Omicron2 Canis Majoris

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For other stars with this Bayer designation, see ο Canis Majoris.
ο² Canis Majoris
Canis Major constellation map.svg
link=ο2 Canis Majoris

Location of ο2 CMa (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Canis Major
Right ascension 07h 03m 01.47211s[1]
Declination –23° 49′ 59.8523″[1]
Apparent magnitude (V) 3.043[2]
Spectral type B3 Ia[3]
U−B color index –0.778[2]
B−V color index –0.107[2]
Variable type α Cyg[4]
Radial velocity (Rv) 48.4[5] km/s
Proper motion (μ) RA: –2.21[1] mas/yr
Dec.: +3.61[1] mas/yr
Parallax (π) 1.18 ± 0.40[1] mas
Distance approx. 2,800 ly
(approx. 800 pc)
Absolute magnitude (MV) –7.3[6]
Mass 21.4 ± 2.2[7] M
Radius 65[6] R
Luminosity 220,000[6] L
Surface gravity (log g) 2.05[6] cgs
Temperature 15,500[6] K
Rotational velocity (v sin i) 58[6] km/s
Age 7.4 ± 1.0[7] Myr
Other designations
24 Canis Majoris, CD −23°4797, FK5 270, HD 53138, HIP 33977, HR 2653, SAO 172839.
Database references

Omicron2 Canis Majoris (ο² CMa, ο² Canis Majoris) is a star in the constellation Canis Major. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified.[3] It has an apparent visual magnitude of 3.043,[2] making it one of the brighter members of the constellation. The distance to this star is roughly 2,800 light years (800 parsecs), with a 34% margin of error.[1]

This is a massive supergiant star with a stellar classification of B3 Ia,[3] indicating that, at the age of around 7 million years,[7] it has exhausted the supply of hydrogen at its core and is now undergoing nuclear fusion of helium to generate energy.[8] It has about 21[7] times the mass of the Sun and 65[6] times the Sun's radius. In all likelihood, it will end its life as a Type II supernova.[8]

Omicron2 Canis Majoris is one of the most luminous stars known, as it radiates about 220,000[6] times as much luminosity as the Sun from its outer envelope at a temperature of 15,500 K.[6] At this heat, the star is glowing with the blue-white hue of a B-type star.[9] This star is classified as an Alpha Cygni-type variable star that undergoes periodic non-radial pulsations, which cause its brightness to cycle from magnitude +2.93 to +3.08 over a 24.44 day interval.[4] It is losing mass from its stellar wind at the rate of around 2 × 10−9 times the mass of the Sun per year, or the equivalent of the Sun's mass every 500 million years.[10]

While this star lies in the field of view of the open cluster named Collinder 121, it is unlikely to be a member. In fact, its optical neighbor, the orange supergiant ο1 Canis Majoris has a much higher likelihood of 23.1% based upon its proper motion being a closer match to the motion of the cluster.[11] Although they are located near each other on the celestial sphere, ο1 CMa and ο2 CMa are most likely not gravitationally bound to each other as they appear to lie many light years apart.[1][8]


In the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, this star was designated Thanih al Adzari (تاني ألعذاري - taanii al-aðārii), which was translated into Latin as Secunda Virginum, meaning the second virgin.[12] This star, along with ε CMa (Adhara), δ CMa (Wezen) and η CMa (Aludra), were Al ʽAdhārā (ألعذاري), the Virgins.[13][14]


  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:0708.1752Freely accessible. 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, 1, Publicaciones Universidad de Chile, Department de Astronomy, pp. 1–17, Bibcode:1966PDAUC...1....1G 
  3. ^ a b c Garrison, R. F. (December 1993), "Anchor Points for the MK System of Spectral Classification", Bulletin of the American Astronomical Society, 25: 1319, Bibcode:1993AAS...183.1710G, retrieved 2012-02-04 
  4. ^ a b Lefèvre, L.; et al. (November 2009), "A systematic study of variability among OB-stars based on HIPPARCOS photometry", Astronomy and Astrophysics, 507 (2): 11411201, Bibcode:2009A&A...507.1141L, doi:10.1051/0004-6361/200912304 
  5. ^ Wielen, R.; et al. (1999), Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions (35), Astronomisches Rechen-Institut Heidelberg, Bibcode:1999VeARI..35....1W 
  6. ^ a b c d e f g h i Crowther, P. A.; Lennon, D. J.; Walborn, N. R. (January 2006), "Physical parameters and wind properties of galactic early B supergiants", Astronomy and Astrophysics, 446 (1): 279–293, arXiv:astro-ph/0509436Freely accessible, Bibcode:2006A&A...446..279C, doi:10.1051/0004-6361:20053685 
  7. ^ a b c d 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:1007.4883Freely accessible, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x 
  8. ^ a b c Kaler, James B., "OMI-2 CMA (Omicron-2 Canis Majoris)", Stars, University of Illinois, retrieved 2012-02-28 
  9. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, retrieved 2012-01-16 
  10. ^ Prinja, R. K.; Massa, D. L. (October 2010), "Signature of wide-spread clumping in B supergiant winds", Astronomy and Astrophysics, 521: L55, arXiv:1007.2744Freely accessible, Bibcode:2010A&A...521L..55P, doi:10.1051/0004-6361/201015252 
  11. ^ Baumgardt, H.; Dettbarn, C.; Wielen, R. (October 2000), "Absolute proper motions of open clusters. I. Observational data", Astronomy and Astrophysics Supplement, 146: 251–258, arXiv:astro-ph/0010306Freely accessible, Bibcode:2000A&AS..146..251B, doi:10.1051/aas:2000362 
  12. ^ 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: 429–438, Bibcode:1895MNRAS..55..429K, doi:10.1093/mnras/55.8.429 
  13. ^ Allen, R. H. (1963), Star Names: Their Lore and Meaning (Reprint ed.), New York: Dover Publications Inc, p. 130, ISBN 0-486-21079-0, retrieved 2010-12-12 
  14. ^ ε CMa as Aoul al Adzari or Prima Virginum (the first virgin), ο2 CMa as Thanih al Adzari or Secunda Virginum (the second virgin) and δ CMa as Thalath al Adzari or Tertia Virginum (the third virgin). η CMa should be Rabah al Adzari or Quarta Virginum (the fourth virgin) consistently, but it was given by the name Aludra, meaning the virgin (same meaning with Adhara (ε CMa) or Al ʽAdhārā)