Editing Wolf–Rayet star (section)

==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/>