View source for Wolf–Rayet star

{{Short description|Heterogeneous class of stars with unusual spectra}}
[[Image:Wolf-Rayet 124 (NIRCam and MIRI composite image).tif|thumb|right|upright=1.4|[[James Webb Space Telescope]] image of the Wolf–Rayet star [[WR 124]] and the nebula [[M1-67|M1–67]] surrounding it. [[NIRCam]] and [[Mid-Infrared Instrument|MIRI]] composite.]]

'''Wolf–Rayet stars''', often abbreviated as '''WR stars''', are a rare heterogeneous set of stars with unusual [[spectroscopy|spectra]] showing prominent broad emission lines of ionised [[helium]] and highly ionised [[nitrogen]] or [[carbon]]. The spectra indicate very high surface enhancement of [[Stellar nucleosynthesis|heavy elements]], depletion of hydrogen, and strong [[stellar wind]]s. The [[Effective temperature|surface temperatures]] of known Wolf–Rayet stars range from 20,000&nbsp;[[Kelvin (unit)|K]] to around 210,000&nbsp;[[Kelvin (unit)|K]], hotter than almost all other kinds of stars. They were previously called '''W-type stars''' referring to their [[Stellar classification|spectral classification]].

Classic (or [[population I]]) Wolf–Rayet stars are [[Stellar evolution|evolved]], massive stars that have completely lost their outer [[hydrogen]] and are [[nuclear fusion|fusing]] helium or heavier elements in the core. A subset of the population&nbsp;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. A separate group of stars with WR spectra are the central stars of [[planetary nebula]]e (CSPNe), post-[[asymptotic giant branch]] stars that were similar to the [[Sun]] while on the [[main sequence]], but have now ceased fusion and shed their atmospheres to reveal a bare carbon-oxygen core.

All Wolf–Rayet stars are highly luminous objects due to their high temperatures—thousands of times the [[bolometer|bolometric]] [[luminosity]] of the Sun ({{solar luminosity}}) for the CSPNe, {{solar luminosity|hundreds of thousands}} for the population&nbsp;I WR stars, to {{solar luminosity|over a million}} for the WNh stars—although not exceptionally bright visually since most of their [[electromagnetic radiation|radiation]] output is in the [[ultraviolet]].

The naked-eye star systems [[Gamma Velorum|γ&nbsp;Velorum]] and [[Theta Muscae|θ&nbsp;Muscae]] both contain Wolf-Rayet stars, and two of the [[List of most massive stars|most massive known stars]], [[BAT99-98]] and [[R136a1]] in [[30 Doradus|30&nbsp;Doradus]], are also Wolf–Rayet stars.

==Observation history==
[[Image: Crescenthunter.jpg|upright=1.0|thumb|left|alt=Crescent Nebula|[[WR 136]], a WN6 star where the atmosphere shed during the red supergiant phase has been shocked by the hot, fast WR winds to form a visible [[NGC 6888|bubble&nbsp;nebula]]]]
In 1867, using the 40&nbsp;cm [[Foucault telescope]] at the [[Paris Observatory]], astronomers [[Charles Wolf (astronomer)|Charles Wolf]] and [[Georges Rayet]]<ref>
{{cite book
 |last=Murdin |first=P.
 |year=2001
 |section=Wolf, Charles J.E. (1827–1918)
 |title=The Encyclopedia of Astronomy and Astrophysics
 |page=4101
 |isbn=978-0333750889 |bibcode=2000eaa..bookE4101.
}}
</ref>
discovered three stars in the constellation [[Cygnus (constellation)|Cygnus]] (HD&nbsp;191765, HD&nbsp;192103 and HD&nbsp;192641, now designated as [[WR 134]], [[WR 135]], and [[WR 137]] respectively) that displayed broad emission bands on an otherwise continuous spectrum.<ref>
{{cite journal
 |last1=Huggins |first1=W.
 |last2=Huggins |first2=Mrs.
 |year=1890
 |title=On Wolf and Rayet's bright-line stars in Cygnus
 |journal=[[Proceedings of the Royal Society of London]]
 |volume=49  |issue=296–301  |pages=33–46
 |s2cid=120014472  |doi=10.1098/rspl.1890.0063
 |url=https://zenodo.org/record/1432081
}}
</ref>
Most stars only display [[absorption line]]s or [[absorption band|band]]s in their spectra, as a result of overlying elements absorbing light energy at specific frequencies, so these were clearly unusual objects.

The nature of the emission bands in the spectra of a Wolf–Rayet star remained a mystery for several decades. [[Edward Charles Pickering|E.C.&nbsp;Pickering]] theorized that the lines were caused by an unusual state of [[hydrogen]], and it was found that this "Pickering series" of lines followed a pattern similar to the [[Balmer series]] when half-integer quantum numbers were substituted. It was later shown that these lines resulted from the presence of [[helium]], the chemical element having just been discovered in 1868.<ref>
{{cite journal
 |last=Fowler |first=A. |author-link=Alfred Fowler
 |date=December 1912
 |title=Observations of the principal and other series of lines in the spectrum of hydrogen (plates 2–4)
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=73 |issue=2 |pages=62–63
 |bibcode=1912MNRAS..73...62F |doi=10.1093/mnras/73.2.62
|doi-access=free}}
</ref>
Pickering noted similarities between Wolf–Rayet spectra and nebular spectra, and this similarity led to the conclusion that some or all Wolf–Rayet stars were the central stars of [[planetary nebula]]e.<ref name=wright>
{{cite journal
 |last=Wright |first=W.H.
 |year=1914
 |title=The relation between the Wolf–Rayet stars and the planetary nebulae
 |journal=[[The Astrophysical Journal]]
 |volume=40 |pages=466
 |bibcode=1914ApJ....40..466W |doi=10.1086/142138
|doi-access=free
 }}</ref>

By 1929, the width of the emission bands was being attributed to [[Doppler broadening]], and hence the gas surrounding these stars must be moving with velocities of 300–2400&nbsp;km/s along the line of sight. The conclusion was that a Wolf–Rayet star is continually ejecting gas into space, producing an expanding envelope of nebulous gas. The force ejecting the gas at the high velocities observed is [[radiation pressure]].<ref name=beals1929>
{{cite journal
 |last=Beals |first=C.S. |author-link=Carlyle Smith Beals
 |year=1929
 |title=On the nature of Wolf–Rayet emission
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=90 |issue=2 |pages=202–212
 |bibcode=1929MNRAS..90..202B |doi = 10.1093/mnras/90.2.202
|doi-access=free}}
</ref>
It was well known that many stars with Wolf–Rayet type spectra were the central stars of planetary nebulae, but also that many were not associated with an obvious planetary nebula or any visible nebulosity at all.<ref name=beals1940>
{{cite journal
 |last=Beals |first=C.S. |author-link=Carlyle Smith Beals
 |year=1940
 |title=On the physical characteristics of the Wolf–Rayet stars and their relation to other objects of early type (with plates&nbsp;VIII, IX)
 |journal=[[Journal of the Royal Astronomical Society of Canada]]
 |volume=34 |page=169
 |bibcode=1940JRASC..34..169B
}}
</ref>

In addition to helium, [[Carlyle Smith Beals]] identified emission lines of carbon, oxygen and nitrogen in the spectra of Wolf–Rayet stars.<ref>
{{cite journal
 |last=Beals |first=C.S. |author-link=Carlyle Smith Beals
 |year=1930
 |journal=Publications of the [[Dominion Astrophysical Observatory]]
 |volume=4 |pages=271–301
 |title=The Wolf–Rayet Stars
 |bibcode=1930PDAO....4..271B
}}
</ref><ref>
{{cite journal
 |last=Beals |first=C.S. |author-link=Carlyle Smith Beals
 |year=1933
 |title=Classification and temperatures of Wolf–Rayet stars
 |journal=[[The Observatory (journal)|The Observatory]]
 |volume=56  |pages=196–197
 |bibcode=1933Obs....56..196B
}}
</ref>
In 1938, the [[International Astronomical Union]] classified the spectra of Wolf–Rayet stars into types WN and WC, depending on whether the spectrum was dominated by lines of nitrogen or carbon-oxygen respectively.<ref>
{{cite journal
 |last1=Swings |first1=P.
 |year=1942
 |title=The spectra of Wolf–Rayet stars and related objects
 |journal=[[The Astrophysical Journal]]
 |volume=95  |page=112
 |bibcode=1942ApJ....95..112S |hdl=2268/72172  |doi=10.1086/144379
 |url=http://orbi.ulg.ac.be/handle/2268/72172
|hdl-access=free
 }}
</ref>

In 1969, several CSPNe with strong oxygen <small>{{thinsp}}VI</small> (O<small>{{thinsp}}VI</small>) emissions lines were grouped under a new "O<small>{{thinsp}}VI</small> sequence", or just OVI type.<ref>
{{cite journal
 |last1=Starrfield |first1=S. |author1-link=Sumner Starrfield
 |last2=Cox        |first2=A.N.
 |last3=Kidman     |first3=R.B.
 |last4=Pensnell   |first4=W.D.
 |year=1985
 |title=An analysis of nonradial pulsations of the central star of the planetary nebula K1-16
 |journal=[[Astrophysical Journal]]
 |volume=293  |pages=L23
 |doi=10.1086/184484 |bibcode=1985ApJ...293L..23S
}}
</ref> Similar stars not associated with planetary nebulae were described shortly after and the WO classification was adopted for them.<ref name=sanduleak>
{{cite journal
 | last1 = Sanduleak  | first1 = N.
 | year = 1971
 | title = On stars having strong O{{sup|VI}} emission
 | journal = The Astrophysical Journal
 | volume = 164
 | pages = L71
 | doi = 10.1086/180694
 | bibcode = 1971ApJ...164L..71S
| doi-access = free
 }}
</ref><ref name=barlow/> The OVI stars were subsequently classified as [WO] stars, consistent with the population&nbsp;I WR stars.<ref name=acker2003>{{cite journal |bibcode=2003A&A...403..659A |title=Quantitative classification of WR nuclei of planetary nebulae |last1=Acker |first1=A. |last2=Neiner |first2=C. |journal=Astronomy and Astrophysics |year=2003 |volume=403 |issue=2 |page=659 |doi=10.1051/0004-6361:20030391 |doi-access=free }}</ref>
 
The understanding that certain late, and sometimes not-so-late, WN stars with [[hydrogen line]]s in their spectra are at a different stage of evolution from hydrogen-free WR stars has led to the introduction of the term ''WNh'' to distinguish these stars generally from other WN stars. They were previously referred to as WNL stars, although there are late-type WN stars without hydrogen as well as WR stars with hydrogen as early as WN5.<ref name=wnh/>

==Classification==
[[File:Wr137 spc.png|right|upright=1.2|thumb|alt=WR 137 spectrum|Spectrum of [[WR 137]], a WC7 star<ref name=wc/> and one of the three original WR stars (horizontal axis : wavelength in Å)]]
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. Initially simply referred to as class W or W-type stars,<ref name="payne2">{{cite journal|bibcode=1930BHarO.878....1P|title=Classification of the O Stars|journal=Harvard College Observatory Bulletin|volume=878|pages=1|last1=Payne|first1=Cecilia H.|year=1930}}</ref><ref>{{cite book |doi=10.1007/978-94-009-9836-0_52 |chapter=Binary Components of Wolf-Rayet Type |title=Double Stars |year=1978 |last1=Heintz |first1=Wulff D. |pages=144–145 |isbn=978-90-277-0886-1 }}</ref> the classification was then split into stars with dominant lines of ionised nitrogen (N<small>{{thinsp}}III</small>, N<small>{{thinsp}}IV</small>, and N<small>{{thinsp}}V</small>) and those with dominant lines of ionised carbon (C<small>{{thinsp}}III</small> and C<small>{{thinsp}}IV</small>) and sometimes oxygen (O<small>{{thinsp}}III</small> – O<small>{{thinsp}}VI</small>), referred to as WN and WC respectively.<ref name=beals1933>
{{cite journal
 |last=Beals |first=C.S. |author-link=Carlyle Smith Beals
 |year=1933
 |title=Classification and temperatures of Wolf–Rayet stars
 |journal=[[The Observatory (journal)|The Observatory]]
 |volume=56 |pages=196
 |bibcode=1933Obs....56..196B
}}
</ref>
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 [[Nanometer|nm]] He<small>{{thinsp}}II</small> and 587.5&nbsp;nm He<small>{{thinsp}}I</small> 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.<ref name=beals1940/> WC and WO spectra are formally distinguished based on the presence or absence of C<small>{{thinsp}}III</small> emission.<ref name=hucht>
{{cite journal
 |last=van der Hucht  |first=Karel A.  |author-link=Karel van der Hucht
 |year=2001
 |title=The VIIth catalogue of galactic Wolf–Rayet stars
 |journal=New Astronomy Reviews
 |volume=45 |issue=3 |pages=135–232
 |doi=10.1016/S1387-6473(00)00112-3
 |bibcode=2001NewAR..45..135V
}}
</ref> WC spectra also generally lack the O<small>{{thinsp}}VI</small> lines that are strong in WO spectra.<ref name="WCO">
{{cite journal
 |last1=Crowther |first1=P.A.
 |last2=de Marco |first2=O.
 |last3=Barlow |first3=M.J.
 |year=1998
 |title=Quantitative classification of WC and WO stars
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=296 |issue=2 |pages=367–378
 |issn=0035-8711 |doi=10.1046/j.1365-8711.1998.01360.x
 |bibcode=1998MNRAS.296..367C
|doi-access=free
 }}
</ref>

The WN spectral sequence was expanded to include WN2–WN9, and the definitions refined based on the relative strengths of the N<small>{{thinsp}}III</small> lines at 463.4–464.1&nbsp;nm and 531.4&nbsp;nm, the N<small>{{thinsp}}IV</small> lines at 347.9–348.4&nbsp;nm and 405.8&nbsp;nm, and the N<small>{{thinsp}}V</small> lines at 460.3&nbsp;nm, 461.9&nbsp;nm, and 493.3–494.4&nbsp;nm.<ref name=smith>
{{cite journal
 |last=Smith |first=Lindsey F.
 |year=1968
 |title=A revised spectral classification system and a new catalogue for galactic Wolf–Rayet stars
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=138 |pages=109–121
 |doi=10.1093/mnras/138.1.109 |bibcode=1968MNRAS.138..109S
|doi-access=free
 }}
</ref>
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.<ref name=smithcrowther>
{{cite journal
 |last1=Crowther |first1=P.A.
 |last2=Smith|first2=L.J.
 |year=1997
 |title=Fundamental parameters of Wolf–Rayet stars. VI.&nbsp;Large Magellanic Cloud WNL stars
 |journal=[[Astronomy and Astrophysics]]
 |volume=320 |page=500
 |bibcode=1997A&A...320..500C
}}
</ref>

The type WN1 was proposed for stars with neither N<small>{{thinsp}}IV</small> nor N<small>{{thinsp}}V</small> lines, to accommodate Brey 1 and Brey 66 which appeared to be intermediate between WN2 and WN2.5.<ref name=conti>
{{cite journal
 | last1=Conti  | first1=Peter S.
 | last2=Massey | first2=Philip
 | year=1989
 | title=Spectroscopic studies of Wolf–Rayet stars. IV – Optical spectrophotometry of the emission lines in galactic and large Magellanic Cloud stars
 | journal=[[The Astrophysical Journal]]
 | volume=337  | page=251
 | doi=10.1086/167101 | bibcode=1989ApJ...337..251C
| doi-access=free
 }}
</ref>
The relative line strengths and widths for each WN sub-class were later quantified, and the ratio between the 541.1&nbsp;nm He<small>{{thinsp}}II</small> and 587.5&nbsp;nm, He<small>{{thinsp}}I</small> 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&nbsp;1 and Brey&nbsp;66 are now classified as WN3b. The somewhat obscure WN2.5 and WN4.5 classes were dropped.<ref name=smith96>
{{cite journal
 | last1=Smith   | first1=L.F.
 | last2=Michael | first2=S.M.
 | last3=Moffat  | first3=A.F.J.
 | year=1996
 | title=A three-dimensional classification for WN stars
 | journal=[[Monthly Notices of the Royal Astronomical Society]]
 | volume=281  | issue=1  | pages=163–191
 | doi=10.1093/mnras/281.1.163
 | bibcode=1996MNRAS.281..163S
| doi-access=free
 }}
</ref>

{| class="wikitable"
|+Classification of WN spectra
! Spectral Type !! Original criteria<ref name=hucht/> !! Updated criteria<ref name=smith96/> !! Other features
|-
| WN2 || N<small>{{thinsp}}V</small> weak or absent || N<small>{{thinsp}}V</small> and N<small>{{thinsp}}IV</small> absent || Strong He<small>{{thinsp}}II</small>, no He<small>{{thinsp}}I</small>
|-
| WN2.5 || N{{thinsp}}<small>{{thinsp}}V</small> present, N<small>{{thinsp}}IV</small> absent || Obsolete class ||
|-
| WN3 || N<small>{{thinsp}}IV</small> ≪ N<small>{{thinsp}}V</small>, N<small>{{thinsp}}III</small> weak or absent || He<small>{{thinsp}}II</small>/He<small>{{thinsp}}I</small> > 10, He<small>{{thinsp}}II</small>/C<small>{{thinsp}}IV</small> > 5 || Peculiar profiles, unpredictable N<small>{{thinsp}}V</small> strength
|-
| WN4 || N<small>{{thinsp}}IV</small> ≈ N<small>{{thinsp}}V</small>, N<small>{{thinsp}}III</small> weak or absent || 4 < He<small>{{thinsp}}II</small>/He<small>{{thinsp}}I</small> < 10, N<small>{{thinsp}}V</small>/N<small>{{thinsp}}III</small> > 2 || C<small>{{thinsp}}IV</small> present
|-
| WN4.5 || N<small>{{thinsp}}IV</small> > N<small>{{thinsp}}V</small>, N<small>{{thinsp}}III</small> weak or absent || Obsolete class ||
|-
| WN5 || N<small>{{thinsp}}III</small> ≈ N<small>{{thinsp}}IV</small> ≈ N<small>{{thinsp}}V</small> || 1.25 < He<small>{{thinsp}}II</small>/He<small>{{thinsp}}I</small> < 8, 0.5 < N<small>{{thinsp}}V</small>/N<small>{{thinsp}}III</small> < 2 || N<small>{{thinsp}}IV</small> or C<small>{{thinsp}}IV</small> > He<small>{{thinsp}}I</small>
|-
| WN6 || N<small>{{thinsp}}III</small> ≈ N<small>{{thinsp}}IV</small>, N<small>{{thinsp}}V</small> weak || 1.25 < He<small>{{thinsp}}II</small>/He<small>{{thinsp}}I</small> < 8, 0.2 < N<small>{{thinsp}}V</small>/N<small>{{thinsp}}III</small> < 0.5 || C<small>{{thinsp}}IV</small> ≈ He<small>{{thinsp}}I</small>
|-
| WN7 || N<small>{{thinsp}}III</small> > N<small>{{thinsp}}IV</small> || 0.65 < He<small>{{thinsp}}II</small>/He<small>{{thinsp}}I</small> < 1.25 || Weak P-Cyg profile He<small>{{thinsp}}I</small>, He<small>{{thinsp}}II</small> > N<small>{{thinsp}}III</small>, C<small>{{thinsp}}IV</small> > He<small>{{thinsp}}I</small>
|-
| WN8 || N<small>{{thinsp}}III</small> ≫ N<small>{{thinsp}}IV</small>|| He<small>{{thinsp}}II</small>/He<small>{{thinsp}}I</small> < 0.65 || Strong P-Cyg profile He<small>{{thinsp}}I</small>, He<small>{{thinsp}}II</small> ≈ N<small>{{thinsp}}III</small>, C<small>{{thinsp}}IV</small> weak
|-
| WN9 || N<small>{{thinsp}}III</small> > N<small>{{thinsp}}II</small>, N<small>{{thinsp}}IV</small> absent|| N<small>{{thinsp}}III</small> > N<small>{{thinsp}}II</small>, N<small>{{thinsp}}IV</small> absent || P-Cyg profile He<small>{{thinsp}}I</small>
|-
| WN10 || N<small>{{thinsp}}III</small> ≈ N<small>{{thinsp}}II</small> || N<small>{{thinsp}}III</small> ≈ N<small>{{thinsp}}II</small> || H Balmer, P-Cyg profile He<small>{{thinsp}}I</small>
|-
| WN11 || N<small>{{thinsp}}III</small> weak or absent, N<small>{{thinsp}}II</small> present|| N<small>{{thinsp}}II</small> ≈ He<small>{{thinsp}}II</small>, N<small>{{thinsp}}III</small> weak or absent, || H Balmer, P-Cyg profile He<small>{{thinsp}}I</small>, Fe<small>{{thinsp}}III</small> present
|}

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<small>{{thinsp}}II</small> 426.7&nbsp;nm, C<small>{{thinsp}}III</small> at 569.6&nbsp;nm, C<small>{{thinsp}}III/IV</small> 465.0&nbsp;nm, C<small>{{thinsp}}IV</small> at 580.1–581.2&nbsp;nm, and the O<small>{{thinsp}}V</small> (and O<small>{{thinsp}}III</small>) blend at 557.2–559.8&nbsp;nm.<ref name=hucht/> 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.<ref name=WCO/>

{| class="wikitable"
|+Classification of WC spectra
!rowspan=2| Spectral type
!rowspan=2| Original criteria<ref name=hucht/>
!colspan=2| Quantitative criteria<ref name=WCO/>
!rowspan=2| Other features
|-
! Primary !! Secondary
|-
| WC4 || C<small>{{thinsp}}IV</small> strong, C<small>{{thinsp}}II</small> weak, O<small>{{thinsp}}V</small> moderate || C<small>{{thinsp}}IV</small>/C<small>{{thinsp}}III</small> > 32 || O<small>{{thinsp}}V</small>/C<small>{{thinsp}}III</small> > 2.5 || O<small>{{thinsp}}VI</small> weak or absent
|-
| WC5 || C<small>{{thinsp}}III</small> ≪ C<small>{{thinsp}}IV</small>, C<small>{{thinsp}}III</small> < O<small>{{thinsp}}V</small> || 12.5 < C<small>{{thinsp}}IV</small>/C<small>{{thinsp}}III</small> < 32 || 0.4 < C<small>{{thinsp}}III</small>/O<small>{{thinsp}}V</small> < 3 || O<small>{{thinsp}}VI</small> weak or absent
|-
| WC6 || C<small>{{thinsp}}III</small> ≪ C<small>{{thinsp}}IV</small>, C<small>{{thinsp}}III</small> > O<small>{{thinsp}}V</small> || 4 < C<small>{{thinsp}}IV</small>/C<small>{{thinsp}}III</small> < 12.5 || 1 < C<small>{{thinsp}}III</small>/O<small>{{thinsp}}V</small> < 5 || O<small>{{thinsp}}VI</small> weak or absent
|-
| WC7 || C<small>{{thinsp}}III</small> < C<small>{{thinsp}}IV</small>, C<small>{{thinsp}}III</small> ≫ O<small>{{thinsp}}V</small> || 1.25 < C<small>{{thinsp}}IV</small>/C<small>{{thinsp}}III</small> < 4 || C<small>{{thinsp}}III</small>/O<small>{{thinsp}}V</small> > 1.25 || O<small>{{thinsp}}VI</small> weak or absent
|-
| WC8 || C<small>{{thinsp}}III</small> > C<small>{{thinsp}}IV</small>, C<small>{{thinsp}}II</small> absent, O<small>{{thinsp}}V</small> weak or absent || 0.5 < C<small>{{thinsp}}IV</small>/C<small>{{thinsp}}III</small> < 1.25 || C<small>{{thinsp}}IV</small>/C<small>{{thinsp}}II</small> > 10 || He<small>{{thinsp}}II</small>/He<small>{{thinsp}}I</small> > 1.25
|-
| WC9 || C<small>{{thinsp}}III</small> > C<small>{{thinsp}}IV</small>, C<small>{{thinsp}}II</small> present, O<small>{{thinsp}}V</small> weak or absent || 0.2 < C<small>{{thinsp}}IV</small>/C<small>{{thinsp}}III</small> < 0.5 || 0.6 < C<small>{{thinsp}}IV</small>/C<small>{{thinsp}}II</small> < 10 || 0.15 < He<small>{{thinsp}}II</small>/He<small>{{thinsp}}I</small> < 1.25
|-
| WC10 || || 0.06 < C<small>{{thinsp}}IV</small>/C<small>{{thinsp}}III</small> < 0.15 || 0.03 < C<small>{{thinsp}}IV</small>/C<small>{{thinsp}}II</small> < 0.6 || He<small>{{thinsp}}II</small>/He<small>{{thinsp}}I</small> < 0.15
|-
| WC11 || || C<small>{{thinsp}}IV</small>/C<small>{{thinsp}}III</small> < 0.06 || C<small>{{thinsp}}IV</small>/C<small>{{thinsp}}II</small> < 0.03 || He<small>{{thinsp}}II</small> absent
|}

For WO-type stars the main lines used are C<small>{{thinsp}}IV</small> at 580.1&nbsp;nm, O<small>{{thinsp}}IV</small> at 340.0&nbsp;nm, O<small>{{thinsp}}V</small> (and O<small>{{thinsp}}III</small>) blend at 557.2–559.8&nbsp;nm, O<small>{{thinsp}}VI</small> at 381.1–383.4&nbsp;nm, O<small>{{thinsp}}VII</small> at 567.0&nbsp;nm, and O<small>{{thinsp}}VIII</small> at 606.8&nbsp;nm. The sequence was expanded to include WO5 and quantified based the relative strengths of the O<small>{{thinsp}}VI</small>/C<small>{{thinsp}}IV</small> and O<small>{{thinsp}}VI</small>/O<small>{{thinsp}}V</small> lines.<ref name=kingsburgh>
{{cite journal
 |last1=Kingsburgh |first1=R.L.
 |last2=Barlow     |first2=M.J.
 |last3=Storey     |first3=P.J.
 |year=1995
 |title=Properties of the WO Wolf–Rayet stars
 |journal=Astronomy and Astrophysics
 |volume=295 |pages=75
 |issn=0004-6361 |bibcode=1995A&A...295...75K
}}
</ref>
A later scheme, designed for consistency across classical WR stars and CSPNe, returned to the WO1 to WO4 sequence and adjusted the divisions.<ref name=WCO/>

{| class="wikitable"
|+Classification of WO spectra
! rowspan=2|Spectral type !! rowspan=2|Original criteria<ref name=hucht/> !! colspan=2|Quantitative criteria<ref name=WCO/> !! rowspan=2|Other features
|-
! Primary !! Secondary
|-
| WO1 || O<small>{{thinsp}}VII</small> ≥ O<small>{{thinsp}}V</small>, O<small>{{thinsp}}VIII</small> present || O<small>{{thinsp}}VI</small>/O<small>{{thinsp}}V</small> > 12.5 || O<small>{{thinsp}}VI</small>/C<small>{{thinsp}}IV</small> > 1.5 || O<small>{{thinsp}}VII</small> ≥ O<small>{{thinsp}}V</small>
|-
| WO2 || O<small>{{thinsp}}VII</small> < O<small>{{thinsp}}V</small>, C<small>{{thinsp}}IV</small> < O<small>{{thinsp}}VI</small> || 4 < O<small>{{thinsp}}VI</small>/O<small>{{thinsp}}V</small> < 12.5 || O<small>{{thinsp}}VI</small>/C<small>{{thinsp}}IV</small> > 1.5 || O<small>{{thinsp}}VII</small> ≤ O<small>{{thinsp}}V</small>
|-
| WO3 || O<small>{{thinsp}}VII</small> weak or absent, C<small>{{thinsp}}IV</small> ≈ O<small>{{thinsp}}VI</small> || 1.8 < O<small>{{thinsp}}VI</small>/O<small>{{thinsp}}V</small> < 4 || 0.1 < O<small>{{thinsp}}VI</small>/C<small>{{thinsp}}IV</small> < 1.5 || O<small>{{thinsp}}VII</small> ≪ O<small>{{thinsp}}V</small>
|-
| WO4 || C<small>{{thinsp}}IV</small> ≫ O<small>{{thinsp}}VI</small> || 0.5 < O<small>{{thinsp}}VI</small>/O<small>{{thinsp}}V</small> < 1.8 || 0.03 < O<small>{{thinsp}}VI</small>/C<small>{{thinsp}}IV</small> < 0.1 || O<small>{{thinsp}}VII</small> ≪ O<small>{{thinsp}}V</small>
|}

Detailed modern studies of Wolf–Rayet stars can identify additional spectral features, indicated by suffixes to the main spectral classification:<ref name=smith96/>
* h for hydrogen emission;
* ha for hydrogen emission and absorption;
* o for no hydrogen emission;
* 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).<ref>
{{cite journal
 | last1=Smith | first1=J.D.T.
 | last2=Houck | first2=J.R.
 | year=2001
 | title=A mid-unfrared spectral survey of galactic Wolf–Rayet stars
 | journal=The Astronomical Journal
 | volume=121 | issue=4 | pages=2115–2123
 | doi=10.1086/319968 | bibcode=2001AJ....121.2115S
| s2cid=122617193
 | doi-access=free
 }}
</ref>

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.<ref name=smith96/>

The hotter WR spectral sub-classes are described as early and the cooler ones as late, consistent with other spectral types. WNE and WCE refer to early type spectra while WNL and WCL refer to late type spectra, with the dividing line approximately at sub-class six or seven. There is no such thing as a late WO-type star. There is a strong tendency for WNE stars to be hydrogen-poor while the spectra of WNL stars frequently include hydrogen lines.<ref name=hucht/><ref name=crowther/>

Spectral types for the central stars of planetary nebulae are qualified by surrounding them with square brackets (e.g. [WC4]).<ref name=hucht/><ref name=hamann96/> They are almost all of the WC sequence with the known [WO] stars representing the hot extension of the carbon sequence. There are also a small number of [WN] and [WC/WN] types, only discovered quite recently.<ref name=todt2010>
{{cite journal
 |last1=Todt |first1=H.
 |display-authors=etal
 |year=2010
 |title=The central star of the planetary nebula PB 8: a Wolf–Rayet-type wind of an unusual WN/WC chemical composition
 |journal=[[Astronomy and Astrophysics]]
 |volume=515 |pages=A83
 |doi=10.1051/0004-6361/200912183 |arxiv=1003.3419
 |bibcode=2010A&A...515A..83T |s2cid=118684886
}}
</ref><ref name=miszalski>
{{cite journal
 |last1=Miszalski |first1=B. |display-authors=etal
 |year=2012
 |title=IC 4663: the first unambiguous [WN] Wolf–Rayet central star of a planetary nebula
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=423 |issue=1 |pages=934–947
 |doi=10.1111/j.1365-2966.2012.20929.x
 |doi-access=free |arxiv=1203.3303 |s2cid=10264296 |bibcode=2012MNRAS.423..934M
}}
</ref><ref name=todt2013>
{{cite journal
 |last1=Todt |first1=H.
 |display-authors=etal
 |year=2013
 |title=Abell&nbsp;48 – a rare WN-type central star of a planetary nebula
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=430 |issue=3 |pages=2301–2312
 |doi=10.1093/mnras/stt056 |doi-access=free
 |arxiv = 1301.1944
 |s2cid=118527324 |bibcode=2013MNRAS.430.2302T
}}
</ref><ref name=frew>
{{cite journal
 |last1=Frew |first1=David J.
 |display-authors=etal
 |year=2014
 |bibcode=2014MNRAS.440.1345F
 |title=The planetary nebula Abell&nbsp;48 and its [WN] nucleus
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=440 |issue=2 |pages=1345–1364
 |doi=10.1093/mnras/stu198 |doi-access=free
 |arxiv = 1301.3994 |s2cid=118489305
}}
</ref>
Their formation mechanism is as yet unclear. Temperatures of the planetary nebula central stars tend to the extremes when compared to population&nbsp;I WR stars, so [WC2] and [WC3] are common and the sequence has been extended to [WC12]. The [WC11] and [WC12] types have distinctive spectra with narrow emission lines and no He<small>{{thinsp}}II</small> and C<small>{{thinsp}}IV</small> lines.<ref name=hamann>
{{cite conference
 |last1=Hamann |first1=W.-R.
 |year=1997
 |title=Spectra of Wolf–Rayet type central stars and their analysis
 |department=Invited Review
<!--
 |book-title=Proceedings of the 180th Symposium of the International Astronomical Union
-->
 |conference=180th Symposium of the [[International Astronomical Union]]
 |page=91
 |bibcode=1997IAUS..180...91H
 |publisher=Kluwer Academic Publishers
}}
</ref><ref name=hamann96>
{{cite journal
 |last1=Hamann |first1=Wolf-Rainer
 |year=1996
 |title=Spectral analysis and model atmospheres of WR central stars
 |department=Invited paper
 |journal=[[Astrophysics and Space Science]]
 |volume=238 |issue=1|pages=31
 |bibcode=1996Ap&SS.238...31H
 |doi=10.1007/BF00645489 |s2cid=118326865
}}
</ref>

[[File:GKPersei-MiniSuperNova-20150316.jpg|right|upright=1.2|thumb|alt=Nova GK Persei|[[GK Persei]] (Nova Persei 1901), which showed Wolf–Rayet features in its spectrum<ref name=beals1929/>]]
Certain supernovae observed before their peak brightness show WR spectra.<ref name=liu>
{{cite journal
 | last1=Liu  | first1=Q.-Z.        | last2=Hu   | first2=J.-Y.
 | last3=Hang | first3=H.-R.        | last4=Qiu  | first4=Y.-L.
 | last5=Zhu  | first5=Z.-X.        | last6=Qiao | first6=Q.-Y.
 | year=2000
 | title=The supernova 1998S in NGC&nbsp;3877: Another supernova with Wolf–Rayet star features in pre-maximum spectrum
 | journal=Astronomy and Astrophysics Supplement Series
 | volume=144  | issue=2  | pages=219–225
 | bibcode=2000A&AS..144..219L
 | doi=10.1051/aas:2000208
 | s2cid=8496705 | url=https://pure.uva.nl/ws/files/3075597/12756_88376y.pdf
}}
</ref>
This is due to the nature of the supernova at this point: a rapidly expanding helium-rich ejecta similar to an extreme Wolf–Rayet wind. The WR spectral features only last a matter of hours, the high ionisation features fading by maximum to leave only weak neutral hydrogen and helium emission, before being replaced with a traditional supernova spectrum. It has been proposed to label these spectral types with an "X", for example XWN5(h).<ref>
{{cite journal
 | last1=Groh | first1=Jose H.
 | year=2014
 | title=Early-time spectra of supernovae and their precursor winds
 | journal=[[Astronomy and Astrophysics]]
 | volume=572 | pages=L11
 | arxiv = 1408.5397 | s2cid=118935040
 | doi=10.1051/0004-6361/201424852 | bibcode=2014A&A...572L..11G
}}
</ref> Similarly, [[classical nova]]e develop spectra consisting of broad emission bands similar to a Wolf–Rayet star. This is caused by the same physical mechanism: rapid expansion of dense gases around an extremely hot central source.<ref name=beals1940/>

==Slash stars==
The separation of Wolf–Rayet stars from spectral [[O-type star|class&nbsp;O stars]] of a similar temperature depends on the existence of strong emission lines of ionised helium, nitrogen, carbon, and oxygen, but there are a number of stars with intermediate or confusing spectral features. For example, high-luminosity O&nbsp;stars can develop helium and nitrogen in their spectra with some emission lines, while some WR stars have hydrogen lines, weak emission, and even absorption components. These stars have been given spectral types such as O3If<sup>∗</sup>/WN6 and are referred to as slash stars.<ref name=crowther2011>
{{cite journal
 |last1=Crowther |first1=Paul A.
 |last2=Walborn  |first2=Nolan R. | author2-link=Nolan R. Walborn
 |year=2011
 |title=Spectral classification of O2–3.5 If*/WN5-7 stars
 |journal=Monthly Notices of the Royal Astronomical Society
 |volume=416 |issue=2 |pages=1311
 |bibcode=2011MNRAS.416.1311C |arxiv=1105.4757
 |doi=10.1111/j.1365-2966.2011.19129.x |doi-access=free
 |s2cid=118455138
}}
</ref>

Class O supergiants can develop emission lines of helium and nitrogen, or emission components to some absorption lines. These are indicated by spectral peculiarity suffix codes specific to this type of star:
* f for N{{smallcaps|{{thinsp}}iii}} and He{{smallcaps|{{thinsp}}ii}} emission
* f<sup>*</sup> for N and He emission with N{{smallcaps|{{thinsp}}iv}} stronger than N{{smallcaps|{{thinsp}}iii}}
* f<sup>+</sup> for emission in Si{{smallcaps|{{thinsp}}iv}} in addition to N and He
* parentheses indicating He{{smallcaps|{{thinsp}}ii}} absorption lines instead of emission, e.g. (f)
* double parentheses indicating strong He{{smallcaps|{{thinsp}}ii}} absorption and N{{smallcaps|{{thinsp}}iii}} emission diluted, e.g. ((f<sup>+</sup>))

These codes may also be combined with more general spectral type qualifiers such as p or a. Common combinations include OIafpe and OIf<sup>*</sup>, and Ofpe. In the 1970s, it was recognised that there was a continuum of spectra from pure absorption class O to unambiguous WR types, and it was unclear whether some intermediate stars should be given a spectral type such as O8Iafpe or WN8-a. The slash notation was proposed to deal with these situations, and the star Sk−67°22 was assigned the spectral type O3If<sup>*</sup>/WN6-A.<ref name=walborn1982a>{{cite journal|bibcode=1982ApJ...254L..15W|title=The O3 stars|journal=Astrophysical Journal|volume=254|pages=L15|last1=Walborn|first1=N. R.|year=1982|doi=10.1086/183747|doi-access=free}}</ref> The criteria for distinguishing OIf<sup>*</sup>, OIf<sup>*</sup>/WN, and WN stars have been refined for consistency. Slash star classifications are used when the H<sub>β</sub> line has a P Cygni profile; this is an absorption line in O supergiants and an emission line in WN stars. Criteria for the following slash star spectral types are given, using the nitrogen emission lines at 463.4–464.1&nbsp;nm, 405.8&nbsp;nm, and 460.3–462.0&nbsp;nm, together with a standard star for each type:<ref name=crowther2011/>
{| class="wikitable"
|+Classification of slash stars
|-
! Spectral type !! Standard star !! Criteria
|-
| O2If<sup>*</sup>/WN5 || [[Melnick 35]] || N{{smallcaps|{{thinsp}}iv}} ≫ N{{smallcaps|{{thinsp}}iii}}, N{{smallcaps|{{thinsp}}v}} ≥ N{{smallcaps|{{thinsp}}iii}}
|-
| O2.5If<sup>*</sup>/WN6 || [[WR 25]] || N{{smallcaps|{{thinsp}}iv}} > N{{smallcaps|{{thinsp}}iii}}, N{{smallcaps|{{thinsp}}v}} < N{{smallcaps|{{thinsp}}iii}}
|-
| O3.5If<sup>*</sup>/WN7 || [[Melnick 51]] || N{{smallcaps|{{thinsp}}iv}} < N{{smallcaps|{{thinsp}}iii}}, N{{smallcaps|{{thinsp}}v}} ≪ N{{smallcaps|{{thinsp}}iii}}
|}

Another set of slash star spectral types is in use for Ofpe/WN stars. These stars have O supergiant spectra plus nitrogen and helium emission, and P Cygni profiles. Alternatively they can be considered to be WN stars with unusually low ionisation levels and hydrogen.<ref name=walborn1082b>{{cite journal|bibcode=1982ApJ...256..452W|title=Ofpe/WN9 circumstellar shells in the Large Magellanic Cloud|journal=Astrophysical Journal|volume=256|pages=452|last1=Walborn|first1=N. R.|year=1982|doi=10.1086/159922}}</ref> The slash notation for these stars was controversial and an alternative was to extend the WR nitrogen sequence to WN10 and WN11<ref name=smith1994>{{cite journal|bibcode=1994A&A...281..833S|title=A study of the luminous blue variable candidate He 3-519 and its surrounding nebula|journal=Astronomy and Astrophysics|volume=281|pages=833|last1=Smith|first1=L. J.|last2=Crowther|first2=P. A.|last3=Prinja|first3=R. K.|year=1994}}</ref> Other authors preferred to use the WNha notation, for example WN9ha for [[WR 108]].<ref name=crowther1997>{{cite journal|bibcode=1997A&A...317..532C|title=The distinction between OIafpe and WNLha stars. A spectral analysis of HD 151804, HD 152408 and HDE 313846|journal=Astronomy and Astrophysics|volume=317|pages=532|last1=Crowther|first1=P. A.|last2=Bohannan|first2=B.|year=1997}}</ref> A recent recommendation is to use an O spectral type such as O8Iaf if the 447.1&nbsp;nm He{{smallcaps|{{thinsp}}i}} line is in absorption and a WR class of WN9h or WN9ha if the line has a P Cygni profile.<ref name=crowther2011/> However, the Ofpe/WN slash notation as well as WN10 and WN11 classifications continue to be widely used.<ref>{{cite journal|bibcode=2015A&A...578A.108V|arxiv=1504.03204|title=The Herschel view of the nebula around the luminous blue variable star AG Carinae|journal=Astronomy & Astrophysics|volume=578|pages=A108|last1=Vamvatira-Nakou|first1=C.|last2=Hutsemékers|first2=D.|last3=Royer|first3=P.|last4=Cox|first4=N. L. J.|last5=Nazé|first5=Y.|last6=Rauw|first6=G.|last7=Waelkens|first7=C.|last8=Groenewegen|first8=M. A. T.|year=2015|doi=10.1051/0004-6361/201425090|s2cid=119160088}}</ref>

A third group of stars with spectra containing features of both O class stars and WR stars has been identified. Nine stars in the Large Magellanic Cloud have spectra that contain both WN3 and O3V features, but do not appear to be binaries. Many of the WR stars in the Small Magellanic Cloud also have very early WN spectra plus high excitation absorption features. It has been suggested that these could be a missing link leading to classical WN stars or the result of tidal stripping by a low-mass companion.<ref name=massey2018/>

==Nomenclature==
[[File:Carina Nebula around the Wolf–Rayet star WR 22.jpg|left|thumb|alt=Carina Nebula around Wolf–Rayet star WR 22|[[WR 22]] in the [[Carina Nebula]]]]
The first three Wolf–Rayet stars to be identified, coincidentally all with hot O-class companions, had already been numbered in the [[Henry Draper Catalogue|Henry Draper catalogue]]. These stars and others were referred to as Wolf–Rayet stars from their initial discovery but specific naming conventions for them would not be created until 1962 in the "fourth" catalogue of galactic Wolf–Rayet stars.<ref name=roberts>{{cite journal|doi=10.1086/108603|title=The galactic distribution of the Wolf–Rayet stars|journal=The Astronomical Journal|volume=67|pages=79|date=1962|last1=Roberts|first1=M. S.|bibcode=1962AJ.....67...79R}}</ref> The first three catalogues were not specifically lists of Wolf–Rayet stars and they used only existing nomenclature.<ref name=campbell>{{cite journal|doi=10.1086/140127|title=Stars whose spectra contain both bright and dark hydrogen lines|journal=The Astrophysical Journal|volume=2|pages=177|date=1895|last1=Campbell|first1=W. W.|bibcode=1895ApJ.....2..177C|doi-access=free}}</ref><ref name=payne>{{cite book|bibcode=1930HarMo...3....1P|title=The stars of high luminosity|series=Harvard Observatory Monographs |volume=3|pages=1|last1=Gaposchkin|first1=Cecilia Payne|date=1930}}</ref><ref name=fleming>{{cite journal|bibcode=1912AnHar..56..165F|title=Stars having peculiar spectra|journal=Annals of the Astronomical Observatory of Harvard College|volume=56|issue=6|pages=165|last1=Fleming|first1=Williamina Paton Stevens|last2=Pickering|first2=Edward Charles|date=1912}}</ref> 

The fourth catalogue of Wolf-Rayet stars numbered them sequentially in order of [[right ascension]]. The fifth catalogue used the same numbers prefixed with MR after the author of the fourth catalogue, plus an additional sequence of numbers prefixed with LS for new discoveries.<ref name="smith" /> Neither of these numbering schemes remains in common use.

{{clear}}
=== Modern WR catalogues ===
The sixth Catalogue of Galactic Wolf–Rayet stars was the first to actually bear that name, as well as to describe the previous five catalogues by that name. It also introduced the WR numbers widely used ever since for galactic WR stars. These are again a numerical sequence from WR 1 to WR 158 in order of right ascension.<ref name="VIth">{{cite journal|doi=10.1007/BF00173260|title=The Sixth Catalogue of galactic Wolf–Rayet stars, their past and present|journal=Space Science Reviews|volume=28|issue=3|pages=227–306|date=1981|last1=Van Der Hucht|first1=Karel A.|author-link1=Karel van der Hucht|last2=Conti|first2=Peter S.|last3=Lundström|first3=Ingemar|last4=Stenholm|first4=Björn|bibcode=1981SSRv...28..227V|s2cid=121477300}}</ref> 

Compiled in 2001, the seventh catalogue and its annex used the same numbering scheme and inserted new stars into the sequence using lower case letter suffixes, for example [[WR 102ka]] for one of the numerous WR stars discovered in the galactic centre.<ref name="hucht" /><ref name="annex">{{cite journal|doi=10.1051/0004-6361:20065819|title=New Galactic Wolf–Rayet stars, and candidates|journal=Astronomy and Astrophysics|volume=458|issue=2|pages=453–459|date=2006|last1=Van Der Hucht|first1=K. A.|author-link1=Karel van der Hucht|bibcode=2006A&A...458..453V|arxiv = astro-ph/0609008 |s2cid=119104786}}</ref> Modern high volume identification surveys use their own numbering schemes for the large numbers of new discoveries.<ref name="shara">{{cite journal|doi=10.1088/0004-6256/143/6/149|title=A Near-Infrared Survey of the Inner Galactic Plane for Wolf–Rayet Stars. Ii. Going Fainter: 71 More New W-R Stars|journal=The Astronomical Journal|volume=143|issue=6|pages=149|date=2012|last1=Shara|first1=Michael M.|last2=Faherty|first2=Jacqueline K.|author2-link=Jackie Faherty|last3=Zurek|first3=David|last4=Moffat|first4=Anthony F. J.|last5=Gerke|first5=Jill|last6=Doyon|first6=René|last7=Artigau|first7=Etienne|last8=Drissen|first8=Laurent|bibcode=2012AJ....143..149S|arxiv = 1106.2196 |s2cid=119186111}}</ref> A 2006 Annex was added to the seventh catalog.  

In 2011, an online Galactic Wolf Rayet Catalogue was set up, hosted by the [[University of Sheffield]]. As of February 2025, it includes 709 stars.<ref name=":1">{{Cite web |title=Wolf-Rayet Star Catalogue |url=https://pacrowther.staff.shef.ac.uk/WRcat/ |access-date=2025-01-17 |website=pacrowther.staff.shef.ac.uk}}</ref>

=== Other numbering schemes ===
Wolf–Rayet stars in external galaxies are numbered using different schemes. In the [[Large Magellanic Cloud]], the most widespread and complete nomenclature for WR stars is from "The Fourth Catalogue of Population I Wolf–Rayet stars in the Large Magellanic Cloud"<ref name=bat99>{{cite journal|doi=10.1051/aas:1999240|title=The fourth catalogue of Population I Wolf–Rayet stars in the Large Magellanic Cloud|journal=Astronomy and Astrophysics Supplement Series|volume=137|pages=117–145|date=1999|last1=Breysacher|first1=J.|last2=Azzopardi|first2=M.|last3=Testor|first3=G.|bibcode=1999A&AS..137..117B|doi-access=free}}</ref> prefixed by {{nowrap|BAT-99}}, for example {{nowrap|[[Melnick 42|BAT-99 105]]}}. Many of these stars are also referred to by their third catalogue number, for example [[Melnick 42|Brey 77]].<ref name=brey>{{cite journal|bibcode=1981A&AS...43..203B|title=Spectral Classification of Wolf–Rayet Stars in the Large Magellanic Cloud|journal=Astronomy and Astrophysics Supplement|volume=43|pages=203|last1=Breysacher|first1=J.|date=1981}}</ref> As of 2018, 154 WR stars are catalogued in the LMC, mostly WN but including about twenty-three WCs as well as three of the extremely rare WO class.<ref name=massey2018>{{Cite journal|arxiv=1807.01209|last1=Neugent|first1=Kathryn F|title=A Modern Search for Wolf–Rayet Stars in the Magellanic Clouds. IV. A Final Census|journal=The Astrophysical Journal|volume=863|issue=2|pages=181|last2=Massey|first2=Philip|last3=Morrell|first3=Nidia|year=2018|doi=10.3847/1538-4357/aad17d|bibcode=2018ApJ...863..181N|s2cid=118988083 |doi-access=free }}</ref><ref name=hainich>{{cite journal|bibcode=2014A&A...565A..27H|title=The Wolf–Rayet stars in the Large Magellanic Cloud. A comprehensive analysis of the WN class|journal=Astronomy & Astrophysics|volume=565|pages=A27|last1=Hainich|first1=R.|last2=Rühling|first2=U.|last3=Todt|first3=H.|last4=Oskinova|first4=L. M.|last5=Liermann|first5=A.|last6=Gräfener|first6=G.|last7=Foellmi|first7=C.|last8=Schnurr|first8=O.|last9=Hamann|first9=W.-R.|date=2014|doi=10.1051/0004-6361/201322696|arxiv = 1401.5474 |s2cid=55123954}}</ref> Many of these stars are often referred to by their RMC (Radcliffe observatory Magellanic Cloud) numbers, frequently abbreviated to just R, for example [[R136a1]].

In the [[Small Magellanic Cloud]] SMC WR numbers are used, usually referred to as AB numbers, for example [[AB7]].<ref name=ab>{{cite journal|bibcode=1979A&A....75..120A|title=A search for new Wolf–Rayet stars in the Small Magellanic Cloud|journal=Astronomy and Astrophysics|volume=75|pages=120|last1=Azzopardi|first1=M.|last2=Breysacher|first2=J.|date=1979}}</ref> There are only twelve known WR stars in the SMC, a very low number thought to be due to the low [[metallicity]] of that galaxy<ref name=massey12th>{{cite journal|doi=10.1086/379024|title=The Discovery of a 12th Wolf-Rayet Star in the Small Magellanic Cloud|journal=Publications of the Astronomical Society of the Pacific|volume=115|issue=813|pages=1265–1268|date=2003|last1=Massey|first1=Philip|last2=Olsen|first2=K. A. G.|last3=Parker|first3=J. Wm.|bibcode=2003PASP..115.1265M|arxiv = astro-ph/0308237 |s2cid=15609362}}</ref><ref name=massey>{{cite journal|doi=10.1086/319818|title=A Search for Wolf-Rayet Stars in the Small Magellanic Cloud|journal=The Astrophysical Journal|volume=550|issue=2|pages=713–723|date=2001|last1=Massey|first1=Philip|last2=Duffy|first2=Alaine S.|bibcode=2001ApJ...550..713M|arxiv = astro-ph/0010420 |s2cid=1579181}}</ref><ref name=smc>{{cite journal|doi=10.1088/0004-6256/140/2/416|title=Spitzersage-Smc Infrared Photometry of Massive Stars in the Small Magellanic Cloud|journal=The Astronomical Journal|volume=140|issue=2|pages=416–429|date=2010|last1=Bonanos|first1=A. Z.|last2=Lennon|first2=D. J.|last3=Köhlinger|first3=F.|last4=Van Loon|first4=J. Th.|last5=Massa|first5=D. L.|last6=Sewilo|first6=M.|last7=Evans|first7=C. J.|last8=Panagia|first8=N.|last9=Babler|first9=B. L.|last10=Block|first10=M.|last11=Bracker|first11=S.|last12=Engelbracht|first12=C. W.|last13=Gordon|first13=K. D.|last14=Hora|first14=J. L.|last15=Indebetouw|first15=R.|last16=Meade|first16=M. R.|last17=Meixner|first17=M.|last18=Misselt|first18=K. A.|last19=Robitaille|first19=T. P.|last20=Shiao|first20=B.|last21=Whitney|first21=B. A.|bibcode=2010AJ....140..416B|arxiv = 1004.0949 |s2cid=119290443}}</ref>

In 2012, an [[IAU]] working group expanded the numbering system from the Catalogue of Galactic Wolf–Rayet stars so that additional discoveries are given the closest existing WR number plus a numeric suffix in order of discovery. This applies to all discoveries since the 2006 annex, although some of these have already been named under the previous nomenclature; thus [[WR 42e]] is now numbered WR 42-1.<ref>{{cite journal |last1=Rosslowe |first1=C. K. |last2=Crowther |first2=P. A. |year=2015 |title=Spatial distribution of Galactic Wolf–Rayet stars and implications for the global population |journal=Monthly Notices of the Royal Astronomical Society |volume=447 |issue=3 |pages=2322–2347 |arxiv=1412.0699 |bibcode=2015MNRAS.447.2322R |doi=10.1093/mnras/stu2525 |doi-access=free |s2cid=28747394}}</ref>

==Properties==
Wolf–Rayet stars are a normal stage in the evolution of very massive stars, in which strong, broad [[spectral line|emission lines]] of helium and nitrogen ("WN" sequence), carbon ("WC" sequence), and oxygen ("WO" sequence) are visible. Due to their strong emission lines they can be identified in nearby galaxies. About 600 Wolf–Rayets have been catalogued in our own [[Milky Way Galaxy]].<ref name=hucht/><ref name=annex/><ref name=shara/><ref name=":1" /> This number has changed dramatically during the last few years as the result of photometric and spectroscopic surveys in the near-infrared dedicated to discovering this kind of object in the [[Galactic plane]].<ref name=shara2009>{{cite journal|doi=10.1088/0004-6256/138/2/402|title=A Near-Infrared Survey of the Inner Galactic Plane for Wolf–Rayet Stars. I. Methods and First Results: 41 New Wr Stars|journal=The Astronomical Journal|volume=138|issue=2|pages=402–420|date=2009|last1=Shara|first1=Michael M.|last2=Moffat|first2=Anthony F. J.|last3=Gerke|first3=Jill|last4=Zurek|first4=David|last5=Stanonik|first5=Kathryn|last6=Doyon|first6=René|last7=Artigau|first7=Etienne|last8=Drissen|first8=Laurent|last9=Villar-Sbaffi|first9=Alfredo|bibcode=2009AJ....138..402S|arxiv = 0905.1967 |s2cid=118370109}}</ref> It is expected that there are fewer than 1,000 WR stars in the rest of the [[Local Group]] galaxies, with around 166 known in the [[Magellanic Clouds]],<ref name="massey2018" /> 206 in the [[Triangulum Galaxy]],<ref name=m33>{{cite journal|doi=10.1088/0004-637X/733/2/123|title=The Wolf–Rayet Content of M33|journal=The Astrophysical Journal|volume=733|issue=2|pages=123|date=2011|last1=Neugent|first1=Kathryn F.|last2=Massey|first2=Philip|bibcode=2011ApJ...733..123N|arxiv = 1103.5549 |s2cid=118507918}}</ref> and 154 in the [[Andromeda Galaxy]].<ref name=m31>{{cite journal|doi=10.1088/0004-637X/759/1/11|title=The Wolf–Rayet Content of M31|journal=The Astrophysical Journal|volume=759|issue=1|pages=11|date=2012|last1=Neugent|first1=Kathryn F.|last2=Massey|first2=Philip|last3=Georgy|first3=Cyril|bibcode=2012ApJ...759...11N|arxiv = 1209.1177 |s2cid=118620069}}</ref> 

Outside the local group, whole galaxy surveys have found thousands more WR stars and candidates. For example, in the [[M101 Group]], over a thousand potential WR stars have been detected, from magnitude 21 to 25,<ref name="bibby">{{cite journal|bibcode=2012AAS...21924213B|title=A Study of the Wolf–Rayet Population of M101 using the Hubble Space Telescope|journal=American Astronomical Society|volume=219|pages=#242.13|last1=Bibby|first1=Joanne|last2=Shara|first2=M.|date=2012}}</ref> and astronomers hope to eventually catalog over ten thousand.<ref>{{Cite journal |last1=Shara |first1=Michael M. |last2=Bibby |first2=Joanne L. |last3=Zurek |first3=David |last4=Crowther |first4=Paul A. |last5=Moffat |first5=Anthony F. J. |last6=Drissen |first6=Laurent |date=2013-12-01 |title=The Vast Population of Wolf-Rayet and Red Supergiant Stars in M101. I. Motivation and First Results |url=https://ui.adsabs.harvard.edu/abs/2013AJ....146..162S |journal=The Astronomical Journal |volume=146 |issue=6 |pages=162 |doi=10.1088/0004-6256/146/6/162 |issn=0004-6256|arxiv=1302.6631 |bibcode=2013AJ....146..162S }}</ref> These stars are expected to be particularly common in the [[starburst galaxy#Types|Wolf–Rayet galaxies]] named after them and in [[starburst galaxy|starburst galaxies]].<ref>{{cite journal|doi=10.1086/305487|title=New Models for Wolf-Rayet and O Star Populations in Young Starbursts|journal=The Astrophysical Journal|volume=497|issue=2|pages=618–644|year=1998|last1=Schaerer|first1=Daniel|last2=Vacca|first2=William D.|bibcode=1998ApJ...497..618S|arxiv = astro-ph/9711140 |s2cid=10201971}}</ref>

Their characteristic emission lines are formed in the extended and dense high-velocity wind region enveloping the very hot stellar [[photosphere]], which produces a flood of [[ultraviolet|UV]] radiation that causes fluorescence in the line-forming wind region.<ref name=wc>{{cite journal|doi=10.1051/0004-6361/201117830|title=The Galactic WC stars|journal=Astronomy & Astrophysics|volume=540|pages=A144|date=2012|last1=Sander|first1=A.|last2=Hamann|first2=W.-R.|last3=Todt|first3=H.|bibcode=2012A&A...540A.144S|arxiv = 1201.6354 |s2cid=119182468}}</ref> This ejection process uncovers in succession, first the nitrogen-rich products of [[CNO cycle]] burning of hydrogen (WN stars), and later the carbon-rich layer due to [[Triple-alpha process|He burning]] (WC and WO-type stars).<ref name=barlow>
{{cite conference
 |author1=Barlow, M.J.
 |author2=Hummer, D.G.
 |year=1982
 |title=The WO Wolf–Rayet stars
 |conference=Wolf–Rayet stars: Observations, physics, evolution
 |volume=99 |pages=387–392
 |publisher=[[International Astronomical Union]]
 |place=Cozumel, Mexico
 |bibcode=1982IAUS...99..387B
 |doi=10.1007/978-94-009-7910-9_51 |isbn=978-90-277-1470-1
}}
</ref>

{| class="wikitable sortable"
|+Physical properties of galactic population I WN stars<ref name=crowther>{{cite journal|doi=10.1146/annurev.astro.45.051806.110615|title=Physical Properties of Wolf–Rayet Stars|journal=Annual Review of Astronomy and Astrophysics|volume=45|issue=1|pages=177–219|year=2007|last1=Crowther|first1=Paul A.|bibcode=2007ARA&A..45..177C|arxiv = astro-ph/0610356 |s2cid=1076292}}</ref><ref name=wn>{{cite journal|doi=10.1051/0004-6361:20065052|title=The Galactic WN stars|journal=Astronomy and Astrophysics|volume=457|issue=3|pages=1015–1031|year=2006|last1=Hamann|first1=W.-R.|last2=Gräfener|first2=G.|last3=Liermann|first3=A.|bibcode=2006A&A...457.1015H|arxiv = astro-ph/0608078 |s2cid=18714731}}</ref><ref name=barniske>
{{cite journal
 |last1=Barniske |first1=A.
 |last2=Hamann   |first2=W.-R.
 |last3=Gräfener |first3=G.
 |year=2006
 |title=Wolf–Rayet stars of the carbon sequence
 |journal=ASP Conference Series
 |publisher=[[Astronomical Society of the Pacific]]
 |volume=353 |page=243
 |bibcode=2006ASPC..353..243B
}}
</ref>
|-
! Spectral<br />type !! Temperature<br />(K) !! Radius<br />({{solar radius}}) !! Mass<br />({{solar mass}}) !! Luminosity<br />({{solar luminosity}}) !! Absolute<br />magnitude !! Example
|-
| WN2 || style="text-align:right; background-color:#{{Color temperature|141000 |hexval}}"|141,000 || style="text-align:right"|0.89 || style="text-align:right"|16 || style="text-align:right"|280,000 || style="text-align:right"|−2.6||[[WR 2]]
|-
| WN3 || style="text-align:right; background-color:#{{Color temperature|85000 |hexval}}"|85,000 || style="text-align:right"|2.3 || style="text-align:right"|19 || style="text-align:right"|220,000 || style="text-align:right"|−3.2||[[WR 46]]
|-
| WN4 || style="text-align:right; background-color:#{{Color temperature|70000 |hexval}}"|70,000 || style="text-align:right"|2.3 || style="text-align:right"|15 || style="text-align:right"|200,000 || style="text-align:right"|−3.8||[[WR 1]]
|-
| WN5 || style="text-align:right; background-color:#{{Color temperature|60000 |hexval}}"|60,000 || style="text-align:right"|3.7 || style="text-align:right"|15 || style="text-align:right"|160,000 || style="text-align:right"|−4.4||[[WR 149]]
|-
| WN5h || style="text-align:right; background-color:#{{Color temperature|50000 |hexval}}"|50,000 || style="text-align:right"|20 || style="text-align:right"|200 || style="text-align:right"|5,000,000 || style="text-align:right"|−8.0||[[R136a1]]
|-
| WN6 || style="text-align:right; background-color:#{{Color temperature|56000 |hexval}}"|56,000 || style="text-align:right"|5.7 || style="text-align:right"|18 || style="text-align:right"|160,000 || style="text-align:right"|−5.1||[[CD Crucis]]
|-
| WN6h || style="text-align:right; background-color:#{{Color temperature|45000 |hexval}}"|45,000 || style="text-align:right"|25 || style="text-align:right"|74 || style="text-align:right"|3,300,000 || style="text-align:right"|−7.5||[[NGC 3603-A1]]
|-
| WN7 || style="text-align:right; background-color:#{{Color temperature|50000 |hexval}}"|50,000 || style="text-align:right"|6.0 || style="text-align:right"|21 || style="text-align:right"|350,000 || style="text-align:right"|−5.7||[[WR 120]]
|-
| WN7h || style="text-align:right; background-color:#{{Color temperature|45000 |hexval}}"|45,000 || style="text-align:right"|23 || style="text-align:right"|52 || style="text-align:right"|2,000,000 || style="text-align:right"|−7.2||[[WR 22]]
|-
| WN8 || style="text-align:right; background-color:#{{Color temperature|45000 |hexval}}"|45,000 || style="text-align:right"|6.6 || style="text-align:right"|11 || style="text-align:right"|160,000 || style="text-align:right"|−5.5||[[WR 123]]
|-
| WN8h || style="text-align:right; background-color:#{{Color temperature|40000 |hexval}}"|40,000 || style="text-align:right"|22 || style="text-align:right"|39 || style="text-align:right"|1,300,000 || style="text-align:right"|−7.2||[[WR 124]]
|-
| WN9h || style="text-align:right; background-color:#{{Color temperature|35000 |hexval}}"|35,000 || style="text-align:right"|23 || style="text-align:right"|33 || style="text-align:right"|940,000 || style="text-align:right"|−7.1||[[WR 102ea]]
|}

It can be seen that the WNh stars are completely different objects from the WN stars without hydrogen. Despite the similar spectra, they are much more massive, much larger, and some of the most luminous stars known. They have been detected as early as WN5h in the Magellanic Clouds. The nitrogen seen in the spectrum of WNh stars is still the product of [[CNO cycle]] fusion in the core, but it appears at the surface of the most massive stars due to rotational and convectional mixing while still in the core hydrogen burning phase, rather than after the outer envelope is lost during core helium fusion.<ref name=wnh>{{cite journal|doi=10.1086/586885|title=On the Role of the WNH Phase in the Evolution of Very Massive Stars: Enabling the LBV Instability with Feedback|journal=The Astrophysical Journal|volume=679|issue=2|pages=1467–1477|date=2008|last1=Smith|first1=Nathan|last2=Conti|first2=Peter S.|bibcode=2008ApJ...679.1467S|arxiv = 0802.1742 |s2cid=15529810}}</ref>

{| class="wikitable sortable"
|+Physical properties of galactic population I WO/C stars<ref name="sander">{{cite journal|last1=Sander|first1=A. A. C.|last2=Hamann|first2=W. -R.|last3=Todt|first3=H.|last4=Hainich|first4=R.|last5=Shenar|first5=T.|last6=Ramachandran|first6=V.|last7=Oskinova|first7=L. M.|year=2019|title=The Galactic WC and WO stars. The impact of revised distances from Gaia DR2 and their role as massive black hole progenitors|journal=Astronomy and Astrophysics|volume=621|pages=A92|arxiv=1807.04293|bibcode=2019A&A...621A..92S|doi=10.1051/0004-6361/201833712|s2cid=67754788}}
</ref>
|-
! Spectral<br />type !! Temperature<br />(K)<ref name=sander/> !! Radius<br />({{solar radius}})<ref name=sander/> !! Mass<br />({{solar mass}})<ref name=sander/> !! Luminosity<br />({{solar luminosity}})<ref name=sander/> !! Absolute<br />magnitude !! Example
|-
| WO2 || style="text-align:right; background-color:#{{Color temperature|200000 |hexval}}"|200,000 || style="text-align:right"|0.7 || style="text-align:right"|22 || style="text-align:right"|630,000 || style="text-align:right"|−1.7||[[WR 142]]
|-
| WC4 || style="text-align:right; background-color:#{{Color temperature|117000 |hexval}}"|117,000 || style="text-align:right"|0.9 || style="text-align:right"|10 || style="text-align:right"|158,000 || style="text-align:right"|−3.28||[[WR 143]]
|-
| WC5 || style="text-align:right; background-color:#{{Color temperature|83000 |hexval}}"|83,000 || style="text-align:right"|3.2 || style="text-align:right"|18 || style="text-align:right" |398,000 || style="text-align:right"|−4.87||[[Theta Muscae]]
|-
| WC6 || style="text-align:right; background-color:#{{Color temperature|78000 |hexval}}"|78,000 || style="text-align:right"|3.6 || style="text-align:right"|18 || style="text-align:right" |501,000 || style="text-align:right"|−4.75||[[WR 45]]
|-
| WC7 || style="text-align:right; background-color:#{{Color temperature|71000 |hexval}}"|71,000 || style="text-align:right"|4.0 || style="text-align:right"|17 || style="text-align:right" |398,000 || style="text-align:right"|−4.8||[[WR 86]]
|-
| WC8 || style="text-align:right; background-color:#{{Color temperature|60000 |hexval}}"|60,000 || style="text-align:right"|6.3 || style="text-align:right"|18 || style="text-align:right" |398,000 || style="text-align:right"|−5.32||[[Gamma Velorum]]
|-
| WC9 || style="text-align:right; background-color:#{{Color temperature|44000 |hexval}}"|44,000 || style="text-align:right"|8.7 || style="text-align:right"|13 || style="text-align:right" |251,000 || style="text-align:right"|−5.57||[[WR 104]]
|}

Some Wolf–Rayet stars of the carbon sequence ("WC"), especially those belonging to the latest types, are noticeable due to their production of [[interstellar dust|dust]]. Usually this takes place on those belonging to [[binary star|binary systems]] as a product of the collision of the [[stellar wind]]s forming the pair,<ref name=hucht/> as is the case of the famous binary [[WR 104]]; however this process occurs on single ones too.<ref name=wc />

A few – roughly 10% – of the central stars of [[planetary nebula]]e, despite their much lower masses – typically ~0.6 ''M''<sub>☉</sub> – are also observationally of the WR-type; i.e. they show emission line spectra with broad lines from helium, carbon and oxygen. Denoted [WR], they are much older objects descended from evolved low-mass stars and are closely related to [[white dwarf]]s, rather than to the very young, very massive [[population I]] stars that comprise the bulk of the WR class.<ref name=tylenda>{{cite journal|bibcode=1993A&AS..102..595T|title=Wolf–Rayet Nuclei of Planetary Nebulae – Observations and Classification|journal=Astronomy and Astrophysics Supplement|volume=102|pages=595|last1=Tylenda|first1=R.|last2=Acker|first2=A.|last3=Stenholm|first3=B.|date=1993}}</ref> These are now generally excluded from the class denoted as Wolf–Rayet stars, or referred to as Wolf–Rayet-type stars.<ref name=crowther/>

===Metallicity===
The numbers and properties of Wolf–Rayet stars vary with the chemical composition of their progenitor stars. A primary driver of this difference is the rate of mass loss at different levels of metallicity. Higher metallicity leads to high mass loss, which affects the evolution of massive stars and also the properties of Wolf–Rayet stars. Higher levels of mass loss cause stars to lose their outer layers before an iron core develops and collapses, so that the more massive red supergiants evolve back to hotter temperatures before exploding as a supernova, and the most massive stars never become red supergiants. In the Wolf–Rayet stage, higher mass loss leads to stronger depletion of the layers outside the convective core, lower hydrogen surface abundances and more rapid stripping of helium to produce a WC spectrum.

These trends can be observed in the various galaxies of the local group, where metallicity varies from near-solar levels in the Milky Way, somewhat lower in M31, lower still in the Large Magellanic Cloud, and much lower in the Small Magellanic Cloud. Strong metallicity variations are seen across individual galaxies, with M33 and the Milky Way showing higher metallicities closer to the centre, and M31 showing higher metallicity in the disk than in the halo. Thus the SMC is seen to have few WR stars compared to its stellar formation rate and no WC stars at all (one star has a WO spectral type), the Milky Way has roughly equal numbers of WN and WC stars and a large total number of WR stars, and the other main galaxies have somewhat fewer WR stars and more WN than WC types. LMC, and especially SMC, Wolf–Rayets have weaker emission and a tendency to higher atmospheric hydrogen fractions. SMC WR stars almost universally show some hydrogen and even absorption lines even at the earliest spectral types, due to weaker winds not entirely masking the photosphere.<ref name=hainichsmc>{{cite journal|last1=Hainich|first1=R.|last2=Pasemann|first2=D.|last3=Todt|first3=H.|last4=Shenar|first4=T.|last5=Sander|first5=A.|last6=Hamann|first6=W.-R.|title=Wolf–Rayet stars in the Small Magellanic Cloud. I. Analysis of the single WN stars|journal=Astronomy & Astrophysics|year=2015|issn=0004-6361|doi=10.1051/0004-6361/201526241|bibcode=2015A&A...581A..21H|volume=581|pages=A21|arxiv = 1507.04000 |s2cid=56230998}}</ref>

The maximum mass of a main-sequence star that can evolve through a red supergiant phase and back to a WNL star is calculated to be around {{solar mass|20}} in the Milky Way, {{solar mass|32}} in the LMC, and {{solar mass|over 50}} in the SMC. The more evolved WNE and WC stages are only reached by stars with an initial mass over {{solar mass|25}} at near-solar metallicity, {{solar mass|over 60}} in the LMC. Normal single star evolution is not expected to produce any WNE or WC stars at SMC metallicity.<ref name=wise/>

===Rotation===
[[File:Hubble spies vast gas disk around unique massive star.tif|thumb|alt=Hubble Spies Vast Gas Disk around Unique Massive Star|Artist's illustration of gas disk around massive hydrogen-rich [[WR 122]]]]
Mass loss is influenced by a star's rotation rate, especially strongly at low metallicity. Fast rotation contributes to mixing of core fusion products through the rest of the star, enhancing surface abundances of heavy elements, and driving mass loss. Rotation causes stars to remain on the main sequence longer than non-rotating stars, evolve more quickly away from the red supergiant phase, or even evolve directly from the main sequence to hotter temperatures for very high masses, high metallicity or very rapid rotation.

[[Stellar mass]] loss produces a loss of angular momentum and this quickly brakes the rotation of massive stars. Very massive stars at near-solar metallicity should be braked almost to a standstill while still on the main sequence, while at SMC metallicity they can continue to rotate rapidly even at the highest observed masses. Rapid rotation of massive stars may account for the unexpected properties and numbers of SMC WR stars, for example their relatively high temperatures and luminosities.<ref name=hainichsmc/>

===Binaries===
Massive stars in binary systems can develop into Wolf–Rayet stars due to stripping by a companion rather than inherent mass loss due to a stellar wind. This process is relatively insensitive to the metallicity or rotation of the individual stars and is expected to produce a consistent set of WR stars across all the local group galaxies. As a result, the fraction of WR stars produced through the binary channel, and therefore the number of WR stars observed to be in binaries, should be higher in low metallicity environments. Calculations suggest that the binary fraction of WR stars observed in the SMC should be as high as 98%, although less than half are actually observed to have a massive companion. The binary fraction in the Milky Way is around 20%, in line with theoretical calculations.<ref name=foellmi>{{Cite journal | doi = 10.1046/j.1365-8711.2003.06052.x| title = Wolf–Rayet binaries in the Magellanic Clouds and implications for massive-star evolution – I. Small Magellanic Cloud| year = 2003| last1 = Foellmi | first1 = C.| last2 = Moffat | first2 = A. F. J.| last3 = Guerrero | first3 = M. A.| journal = Monthly Notices of the Royal Astronomical Society| volume = 338| issue = 2| pages = 360–388| bibcode = 2003MNRAS.338..360F| s2cid = 124628247| doi-access = free}}</ref>

==Nebulae==
[[File:LHA 115 - N 76A - Eso0310a.jpg|upright=1.0|thumb|alt=LHA 115 - N76A|[[AB7]] produces one of the highest excitation nebulae in the [[Magellanic Clouds]].]]
{{main|Wolf–Rayet nebula}}
A significant proportion of WR stars are surrounded by nebulosity associated directly with the star, not just the normal background nebulosity associated with any massive star forming region, and not a [[planetary nebula]] formed by a post-[[Asymptotic giant branch|AGB]] star. The nebulosity presents a variety of forms and classification has been difficult. Many were originally catalogued as planetary nebulae and sometimes only a careful multi-wavelength study can distinguish a planetary nebula around a low mass post-AGB star from a similarly shaped nebula around a more massive core helium-burning star.<ref name=wise>{{Cite journal|arxiv=1503.06878|last1= Toalá|first1= J. A.|title= WISE morphological study of Wolf–Rayet nebulae|journal= Astronomy & Astrophysics|volume= 578|pages= A66|last2= Guerrero|first2= M. A.|last3= Ramos-Larios|first3= G.|last4= Guzmán|first4= V.|year= 2015|doi= 10.1051/0004-6361/201525706|bibcode=2015A&A...578A..66T|s2cid= 55776698}}</ref><ref name=frew_parker>{{cite journal|bibcode=2010PASA...27..129F|title=Planetary Nebulae: Observational Properties, Mimics and Diagnostics|journal=Publications of the Astronomical Society of Australia|volume=27|issue=2|pages=129–148|last1=Frew|first1=David J.|last2=Parker|first2=Quentin A.|date=2010|doi=10.1071/AS09040|arxiv = 1002.1525 |s2cid=59429975}}</ref>

==Wolf–Rayet galaxies==
{{main|Wolf–Rayet galaxy}}
A Wolf–Rayet galaxy is a type of [[starburst galaxy]] where a sufficient number of WR stars exist that their characteristic emission line spectra become visible in the overall spectrum of the galaxy.<ref name=conti1994>{{cite journal|bibcode=1994ApJ...423L..97C|title=HST UV Imaging of the Starburst Regions in the Wolf–Rayet Galaxy He 2-10: Newly Formed Globular Clusters?|journal=Astrophysical Journal Letters |volume=423|pages=L97|last1=Conti|first1=Peter S.|last2=Vacca|first2=William D.|year=1994|doi=10.1086/187245|doi-access=free}}</ref> Specifically a broad emission feature due to the 468.6&nbsp;nm He{{smallcaps|{{thinsp}}ii}} and nearby spectral lines is the defining characteristic of a Wolf–Rayet galaxy. The relatively short lifetime of WR stars means that the [[Starburst region|starburst]]s in such galaxies must have occurred within the last few million years, and must have lasted less than a million years or else the WR emission would be swamped by large numbers of other luminous stars.<ref name=leitherer>{{cite journal|bibcode=1996ApJ...465..717L|title=Hubble Space Telescope Ultraviolet Imaging and Spectroscopy of the Bright Starburst in the Wolf–Rayet Galaxy NGC 4214|journal=Astrophysical Journal |volume=465|pages=717|last1=Leitherer|first1=Claus|last2=Vacca|first2=William D.|last3=Conti|first3=Peter S.|last4=Filippenko|first4=Alexei V.|last5=Robert|first5=Carmelle|author5-link=Carmelle Robert|last6=Sargent|first6=Wallace L. W.|year=1996|doi=10.1086/177456|doi-access=free}}</ref>

==Evolution==
Theories about how WR stars form, develop, and die have been slow to form compared to the explanation of less extreme [[stellar evolution]]. They are rare, distant, and often obscured, and even into the 21st century many aspects of their lives are unclear.

===History===
Although Wolf–Rayet stars have been clearly identified as an unusual and distinctive class of stars since the 19th century,<ref name=campbell1894>
{{cite journal
 |last=Campbell |first=W.W.
 |year=1894
 |title=The Wolf–Rayet stars
 |journal=Astronomy and Astro-Physics
 |volume=13 |pages=448
 |bibcode=1894AstAp..13..448C
}}
</ref>
the nature of these stars was uncertain until towards the end of the 20th century. Before the 1960s, even the classification of WR stars was highly uncertain, and their nature and evolution was essentially unknown. The very similar appearance of the central stars of planetary nebulae (CSPNe) and the much more luminous classical WR stars contributed to the uncertainty.<ref name=zanstra>
{{cite journal
 |last1=Zanstra |first1=H.
 |last2=Weenen  |first2=J.
 |year=1950
 |title=On physical processes in Wolf–Rayet stars. Paper 1: Wolf–Rayet stars and Beals' hypothesis of pure recombination (Errata: 11 357)
 |journal=Bulletin of the Astronomical Institutes of the Netherlands
 |volume=11 |pages=165
 |bibcode=1950BAN....11..165Z
}}
</ref>

By about 1960, the distinction between CSPNe and massive luminous classical WR stars was more clear. Studies showed that they were small dense stars surrounded by extensive circumstellar material, but not yet clear whether the material was expelled from the star or contracting onto it.<ref name=limber>
{{cite journal
 |last=Limber |first=D. Nelson
 |year=1964
 |title=The Wolf–Rayet Phenomenon
 |journal=[[The Astrophysical Journal]]
 |volume=139 |pages=1251
 |doi=10.1086/147863  |bibcode=1964ApJ...139.1251L
}}
</ref><ref name=underhill1968>
{{cite journal
 |last1=Underhill  |first1=Anne B.
 |year=1968
 |title=The Wolf–Rayet Stars
 |journal=[[Annual Review of Astronomy and Astrophysics]]
 |volume=6 |pages=39–78
 |doi=10.1146/annurev.aa.06.090168.000351
 |bibcode=1968ARA&A...6...39U
}}
</ref>
The unusual abundances of nitrogen, carbon, and oxygen, as well as the lack of hydrogen, were recognised, but the reasons remained obscure.<ref name=underhill1960>
{{cite journal
 |last=Underhill  |first=Anne B.
 |year=1960
 |title=A study of the Wolf–Rayet stars HD&nbsp;192103 and HD&nbsp;192163
 |journal=Publications of the [[Dominion Astrophysical Observatory]] Victoria
 |volume=11 |pages=209
 |bibcode=1960PDAO...11..209U
}}
</ref>
It was recognised that WR stars were very young and very rare, but it was still open to debate whether they were evolving towards or away from the main sequence.<ref name=sahade>
{{cite journal
 |last=Sahade |first=J.
 |year=1958
 |title=On the nature of the Wolf–Rayet stars
 |journal=[[The Observatory (journal)|The Observatory]]
 |volume=78 |pages=79
 |bibcode=1958Obs....78...79S
}}
</ref><ref name=westerlund1964>
{{cite journal
 |last1=Westerlund |first1=B.E.
 |last2=Smith      |first2=L.F.
 |year=1964
 |title=Worlf–Rayet stars in the Large Magellanic Cloud
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=128 |issue=4 |pages=311–325
 |bibcode=1964MNRAS.128..311W |doi=10.1093/mnras/128.4.311
|doi-access=free}}
</ref>

By the 1980s, WR stars were accepted as the descendants of massive OB stars, although their exact evolutionary state in relation to the main sequence and other evolved massive stars was still unknown.<ref name=abbott>
{{cite journal
 |last1=Abbott |first1=David C.
 |last2=Conti  |first2=Peter S.
 |year=1987
 |title=Wolf–Rayet stars
 |journal=Annual Review of Astronomy and Astrophysics
 |volume=25 |pages=113–150
 |doi=10.1146/annurev.aa.25.090187.000553
 |bibcode=1987ARA&A..25..113A
}}
</ref>
Theories that the preponderance of WR stars in massive binaries and their lack of hydrogen could be due to gravitational stripping had been largely ignored or abandoned.<ref>
{{cite journal
 |last=Paczyński |first=B.
 |year=1967
 |title=Evolution of close binaries. V. The evolution of nassive binaries and the formation of the Wolf–Rayet stars
 |journal=[[Acta Astronomica]]
 |volume=17 |pages=355
 |bibcode=1967AcA....17..355P
}}
</ref>
WR stars were being proposed as possible progenitors of supernovae, and particularly the newly-discovered type&nbsp;Ib supernovae, lacking hydrogen but apparently associated with young massive stars.<ref name=abbott/>

By the start of the 21st century, WR stars were largely accepted as massive stars that had exhausted their core hydrogen, left the main sequence, and expelled most of their atmospheres, leaving behind a small hot core of helium and heavier fusion products.<ref name=nugis>
{{cite journal
 |last1=Nugis  |first1=T.
 |last2=Lamers |first2=H.J.G.L.M.
 |year=2000
 |title=Mass-loss rates of Wolf–Rayet stars as a function of stellar parameters
 |journal=[[Astronomy and Astrophysics]]
 |volume=360 |pages=227
 |bibcode=2000A&A...360..227N
}}
</ref><ref name=review>
{{cite conference
 |last=Humphreys |first=R.M.  |author-link=Roberta M. Humphreys
 |year=1991
 |title=The Wolf–Rayet connection – luminous blue variables and evolved supergiants (review)
 |conference=143rd Symposium of the [[International Astronomical Union]]
 |volume=143 |pages=485
 |bibcode=1991IAUS..143..485H
}}
</ref>

===Current models===
[[File:WR 31a.jpg|thumb|left|upright=1.2|alt=Blue bubble in Carina|[[WR 31a]] is surrounded by a blue bubble created by a powerful [[stellar wind]] impacting material expelled during earlier stages of the star's life (ESA/Hubble & NASA Acknowledgement: Judy Schmidt)]]
Most WR stars, the classical population I type, are now understood as being a natural stage in the evolution of the most massive stars (not counting the less common planetary nebula central stars), either after a period as a red supergiant, after a period as a blue supergiant, or directly from the most massive main-sequence stars. Only the lower mass red supergiants are expected to explode as a supernova at that stage, while more massive red supergiants progress back to hotter temperatures as they expel their atmospheres. Some explode while at the yellow hypergiant or LBV stage, but many become Wolf–Rayet stars.<ref name=groh13>
{{cite journal
 |last1=Groh   |first1=Jose H.      |last2=Meynet  |first2=Georges
 |last3=Georgy |first3=Cyril        |last4=Ekström |first4=Sylvia
 |year=2013
 |title=Fundamental properties of core-collapse supernova and GRB progenitors: Predicting the look of massive stars before death
 |journal=[[Astronomy & Astrophysics]]
 |volume=558 |pages=A131
 |doi=10.1051/0004-6361/201321906 |arxiv=1308.4681
 |bibcode=2013A&A...558A.131G |s2cid=84177572
}}
</ref>
They have lost or burnt almost all of their hydrogen and are now fusing helium in their cores, or heavier elements for a very brief period at the end of their lives.<ref name=groh13/>

Massive main-sequence stars create a very hot core which fuses hydrogen very rapidly via the CNO process and results in strong convection throughout the whole star. This causes mixing of helium to the surface, a process that is enhanced by rotation, possibly by [[differential rotation]] where the core is spun up to a faster rotation than the surface. Such stars also show nitrogen enhancement at the surface at a very young age, caused by changes in the proportions of carbon and nitrogen due to the CNO cycle. The enhancement of heavy elements in the atmosphere, as well as increases in luminosity, create strong stellar winds which are the source of the emission line spectra. These stars develop an Of spectrum, Of* if they are sufficiently hot, which develops into a WNh spectrum as the stellar winds increase further. This explains the high mass and luminosity of the WNh stars, which are still burning hydrogen at the core and have lost little of their initial mass. These will eventually expand into blue supergiants (LBVs?) as hydrogen at the core becomes depleted, or if mixing is efficient enough (e.g. through rapid rotation) they may progress directly to WN stars without hydrogen.

WR stars are likely to end their lives violently rather than fade away to a white dwarf. Thus every star with an initial mass more than about 9 times the Sun would inevitably result in a supernova explosion (with the exception of direct collapse<ref name=":0">{{Cite journal |last1=Heger |first1=A. |last2=Fryer |first2=C. L. |last3=Woosley |first3=S. E. |last4=Langer |first4=N. |last5=Hartmann |first5=D. H. |year=2003 |title=How Massive Single Stars End their Life |journal=The Astrophysical Journal |volume=591 |issue=1 |pages=288–300 |doi=10.1086/375341 |arxiv=astro-ph/0212469 |bibcode=2003ApJ...591..288H |s2cid=15539500 |issn=0004-637X}}</ref>), many of them from the WR stage.<ref name=crowther/><ref name=groh13/><ref>{{cite journal|version=v1|author1=Georges Meynet|author2=Cyril Georgy|author3=Raphael Hirschi|author4=Andre Maeder|author5=Phil Massey|author6=Norbert Przybilla|author7=M-Fernanda Nieva|title=Red Supergiants, Luminous Blue Variables and Wolf–Rayet stars: The single massive star perspective|pages=266–278|volume=80|issue=39|journal=Bulletin de la Société Royale des Sciences de Liège|arxiv=1101.5873|bibcode = 2011BSRSL..80..266M|date=2011 }}</ref>

A simple progression of WR stars from low to hot temperatures, resulting finally in WO-type stars, is not supported by observation. WO-type stars are extremely rare and all the known examples are more luminous and more massive than the relatively common WC stars. Alternative theories suggest either that the WO-type stars are only formed from the most massive main-sequence stars,<ref name="wc" /> and/or that they form an extremely short-lived end stage of just a few thousand years before exploding, with the WC phase corresponding to the core [[helium burning]] phase and the WO phase to nuclear burning stages beyond. It is still unclear whether the WO spectrum is purely the result of ionisation effects at very high temperature, reflects an actual chemical abundance difference, or if both effects occur to varying degrees.<ref name=groh13/><ref name=tramper>{{cite journal|bibcode=2013msao.confE.187T|title=The nature of WO stars: VLT/X-Shooter spectroscopy of DR1|journal=Massive Stars: From Alpha to Omega|pages=187|last1=Tramper|first1=Frank|date=2013|arxiv = 1312.1555 }}</ref><ref name=eldridge>{{cite journal|bibcode=2013MNRAS.436..774E|title=The death of massive stars – II. Observational constraints on the progenitors of Type Ibc supernovae|journal=Monthly Notices of the Royal Astronomical Society|volume=436|issue=1|pages=774–795|last1=Eldridge|first1=John J.|last2=Fraser|first2=Morgan|last3=Smartt|first3=Stephen J.|last4=Maund|first4=Justyn R.|last5=Crockett|first5=R. Mark|date=2013|doi=10.1093/mnras/stt1612|doi-access=free |arxiv = 1301.1975 |s2cid=118535155}}</ref><ref name=groh0114>{{Cite journal|arxiv=1401.7322|last1=Groh|first1=Jose|title=The evolution of massive stars and their spectra I. A non-rotating 60 Msun star from the zero-age main sequence to the pre-supernova stage|journal=Astronomy & Astrophysics|volume=564|pages=A30|last2=Meynet|first2=Georges|last3=Ekstrom|first3=Sylvia|last4=Georgy|first4=Cyril|date=2014|doi=10.1051/0004-6361/201322573|bibcode=2014A&A...564A..30G|s2cid=118870118}}</ref>

{| class="wikitable"
|+Schematic evolution of stars by initial mass (at solar metallicity) {{citation needed|date=July 2023}}
|-
! Initial mass ({{Solar mass}}) !! Evolutionary sequence !! [[Supernova]] type
|-
|~250+
|
|None<ref name=":0" />
|-
|~140–{{thinsp}}~250
|WNh-WNE-WO
|Ic/Pair-instability
|-
| 120–{{thinsp}}~140 || WNh&nbsp;→&nbsp;WN&nbsp;→&nbsp;WC&nbsp;→&nbsp;WO || Ic
|-
| 60–120 || O&nbsp;→&nbsp;Of&nbsp;→&nbsp;WNh&nbsp;↔&nbsp;LBV&nbsp;→[WNL] || IIn {{citation needed|date=July 2023}}
|-
| 45–60 || O&nbsp;→&nbsp;WNh&nbsp;→&nbsp;LBV/WNE?&nbsp;→&nbsp;WO || Ib/c
|-
| 20–45 || O&nbsp;→&nbsp;RSG&nbsp;→&nbsp;WNE&nbsp;→&nbsp;WC || Ib
|-
| 15–20 || O&nbsp;→&nbsp;RSG&nbsp;↔&nbsp;(YHG)&nbsp;↔&nbsp;BSG&nbsp;(blue&nbsp;loops) || II-L (or IIb)
|-
| 8–15 || B&nbsp;→&nbsp;RSG || II-P
|}

Key:
* O: [[O-type main-sequence star]]
* Of: evolved O-type showing N and He emission
* BSG: [[blue supergiant]]
* RSG: [[red supergiant]]
* YHG: [[yellow hypergiant]]
* LBV: [[luminous blue variable]]
* WNh: WN plus hydrogen lines
* WNL: "late" WN-class Wolf–Rayet star (about WN6 to WN11)
* WNE: "early" WN-class Wolf–Rayet star (about WN2 to WN6)
* WN/WC: Transitional (transitioning from WN to WC) Wolf–Rayet star (may be WN#/WCE or WC#/WN)
* WC: WC-class Wolf–Rayet star
* WO: WO-class Wolf–Rayet star

Wolf–Rayet stars form from massive stars, although the evolved population I stars have lost half or more of their initial masses by the time they show a WR appearance. For example, [[Gamma Velorum|γ<sup>2</sup> Velorum A]] currently has a mass around 9 times the Sun, but began with a mass at least 40 times the Sun.<ref>{{cite journal|bibcode=2000A&A...353..715O|title=COMPTEL limits on 26Al 1.809 MeV line emission from gamma2 Velorum|journal=Astronomy and Astrophysics|volume=353|pages=715|last1=Oberlack|first1=U.|last2=Wessolowski|first2=U.|last3=Diehl|first3=R.|last4=Bennett|first4=K.|last5=Bloemen|first5=H.|last6=Hermsen|first6=W.|last7=Knödlseder|first7=J.|last8=Morris|first8=D.|last9=Schönfelder|first9=V.|last10=von Ballmoos|first10=P.|date=2000|arxiv = astro-ph/9910555 }}</ref> High-mass stars are very rare, both because they form less often and because they have short lives. This means that Wolf–Rayet stars themselves are extremely rare because they only form from the most massive main-sequence stars and because they are a relatively short-lived phase in the lives of those stars. This also explains why type Ib/c supernovae are less common than type II, since they result from higher-mass stars.

WNh stars, spectroscopically similar but actually a much less evolved star which has only just started to expel its atmosphere, are an exception and still retain much of their initial mass. The [[most massive stars]] currently known are all WNh stars rather than O-type main-sequence stars, an expected situation because such stars show helium and nitrogen at the surface only a few thousand years after they form, possibly before they become visible through the surrounding gas cloud. An alternative explanation is that these stars are so massive that they could not form as normal main-sequence stars, instead being the result of mergers of less extreme stars.<ref name=merger>{{cite journal|bibcode=2012MNRAS.426.1416B|title=The emergence of super-canonical stars in R136-type starburst clusters|journal=Monthly Notices of the Royal Astronomical Society|volume=426|issue=2|pages=1416–1426|last1=Banerjee|first1=Sambaran|last2=Kroupa|first2=Pavel|last3=Oh|first3=Seungkyung|date=2012|doi=10.1111/j.1365-2966.2012.21672.x|doi-access=free |arxiv = 1208.0826 |s2cid=119202197}}</ref>

The difficulties of modelling the observed numbers and types of Wolf–Rayet stars through single star evolution have led to theories that they form through binary interactions which could accelerate loss of the outer layers of a star through mass exchange. [[WR 122]] is a potential example that has a flat disk of gas encircling the star, almost 2 trillion miles wide, and may have a companion star that stripped its outer envelope.<ref>{{Cite journal|title=Multiwavelength Observations of NaSt1 (WR 122): Equatorial Mass Loss and X-rays from an Interacting Wolf–Rayet Binary|volume=1502|issue=3|pages=1794|bibcode=2015MNRAS.450.2551M|last1=Mauerhan|first1=Jon C.|last2=Smith|first2=Nathan|last3=Van Dyk|first3=Schuyler D.|last4=Morzinski|first4=Katie M.|last5=Close|first5=Laird M.|last6=Hinz|first6=Philip M.|last7=Males|first7=Jared R.|last8=Rodigas|first8=Timothy J.|year=2015|arxiv=1502.01794|doi=10.1093/mnras/stv257|journal=Monthly Notices of the Royal Astronomical Society|doi-access=free |s2cid=40573971}}</ref>

===Supernovae===
{{main|supernova}}
It is widely suspected that many type Ib and type Ic supernova progenitors are WR stars, although no conclusive identification has been made of such a progenitor.

Type Ib supernovae lack hydrogen lines in their spectra. The more common type Ic supernovae lack both hydrogen and helium lines in their spectra. The expected progenitors for such supernova are massive stars that respectively lack hydrogen in their outer layers, or lack both hydrogen and helium. WR stars are just such objects. All WR stars lack hydrogen and in some WR stars, most notably the WO group, helium is also strongly depleted. WR stars are expected to experience core collapse when they have generated an iron core, and resulting supernova explosions would be of type Ib or Ic. In some cases it is possible that direct collapse of the core to a black hole would not produce a visible explosion.<ref name=dessart>{{cite journal|bibcode=2011MNRAS.414.2985D|arxiv=1102.5160|title=Core-collapse explosions of Wolf–Rayet stars and the connection to Type IIb/Ib/Ic supernovae|journal=Monthly Notices of the Royal Astronomical Society|volume=414|issue=4|pages=2985|last1=Dessart|first1=Luc|last2=Hillier|first2=D. John|last3=Livne|first3=Eli|last4=Yoon|first4=Sung-Chul|last5=Woosley|first5=Stan|last6=Waldman|first6=Roni|last7=Langer|first7=Norbert|year=2011|doi=10.1111/j.1365-2966.2011.18598.x|doi-access=free |s2cid=119257348}}</ref>

WR stars are very luminous due to their high temperatures but not visually bright, especially the hottest examples that are expected to make up most supernova progenitors. Theory suggests that the progenitors of type Ibc supernovae observed to date would not be bright enough to be detected, although they place constraints on the properties of those progenitors.<ref name=eldridge/> A possible progenitor star which has disappeared at the location of supernova [[iPTF13bvn]] may be a single WR star,<ref name=groh>{{cite journal|doi=10.1051/0004-6361/201322369|title=Progenitors of supernova Ibc: A single Wolf–Rayet star as the possible progenitor of the SN Ib iPTF13bvn|journal=Astronomy & Astrophysics|volume=558|pages=L1|date=2013|last1=Groh|first1=Jose H.|last2=Georgy|first2=Cyril|last3=Ekström|first3=Sylvia|bibcode=2013A&A...558L...1G|arxiv = 1307.8434 |s2cid=58911704}}</ref> although other analyses favour a less massive binary system with a stripped star or helium giant.<ref name=neutron>{{cite book|doi=10.1007/978-3-319-97616-7_1|chapter=Neutron Stars Formation and Core Collapse Supernovae|title=The Physics and Astrophysics of Neutron Stars|volume=457|pages=1–56|series=Astrophysics and Space Science Library|year=2018|last1=Cerda-Duran|first1=Pablo|last2=Elias-Rosa|first2=Nancy|isbn=978-3-319-97615-0|arxiv=1806.07267|s2cid=119340817}}</ref><ref name=milisavljevic>{{cite book|bibcode=2013nha..confE...9M |pages=9|last1=Milisavljevic|first1=D.|title=Proceedings of Frank N. Bash Symposium 2013: New Horizons in Astronomy — PoS(BASH 2013)|chapter=The Progenitor Systems and Explosion Mechanisms of Supernovae|year=2013|doi=10.22323/1.206.0009 |doi-access=free}}</ref> The only other possible WR supernova progenitor is for [[SN 2017ein]], and again it is uncertain whether the progenitor is a single massive WR star or binary system.<ref name=kilpatrick>{{cite journal|doi=10.1093/mnras/sty2022|title=A potential progenitor for the Type Ic supernova 2017ein|journal=Monthly Notices of the Royal Astronomical Society|volume=480|issue=2|pages=2072–2084|year=2018|last1=Kilpatrick|first1=Charles D.|last2=Takaro|first2=Tyler|last3=Foley|first3=Ryan J.|last4=Leibler|first4=Camille N.|last5=Pan|first5=Yen-Chen|last6=Campbell|first6=Randall D.|last7=Jacobson-Galan|first7=Wynn V.|last8=Lewis|first8=Hilton A.|last9=Lyke|first9=James E.|last10=Max|first10=Claire E.|last11=Medallon|first11=Sophia A.|last12=Rest|first12=Armin|doi-access=free |bibcode=2018MNRAS.480.2072K|arxiv=1808.02989|s2cid=73695137}}</ref>

In 2022 astronomers from the [[Gran Telescopio Canarias]] reported the first supernova explosion of a Wolf–Rayet star. SN 2019hgp was a type Icn supernova and is also the first in which the element [[neon]] has been detected.<ref>{{Cite journal|last1=Gal-Yam|first1=A.|last2=Bruch|first2=R.|last3=Schulze|first3=S.|last4=Yang|first4=Y.|last5=Perley|first5=D. A.|last6=Irani|first6=I.|last7=Sollerman|first7=J.|last8=Kool|first8=E. C.|last9=Soumagnac|first9=M. T.|last10=Yaron|first10=O.|last11=Strotjohann|first11=N. L.|date=2022-01-13|title=A WC/WO star exploding within an expanding carbon–oxygen–neon nebula|url=https://www.nature.com/articles/s41586-021-04155-1|journal=Nature|language=en|volume=601|issue=7892|pages=201–204|doi=10.1038/s41586-021-04155-1|pmid=35022591|arxiv=2111.12435 |bibcode=2022Natur.601..201G |s2cid=244527654|issn=0028-0836}}</ref><ref>{{Cite web|title=Astronomers discover first supernova explosion of a Wolf-Rayet star|url=https://www.iac.es/en/outreach/news/astronomers-discover-first-supernova-explosion-wolf-rayet-star|access-date=2022-02-09|website=Instituto de Astrofísica de Canarias • IAC|date=12 January 2022 |language=en}}</ref><ref>{{Cite web|title=SN2019hgp – The Open Supernova Catalog|url=https://sne.space/sne/SN2019hgp/|access-date=2022-02-09|language=en-US|archive-date=2022-02-09|archive-url=https://web.archive.org/web/20220209212055/https://sne.space/sne/SN2019hgp/|url-status=dead}}</ref>

==Examples==
{{main|List of Wolf-Rayet stars}}
By far the most visible example of a Wolf–Rayet star is [[Gamma Velorum|γ<sup>2</sup> Velorum]] (WR 11), which is a bright naked eye star for those located south of 40 degrees northern [[latitude]], although most of the light comes from an O7.5 giant companion. Due to the exotic nature of its spectrum (bright [[emission line]]s in lieu of dark [[absorption line]]s) it is dubbed the "Spectral Gem of the Southern Skies". The only other Wolf–Rayet star brighter than magnitude 6 is [[Theta Muscae|θ Muscae]] (WR 48), a triple star with two O class companions. Both are WC stars. The "ex" WR star WR 79a ([[HD 152408|HR 6272]]) is brighter than magnitude 6 but is now considered to be a peculiar O8 supergiant with strong emission. The next brightest at magnitude 6.4 is [[WR 22]], a massive binary with a WN7h primary.<ref name=hucht/>

The most massive and most luminous star currently known, [[R136a1]], is also a Wolf–Rayet star of the WNh type that is still fusing hydrogen in its core. This type of star, which includes many of the [[List of most luminous stars|most luminous]] and [[List of most massive stars|most massive]] stars, is very young and usually found only in the centre of the densest star clusters. Occasionally a runaway WNh star such as [[VFTS 682]] is found outside such clusters, probably having been ejected from a multiple system or by interaction with other stars.

An example of a triple star system containing a Wolf–Rayet binary is [[Apep (star system)|Apep]]. It releases huge amounts of carbon dust driven by their extreme stellar winds. As the two stars orbit one another, the dust gets wrapped into a glowing sooty tail.

All of the very [[List of hottest stars|hottest non-degenerate stars]] (the hottest few) are Wolf–Rayet stars, the hottest of which being [[WR 102]], which seems to be as hot as 210,000 K, followed by [[WR 142]] which is around 200,000 K in temperature. [[LMC195-1]], located in the [[Large Magellanic Cloud]], should have a similar temperature, but at the moment this temperature is unknown.

[[File:A giant, smouldering star.jpg|thumb|upright=1.2|alt=a giant smouldering star|[[HD 184738]], also known as Campbell's star. This is actually a planetary nebula and the central star is an old low-mass star unlike the main class of massive Wolf–Rayet stars.<ref>{{cite journal|doi=10.1051/0004-6361:20030391 | bibcode=2003A&A...403..659A | volume=403 | issue=2 | title=Quantitative classification of WR nuclei of planetary nebulae | journal=Astronomy and Astrophysics | pages=659–673| year=2003 | last1=Acker | first1=A. | last2=Neiner | first2=C. | doi-access=free }}</ref>]]
[[HD 45166]] has been described as the most magnetic massive star known and as the first magnetic known Wolf-Rayet star.<ref>{{Cite conference |last=Shenar |first=Tomer |date=2022-11-15 |title=HD 45166: The first magnetic Wolf-Rayet star |url=https://zenodo.org/record/7326750 |doi=10.5281/ZENODO.7326750}}</ref>

Only a minority of planetary nebulae have WR type central stars, but a considerable number of well-known planetary nebulae do have them.

{| class="wikitable"
|+Planetary nebulae with WR type central stars<ref>{{cite journal|bibcode=2013RMxAA..49...87P|title=Galactic kinematics of Planetary Nebulae with [WC] central star|journal=Revista Mexicana de Astronomía y Astrofísica |volume=49|pages=87|last1=Peña|first1=M.|author1-link=Miriam Peña Cárdenas|last2=Rechy-García|first2=J. S.|last3=García-Rojas|first3=J.|year=2013|arxiv = 1301.3657 }}</ref>
|-
! Planetary nebula !! Central star type
|-
| [[NGC 2452]] || [WO1]
|-
| [[NGC 2867]] || [WO2]
|-
| [[NGC 5189]] (Spiral Planetary Nebula) || [WO1]
|-
| [[NGC 2371-2]] || [WO1]
|-
| [[NGC 5315]] || [WO4]
|-
| [[NGC 40]] || [WC8]
|-
| [[NGC 7026]] || [WO3]
|-
| [[NGC 1501]] || [WO4]
|-
| [[NGC 6751]] || [WO4]
|-
| [[NGC 6369]] (Little Ghost Nebula) || [WO3]
|-
| [[MyCn18]] (Hourglass Nebula) || [WC]–[[PG1159 star|PG1159]]
|}

==See also==
* [[Markarian 50]]

==References==
{{reflist|25em}}

==Further reading==
* {{cite conference
 |last1=Tuthill |first1=Peter G.
 |last2=Monnier |first2=John D.
 |last3=Danchi  |first3=William C.
 |last4=Turner  |first4=Nils H.
 |date=24–28 June 2002
 |publication-date=2003
 |title=High-resolution near-IR imaging of the WCd(+OB) environments: Pinwheels
 |conference=212th Symposium of the [[International Astronomical Union]]
 |place=Costa Teguise, Lanzarote, Canary Islands
 |book-title=A Massive Star Odyssey: From main sequence to supernova
 |editor1=van der Hucht, K.A. 
 |editor2=Herrero, A. 
 |editor3=Esteban, C. 
 |volume=212  |pages=121
 |bibcode=2003IAUS..212..121T
}}
* {{cite journal
 |last1=Monnier |first1=J.D.
 |last2=Tuthill |first2=P.G.
 |last3=Danchi  |first3=W.C.
 |year=1999
 |title=Pinwheel Nebula around WR&nbsp;98[CLC]a[/CLC]
 |journal=The Astrophysical Journal
 |volume=525  |issue=2  |pages=L97–L100
 |bibcode=1999ApJ...525L..97M
 |doi=10.1086/312352  |pmid=10525463
 |arxiv = astro-ph/9909282  |s2cid=2811347
}}
* {{cite journal
 |last1=Dougherty   |first1=S.M.       |last2=Beasley  |first2=A.J.
 |last3=Claussen    |first3=M.J.       |last4=Zauderer |first4=B.A.
 |last5=Bolingbroke |first5=N.J.
 |year=2005
 |title=High-resolution radio observations of the colliding-wind binary WR&nbsp;140
 |journal=The Astrophysical Journal
 |volume=623  |issue=1  |pages=447–459
 |doi=10.1086/428494  |arxiv = astro-ph/0501391
 |s2cid=17035675  |bibcode=2005ApJ...623..447D
}}

==External links==
{{Commons category|Wolf-Rayet stars}}
* [http://www.physics.usyd.edu.au/~gekko/wr104.html The Twisted Tale of Wolf–Rayet 104]
* [http://cfa-www.harvard.edu/~pberlind/atlas/htmls/wrstars.html Perry Berlind's page on Wolf–Rayet Spectral Classifications]
* [http://pacrowther.staff.shef.ac.uk/WRcat/index.php Online catalog of galactic Wolf–Rayet Stars]
* [http://www.nasa.gov/centers/goddard/news/topstory/2004/0105wrstar.html Big Old Stars Don't Die Alone (NASA)] {{Webarchive|url=https://web.archive.org/web/20131030080054/http://www.nasa.gov/centers/goddard/news/topstory/2004/0105wrstar.html |date=2013-10-30 }}
* [https://www.nasa.gov/feature/hubble-observes-one-of-a-kind-star-nicknamed-nasty Hubble observes Nasty 1]

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{{Variable star topics}}
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{{DEFAULTSORT:Wolf-Rayet Star}}
[[Category:Star types]]
[[Category:Wolf–Rayet stars|*]]
[[Category:Astronomical objects discovered in 1867]]