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B-Type ○｜Definition｜1st｜20260908163816-00-⌔

Stellar classification - Wikipedia#Class_B

Class B

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B-type stars are very luminous and blue. Their spectra have neutral helium lines, which are most prominent at the B2 subclass, and moderate hydrogen lines. As O- and B-type stars are so energetic, they only live for a relatively short time. Thus, due to the low probability of kinematic interaction during their lifetime, they are unable to stray far from the area in which they formed, apart from runaway stars.

The transition from class O to class B was originally defined to be the point at which the He II λ4541 disappears. However, with modern equipment, the line is still apparent in the early B-type stars. Today for main-sequence stars, the B class is instead defined by the intensity of the He I violet spectrum, with the maximum intensity corresponding to class B2. For supergiants, lines of silicon are used instead; the Si IV λ4089 and Si III λ4552 lines are indicative of early B. At mid-B, the intensity of the latter relative to that of Si II λλ4128-30 is the defining characteristic, while for late B, it is the intensity of Mg II λ4481 relative to that of He I λ4471.1

These stars tend to be found in their originating OB associations, which are associated with giant molecular clouds. The Orion OB1 association occupies a large portion of a spiral arm of the Milky Way and contains many of the brighter stars of the constellation Orion. About 1 in 800 (0.125%) of the main-sequence stars in the solar neighborhood are B-type main-sequence stars.23 B-type stars are relatively uncommon and the closest is Regulus, at around 80 light years.4

Massive yet non- supergiant stars known as Be stars have been observed to show one or more Balmer lines in emission, with the hydrogen -related electromagnetic radiation series projected out by the stars being of particular interest. Be stars are generally thought to feature unusually strong stellar winds, high surface temperatures, and significant attrition of stellar mass as the objects rotate at a curiously rapid rate.5

Objects known as B[e] stars – or B(e) stars for typographic reasons – possess distinctive neutral or low ionisation emission lines that are considered to have forbidden mechanisms, undergoing processes not normally allowed under current understandings of quantum mechanics.

Example spectral standards:6

  • B0V – Upsilon Orionis
  • B0Ia – Alnilam
  • B2Ia – Chi Orionis
  • B2Ib – 9 Cephei
  • B3V – Alkaid
  • B3V – Eta Aurigae
  • B3Ia – Omicron Canis Majoris
  • B5Ia – Eta Canis Majoris
  • B8Ia – Rigel

Printed 2026-09-08.

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Footnotes

  1. 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. ↩

  2. 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. ↩

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

  4. Howell, Elizabeth (21 September 2013). “Regulus: The Kingly Star”. Space.com. Retrieved 13 April 2022. ↩

  5. Slettebak, Arne (July 1988). “The Be Stars”. Publications of the Astronomical Society of the Pacific. 100: 770–784. Bibcode:1988PASP..100..770S. doi:10.1086/132234. ↩

  6. 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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