta Velorum is member of 天社 (Tiān Shè), meaning Celestial Earth God's Temple asterism and Omega Velorum is not a member of any asterisms. 天社 (Tiān Shè) is westernized into Tseen She and R. H. Allen used the term Tseen She for Chinese name of η Carinae. See Richard Hinckley Allen: Star Names — Their Lore and Meaning: Argo Navis and (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 17 日.
  19. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 17 日
  20. ^ Perryman, Michael (2010), "The Making of History's Greatest Star Map", The Making of History's Greatest Star Map, Astronomers' Universe, Heidelberg: Springer-Verlag, Bibcode:2010mhgs.book.....P, doi:10.1007/978-3-642-11602-5, ISBN 978-3-642-11601-8
  21. ^ Wilkinson, E.; Green, J. C.; McLean, R.; Welsh, B. (1996). "Extreme Ultraviolet Spectrum of ɛ Canis Majoris Between 600-920 Å". Bull. Am. Astron. Soc. 28 (2): 915. Bibcode:1996BAAS...28..915W.
  22. ^ Vallerga, J. V.; Welsh, B. Y. (1995). "Epsilon Canis Majoris and the ionization of the local cloud". Astrophys. J. 444: 702–707. Bibcode:1995ApJ...444..702V. doi:10.1086/175643.
  23. ^ "Astronomy of the Brazilian Flag". FOTW Flags Of The World website. Archived from the original on March 30, 2014.