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O-Type ○｜Definition｜1st｜20260908165233-00-⌔

Stellar classification - Wikipedia#Class_O

Class O

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O-type stars are very hot and extremely luminous, with most of their radiated output in the ultraviolet range. These are the rarest of all main-sequence stars. About 1 in 3,000,000 (0.00003%) of the main-sequence stars in the solar neighborhood are O-type stars.12 Some of the most massive stars lie within this spectral class. O-type stars frequently have complicated surroundings that make measurement of their spectra difficult.

O-type spectra formerly were defined by the ratio of the strength of the He II λ4541 relative to that of He I λ4471, where λ is the radiation wavelength. Spectral type O7 was defined to be the point at which the two intensities are equal, with the He I line weakening towards earlier types. Type O3 was, by definition, the point at which said line disappears altogether, although it can be seen very faintly with modern technology. Due to this, the modern definition uses the ratio of the nitrogen line N IV λ4058 to N III λλ4634-40-42.3

O-type stars have dominant lines of absorption and sometimes emission for He II lines, prominent ionized (Si IV, O III, N III, and C III) and neutral helium lines, strengthening from O5 to O9, and prominent hydrogen Balmer lines, although not as strong as in later types. Higher-mass O-type stars do not retain extensive atmospheres due to the extreme velocity of their stellar wind, which may reach 2,000 km/s. Because they are so massive, O-type stars have very hot cores and burn through their hydrogen fuel very quickly, so they are the first stars to leave the main sequence.

When the MKK classification scheme was first described in 1943, the only subtypes of class O used were O5 to O9.5.4 The MKK scheme was extended to O9.7 in 19715 and O4 in 1978,6 and new classification schemes that add types O2, O3, and O3.5 have subsequently been introduced.7

Example spectral standards:8

  • O7V – S Monocerotis
  • O9V – 10 Lacertae

Printed 2026-09-08.

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Footnotes

  1. These proportions are fractions of stars brighter than absolute magnitude 16; lowering this limit will render earlier types even rarer, whereas generally adding only to the M class. The proportions are calculated ignoring the value of 800 in the total column since the actual numbers add up to 824. ↩

  2. Ledrew, Glenn (February 2001). “The Real Starry Sky”. Journal of the Royal Astronomical Society of Canada. 95: 32. Bibcode:2001JRASC..95…32L. ↩

  3. Walborn, N. R. (2008). “Multiwavelength Systematics of OB Spectra”. Massive Stars: Fundamental Parameters and Circumstellar Interactions (Eds. P. Benaglia. 33: 5. Bibcode:2008RMxAC..33…5W. ↩

  4. An atlas of stellar spectra, with an outline of spectral classification, W. W. Morgan, P. C. Keenan and E. Kellman, Chicago: The University of Chicago Press, 1943. ↩

  5. Walborn, N. R. (1971). “Some Spectroscopic Characteristics of the OB Stars: An Investigation of the Space Distribution of Certain OB Stars and the Reference Frame of the Classification”. The Astrophysical Journal Supplement Series. 23: 257. Bibcode:1971ApJS…23..257W. doi:10.1086/190239. ↩

  6. Morgan, W. W.; Abt, Helmut A.; Tapscott, J. W. (1978). “Revised MK Spectral Atlas for stars earlier than the sun”. Williams Bay: Yerkes Observatory. Bibcode:1978rmsa.book…M. ↩

  7. Walborn, Nolan R.; Howarth, Ian D.; Lennon, Daniel J.; Massey, Philip; Oey, M. S.; Moffat, Anthony F. J.; Skalkowski, Gwen; Morrell, Nidia I.; Drissen, Laurent; Parker, Joel Wm. (2002). “A New Spectral Classification System for the Earliest O Stars: Definition of Type O2” (PDF). The Astronomical Journal. 123 (5): 2754–2771. Bibcode:2002AJ…123.2754W. doi:10.1086/339831. S2CID 122127697. ↩

  8. Garrison, R. F. (1994). “A Hierarchy of Standards for the MK Process” (PDF). In Corbally, C. J.; Gray, R. O.; Garrison, R. F. (eds.). The MK Process at 50 Years: A Powerful Tool for Astrophysical Insight. Astronomical Society of the Pacific conference series. Vol. 60. San Francisco: Astronomical Society of the Pacific. pp. 3–14. ISBN 978-1-58381-396-6. OCLC 680222523. ↩

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