page_title: Wolf–Rayet star
section_title: Classification
hierarchical_section_title: Wolf–Rayet star / Classification
caption_attribution_description: English: Spectrum of Wolf-Rayet star WR137, of WC7 spectral type. Spectrum made at T60 Pic du MidiFrançais&#160;: Spectre de l'étoile Wolf-Rayet WR137, de type spectral WC7. Spectre réalisé au T60 du Pic du Midi
context_page_description: Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon. The spectra indicate very high surface enhancement of heavy elements, depletion of hydrogen, and strong stellar winds. Their surface temperatures range from 30,000 K to around 210,000 K, hotter than almost all other stars. They were previously called W-type stars referring to their spectral classification.
  Classic Wolf–Rayet stars are evolved, massive stars that have completely lost their outer hydrogen and are fusing helium or heavier elements in the core. A subset of the population I WR stars show hydrogen lines in their spectra and are known as WNh stars; they are young extremely massive stars still fusing hydrogen at the core, with helium and nitrogen exposed at the surface by strong mixing and radiation-driven mass loss.
context_section_description: Wolf–Rayet stars were named on the basis of the strong broad emission lines in their spectra, identified with helium, nitrogen, carbon, silicon, and oxygen, but with hydrogen lines usually weak or absent. The first system of classification split these into stars with dominant lines of ionised nitrogen (N III, N IV, and N V) and those with dominant lines of ionised carbon (C III and C IV) and sometimes oxygen (O III – O VI), referred to as WN and WC respectively. The two classes WN and WC were further split into temperature sequences WN5–WN8 and WC6–WC8 based on the relative strengths of the 541.1 nm He II and 587.5 nm He I lines. Wolf–Rayet emission lines frequently have a broadened absorption wing (P Cygni profile) suggesting circumstellar material. A WO sequence has also been separated from the WC sequence for even hotter stars where emission of ionised oxygen dominates that of ionised carbon, although the actual proportions of those elements in the stars are likely to be comparable. WC and WO spectra are formally distinguished based on the presence or absence of C III emission. WC spectra also generally lack the O VI lines that are strong in WO spectra.
  The WN spectral sequence was expanded to include WN2–WN9, and the definitions refined based on the relative strengths of the N III lines at 463.4–464.1 nm and 531.4 nm, the N IV lines at 347.9–348.4 nm and 405.8 nm, and the N V lines at 460.3 nm, 461.9 nm, and 493.3–494.4 nm. These lines are well separated from areas of strong and variable He emission and the line strengths are well correlated with temperature. Stars with spectra intermediate between WN and Ofpe have been classified as WN10 and WN11 although this nomenclature is not universally accepted.
  The type WN1 was proposed for stars with neither N IV nor N V lines, to accommodate Brey 1 and Brey 66 which appeared to be intermediate between WN2 and WN2.5. The relative line strengths and widths for each WN sub-class were later quantified, and the ratio between the 541.1 nm He II and 587.5m, He I lines was introduced as the primary indicator of the ionisation level and hence of the spectral sub-class. The need for WN1 disappeared and both Brey 1 and Brey 66 are now classified as WN3b. The somewhat obscure WN2.5 and WN4.5 classes were dropped.
  The WC spectral sequence was expanded to include WC4–WC11, although some older papers have also used WC1–WC3. The primary emission lines used to distinguish the WC sub-types are C II 426.7 nm, C III at 569.6 nm, C III/IV465.0 nm, C IV at 580.1–581.2 nm, and the O V (and O III) blend at 557.2–559.8 nm. The sequence was extended to include WC10 and WC11, and the subclass criteria were quantified based primarily on the relative strengths of carbon lines to rely on ionisation factors even if there were abundance variations between carbon and oxygen.
  For WO-type stars the main lines used are C IV at 580.1 nm, O IV at 340.0 nm, O V (and O III) blend at 557.2–559.8 nm, O VI at 381.1–383.4 nm, O VII at 567.0 nm, and O VIII at 606.8 nm. The sequence was expanded to include WO5 and quantified based the relative strengths of the O VI/C IV and O VI/O V lines. A later scheme, designed for consistency across classical WR stars and CSPNe, returned to the WO1 to WO4 sequence and adjusted the divisions.
  Detailed modern studies of Wolf–Rayet stars can identify additional spectral features, indicated by suffixes to the main spectral classification:
  h for hydrogen emission;
  ha for hydrogen emission and absorption;
  w for weak lines;
  s for strong lines;
  b for broad strong lines;
  d for dust (occasionally vd, pd, or ed for variable, periodic, or episodic dust).
  The classification of Wolf–Rayet spectra is complicated by the frequent association of the stars with dense nebulosity, dust clouds, or binary companions. A suffix of "+OB" is used to indicate the presence of absorption lines in the spectrum likely to be associated with a more normal companion star, or "+abs" for absorption lines with an unknown origin.
  The hotter WR spectral sub-classes are described as early and the cooler ones as late, cons
caption: Spectrum of WR 137, a WC7 star[14] and one of the three original WR stars (horizontal axis : wavelength in Å).
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