Wolf–Rayet star#Classification

Image: Crescenthunter.jpg, 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 bubble nebula]]

In 1867, using the 40 cm Foucault telescope at the Paris Observatory, astronomers Charles Wolf and Georges Rayet

{{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.

}}

discovered three stars in the constellation Cygnus (HD 191765, HD 192103 and HD 192641, now designated as WR 134, WR 135, and WR 137 respectively) that displayed broad emission bands on an otherwise continuous spectrum.

{{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

}}

Most stars only display absorption lines or bands 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. E.C. 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.

{{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}}

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 nebulae.

{{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

}}

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 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.

{{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}}

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.

{{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 VIII, IX)

|journal=Journal of the Royal Astronomical Society of Canada

|volume=34 |page=169

|bibcode=1940JRASC..34..169B

}}

In addition to helium, Carlyle Smith Beals identified emission lines of carbon, oxygen and nitrogen in the spectra of Wolf–Rayet stars.

{{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

}}

{{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

|volume=56 |pages=196–197

|bibcode=1933Obs....56..196B

}}

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.

{{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

}}

In 1969, several CSPNe with strong oxygen {{thinsp}}VI (O{{thinsp}}VI) emissions lines were grouped under a new "O{{thinsp}}VI sequence", or just OVI type.

{{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

}}

Similar stars not associated with planetary nebulae were described shortly after and the WO classification was adopted for them.

{{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

}}

The OVI stars were subsequently classified as [WO] stars, consistent with the population I WR stars.{{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 }}

The understanding that certain late, and sometimes not-so-late, WN stars with hydrogen lines 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.

File:Wr137 spc.png, a WC7 star 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,{{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}}{{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 }} the classification was then split into stars with dominant lines of ionised nitrogen (N{{thinsp}}III, N{{thinsp}}IV, and N{{thinsp}}V) and those with dominant lines of ionised carbon (C{{thinsp}}III and C{{thinsp}}IV) and sometimes oxygen (O{{thinsp}}III – O{{thinsp}}VI), referred to as WN and WC respectively.

{{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

|volume=56 |pages=196

|bibcode=1933Obs....56..196B

}}

The two classes WN and WC were further split into temperature sequences WN5–WN8 and WC6–WC8 based on the relative strengths of the 541.1 nm He{{thinsp}}II and 587.5 nm He{{thinsp}}I lines. Wolf–Rayet emission lines frequently have a broadened absorption wing (P Cygni profile) suggesting circumstellar material. A WO sequence has also been separated from the WC sequence for even hotter stars where emission of ionised oxygen dominates that of ionised carbon, although the actual proportions of those elements in the stars are likely to be comparable. WC and WO spectra are formally distinguished based on the presence or absence of C{{thinsp}}III emission.

{{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

}}

WC spectra also generally lack the O{{thinsp}}VI lines that are strong in WO spectra.

{{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

}}

The WN spectral sequence was expanded to include WN2–WN9, and the definitions refined based on the relative strengths of the N{{thinsp}}III lines at 463.4–464.1 nm and 531.4 nm, the N{{thinsp}}IV lines at 347.9–348.4 nm and 405.8 nm, and the N{{thinsp}}V lines at 460.3 nm, 461.9 nm, and 493.3–494.4 nm.

{{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

}}

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.

{{cite journal

|last1=Crowther |first1=P.A.

|last2=Smith|first2=L.J.

|year=1997

|title=Fundamental parameters of Wolf–Rayet stars. VI. Large Magellanic Cloud WNL stars

|journal=Astronomy and Astrophysics

|volume=320 |page=500

|bibcode=1997A&A...320..500C

}}

The type WN1 was proposed for stars with neither N{{thinsp}}IV nor N{{thinsp}}V lines, to accommodate Brey 1 and Brey 66 which appeared to be intermediate between WN2 and WN2.5.

{{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

}}

The relative line strengths and widths for each WN sub-class were later quantified, and the ratio between the 541.1 nm He{{thinsp}}II and 587.5 nm, He{{thinsp}}I lines was introduced as the primary indicator of the ionisation level and hence of the spectral sub-class. The need for WN1 disappeared and both Brey 1 and Brey 66 are now classified as WN3b. The somewhat obscure WN2.5 and WN4.5 classes were dropped.

{{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

}}

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{{thinsp}}II 426.7 nm, C{{thinsp}}III at 569.6 nm, C{{thinsp}}III/IV 465.0 nm, C{{thinsp}}IV at 580.1–581.2 nm, and the O{{thinsp}}V (and O{{thinsp}}III) blend at 557.2–559.8 nm. The sequence was extended to include WC10 and WC11, and the subclass criteria were quantified based primarily on the relative strengths of carbon lines to rely on ionisation factors even if there were abundance variations between carbon and oxygen.

For WO-type stars the main lines used are C{{thinsp}}IV at 580.1 nm, O{{thinsp}}IV at 340.0 nm, O{{thinsp}}V (and O{{thinsp}}III) blend at 557.2–559.8 nm, O{{thinsp}}VI at 381.1–383.4 nm, O{{thinsp}}VII at 567.0 nm, and O{{thinsp}}VIII at 606.8 nm. The sequence was expanded to include WO5 and quantified based the relative strengths of the O{{thinsp}}VI/C{{thinsp}}IV and O{{thinsp}}VI/O{{thinsp}}V lines.

{{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

}}

A later scheme, designed for consistency across classical WR stars and CSPNe, returned to the WO1 to WO4 sequence and adjusted the divisions.

Detailed modern studies of Wolf–Rayet stars can identify additional spectral features, indicated by suffixes to the main spectral classification:

• h for hydrogen emission;
• ha for hydrogen emission and absorption;
• 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).

{{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

}}

The classification of Wolf–Rayet spectra is complicated by the frequent association of the stars with dense nebulosity, dust clouds, or binary companions. A suffix of "+OB" is used to indicate the presence of absorption lines in the spectrum likely to be associated with a more normal companion star, or "+abs" for absorption lines with an unknown origin.

The hotter WR spectral sub-classes are described as early and the cooler ones as late, 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.

Spectral types for the central stars of planetary nebulae are qualified by surrounding them with square brackets (e.g. [WC4]). 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.

{{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

}}

{{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

}}

{{cite journal

|last1=Todt |first1=H.

|display-authors=etal

|year=2013

|title=Abell 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

}}

{{cite journal

|last1=Frew |first1=David J.

|display-authors=etal

|year=2014

|bibcode=2014MNRAS.440.1345F

|title=The planetary nebula Abell 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

}}

Their formation mechanism is as yet unclear. Temperatures of the planetary nebula central stars tend to the extremes when compared to population 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{{thinsp}}II and C{{thinsp}}IV lines.

{{cite conference

|last1=Hamann |first1=W.-R.

|year=1997

|title=Spectra of Wolf–Rayet type central stars and their analysis

|department=Invited Review

|conference=180th Symposium of the International Astronomical Union

|page=91

|bibcode=1997IAUS..180...91H

|publisher=Kluwer Academic Publishers

}}

{{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

}}

File:GKPersei-MiniSuperNova-20150316.jpg (Nova Persei 1901), which showed Wolf–Rayet features in its spectrum]]

Certain supernovae observed before their peak brightness show WR spectra.

{{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 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

}}

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).

{{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

}}

Similarly, classical novae 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.

The separation of Wolf–Rayet stars from spectral class 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 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^∗/WN6 and are referred to as slash stars.

{{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

}}

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^* for N and He emission with N{{smallcaps|{{thinsp}}iv}} stronger than N{{smallcaps|{{thinsp}}iii}}
• f⁺ 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⁺))

These codes may also be combined with more general spectral type qualifiers such as p or a. Common combinations include OIafpe and OIf^*, 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^*/WN6-A.{{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}} The criteria for distinguishing OIf^*, OIf^*/WN, and WN stars have been refined for consistency. Slash star classifications are used when the H_β 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 nm, 405.8 nm, and 460.3–462.0 nm, together with a standard star for each type:

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.{{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}} The slash notation for these stars was controversial and an alternative was to extend the WR nitrogen sequence to WN10 and WN11{{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}} Other authors preferred to use the WNha notation, for example WN9ha for WR 108.{{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}} A recent recommendation is to use an O spectral type such as O8Iaf if the 447.1 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. However, the Ofpe/WN slash notation as well as WN10 and WN11 classifications continue to be widely used.{{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}}

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.

File:Carina Nebula around the Wolf–Rayet star WR 22.jpg 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. 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.{{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}} The first three catalogues were not specifically lists of Wolf–Rayet stars and they used only existing nomenclature.{{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}}{{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}}{{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}}

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. Neither of these numbering schemes remains in common use.

{{clear}}

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.{{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}}

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.{{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}} Modern high volume identification surveys use their own numbering schemes for the large numbers of new discoveries.{{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}} 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.{{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}}

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"{{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}} prefixed by {{nowrap|BAT-99}}, for example {{nowrap|BAT-99 105}}. Many of these stars are also referred to by their third catalogue number, for example Brey 77.{{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}} 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.{{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 }}{{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}} 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.{{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}} 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{{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}}{{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}}{{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}}

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.{{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}}

Wolf–Rayet stars are a normal stage in the evolution of very massive stars, in which strong, broad 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. 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.{{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}} 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, 206 in the Triangulum Galaxy,{{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}} and 154 in the Andromeda Galaxy.{{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}}

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,{{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}} and astronomers hope to eventually catalog over ten thousand.{{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 }} These stars are expected to be particularly common in the Wolf–Rayet galaxies named after them and in starburst galaxies.{{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}}

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 UV radiation that causes fluorescence in the line-forming wind region.{{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}} 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 He burning (WC and WO-type stars).

{{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

}}

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.{{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}}

Some Wolf–Rayet stars of the carbon sequence ("WC"), especially those belonging to the latest types, are noticeable due to their production of dust. Usually this takes place on those belonging to binary systems as a product of the collision of the stellar winds forming the pair, as is the case of the famous binary WR 104; however this process occurs on single ones too.

A few – roughly 10% – of the central stars of planetary nebulae, despite their much lower masses – typically ~0.6 M_☉ – 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 dwarfs, rather than to the very young, very massive population I stars that comprise the bulk of the WR class.{{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}} These are now generally excluded from the class denoted as Wolf–Rayet stars, or referred to as Wolf–Rayet-type stars.

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.{{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}}

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.

File:Hubble spies vast gas disk around unique massive star.tif]]

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.

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.{{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}}

File:LHA 115 - N 76A - Eso0310a.jpg 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-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.{{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}}{{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}}

{{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.{{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}} Specifically a broad emission feature due to the 468.6 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 starbursts 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.{{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}}

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.

Although Wolf–Rayet stars have been clearly identified as an unusual and distinctive class of stars since the 19th century,

{{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

}}

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.

{{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

}}

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.

{{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

}}

{{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

}}

The unusual abundances of nitrogen, carbon, and oxygen, as well as the lack of hydrogen, were recognised, but the reasons remained obscure.

{{cite journal

|last=Underhill |first=Anne B.

|year=1960

|title=A study of the Wolf–Rayet stars HD 192103 and HD 192163

|journal=Publications of the Dominion Astrophysical Observatory Victoria

|volume=11 |pages=209

|bibcode=1960PDAO...11..209U

}}

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.

{{cite journal

|last=Sahade |first=J.

|year=1958

|title=On the nature of the Wolf–Rayet stars

|journal=The Observatory

|volume=78 |pages=79

|bibcode=1958Obs....78...79S

}}

{{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}}

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.

{{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

}}

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.

{{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

}}

WR stars were being proposed as possible progenitors of supernovae, and particularly the newly-discovered type Ib supernovae, lacking hydrogen but apparently associated with young massive stars.

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.

{{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

}}

{{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

}}

File:WR 31a.jpg 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.

{{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

}}

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.

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{{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}}), many of them from the WR stage.{{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 }}

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, 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.{{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 }}{{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}}{{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}}

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, γ² Velorum A currently has a mass around 9 times the Sun, but began with a mass at least 40 times the Sun.{{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 }} 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.{{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}}

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.{{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}}

{{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.{{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}}

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. A possible progenitor star which has disappeared at the location of supernova iPTF13bvn may be a single WR star,{{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}} although other analyses favour a less massive binary system with a stripped star or helium giant.{{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}}{{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}} 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.{{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}}

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.{{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}}{{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}}{{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}}

{{main|List of Wolf-Rayet stars}}

By far the most visible example of a Wolf–Rayet star is γ² 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 lines in lieu of dark absorption lines) it is dubbed the "Spectral Gem of the Southern Skies". The only other Wolf–Rayet star brighter than magnitude 6 is θ Muscae (WR 48), a triple star with two O class companions. Both are WC stars. The "ex" WR star WR 79a (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.

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 most luminous and 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. 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 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, 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.{{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 }}]]

HD 45166 has been described as the most magnetic massive star known and as the first magnetic known Wolf-Rayet star.{{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}}

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

• Markarian 50

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{{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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Category:Star types

*

Category:Astronomical objects discovered in 1867