Editing Stellar classification (section)

===Hot blue emission star classes===
[[File:Galactic refurbishment.jpg|thumb|[[UGC 5797]], an emission-line galaxy where massive bright blue stars are formed<ref>{{cite web |title=Galactic refurbishment |url=http://www.spacetelescope.org/images/potw1517a/ |website=www.spacetelescope.org |publisher=ESA/Hubble |access-date=29 April 2015}}</ref>]]

Spectra of some very hot and bluish stars exhibit marked emission lines from carbon or nitrogen, or sometimes oxygen.

====Class WR (or W): Wolf–Rayet====
{{Main|Wolf–Rayet star}}
[[File:M1-67 & WR124.png|thumb|[[Hubble Space Telescope]] image of the nebula M1-67 and the Wolf–Rayet star [[WR 124]] in the center]]

Once included as type O stars, the [[Wolf–Rayet star]]s of class&nbsp;W<ref name=payne>{{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> or WR are notable for spectra lacking hydrogen lines. Instead their spectra are dominated by broad emission lines of highly ionized helium, nitrogen, carbon, and sometimes oxygen. They are thought to mostly be dying supergiants with their hydrogen layers blown away by [[stellar wind]]s, thereby directly exposing their hot helium shells. Class&nbsp;WR is further divided into subclasses according to the relative strength of nitrogen and carbon emission lines in their spectra (and outer layers).<ref name=WR>{{cite journal |bibcode=2007ARA&A..45..177C |title=Physical Properties of Wolf-Rayet Stars |journal=Annual Review of Astronomy & Astrophysics |volume=45 |issue=1 |pages=177–219 |last1=Crowther |first1=Paul A. |year=2007 |doi=10.1146/annurev.astro.45.051806.110615 |arxiv=astro-ph/0610356|s2cid=1076292 }}</ref>

WR spectra range is listed below:<ref>{{Cite journal |doi=10.1086/304488 |title=AK-Band Spectral Atlas of Wolf-Rayet Stars |year=1997 |last1=Figer |first1=Donald F. |last2=McLean |first2=Ian S. |last3=Najarro |first3=Francisco |journal=The Astrophysical Journal |volume=486 |issue=1 |pages=420–434 |bibcode=1997ApJ...486..420F|doi-access=free }}</ref><ref>{{cite journal |bibcode=1995A&A...295...75K |title=Properties of the WO Wolf-Rayet stars |journal=Astronomy and Astrophysics |volume=295 |pages=75 |last1=Kingsburgh |first1=R. L. |last2=Barlow |first2=M. J. |last3=Storey |first3=P. J. |year=1995}}</ref>
* WN<ref name=WR/> – spectrum dominated by N&nbsp;III-V and He&nbsp;I-II lines
** WNE (WN2 to WN5 with some WN6) – hotter or "early"
** WNL (WN7 to WN9 with some WN6) – cooler or "late"
** Extended WN classes WN10 and WN11 sometimes used for the Ofpe/WN9 stars<ref name=WR/>
** h tag used (e.g. WN9h) for WR with hydrogen emission and ha (e.g. WN6ha) for both hydrogen emission and absorption
* WN/C – WN stars plus strong C&nbsp;IV lines, intermediate between WN and WC stars<ref name=WR/>
* WC<ref name=WR/> – spectrum with strong C&nbsp;II-IV lines
** WCE (WC4 to WC6) – hotter or "early"
** WCL (WC7 to WC9) – cooler or "late"
* WO (WO1 to WO4) – strong O&nbsp;VI lines, extremely rare, extension of the WCE class into incredibly hot temperatures (up to 200 kK or more)

Although the central stars of most planetary nebulae (CSPNe) show O-type spectra,<ref name=tinkler>{{Cite journal |last1=Tinkler |first1=C. M. |last2=Lamers |first2=H. J. G. L. M. |doi=10.1051/0004-6361:20020061 |title=Mass-loss rates of H-rich central stars of planetary nebulae as distance indicators? |journal=Astronomy and Astrophysics |volume=384 |issue=3 |pages=987–998 |year=2002 |bibcode=2002A&A...384..987T|doi-access=free }}</ref> around 10% are hydrogen-deficient and show WR spectra.<ref name=mizsalski>{{Cite journal |last1=Miszalski |first1=B. |last2=Crowther |first2=P. A. |last3=De Marco |first3=O. |last4=Köppen |first4=J. |last5=Moffat |first5=A. F. J. |last6=Acker |first6=A. |last7=Hillwig |first7=T. C. |title=IC 4663: The first unambiguous [WN] Wolf-Rayet central star of a planetary nebula |doi=10.1111/j.1365-2966.2012.20929.x |journal=Monthly Notices of the Royal Astronomical Society |volume=423 |issue=1 |pages=934–947 |year=2012 |doi-access=free |arxiv=1203.3303 |bibcode=2012MNRAS.423..934M|s2cid=10264296 }}</ref> These are low-mass stars and to distinguish them from the massive Wolf–Rayet stars, their spectra are enclosed in square brackets: e.g. [WC]. Most of these show [WC] spectra, some [WO], and very rarely [WN].

====Slash stars====
{{Main|Slash star}}
The ''slash'' stars are O-type stars with WN-like lines in their spectra. The name "slash" comes from their printed spectral type having a slash in it (e.g. "Of/WNL")<ref name="Walborn">{{cite journal |bibcode=2008RMxAC..33....5W |title=Multiwavelength Systematics of OB Spectra |journal=Massive Stars: Fundamental Parameters and Circumstellar Interactions (Eds. P. Benaglia |volume=33 |pages=5 |last1=Walborn |first1=N. R. |year=2008}}</ref>).

There is a secondary group found with these spectra, a cooler, "intermediate" group designated "Ofpe/WN9".<ref name="Walborn"/> These stars have also been referred to as WN10 or WN11, but that has become less popular with the realisation of the evolutionary difference from other Wolf–Rayet stars. Recent discoveries of even rarer stars have extended the range of slash stars as far as O2-3.5If<sup>*</sup>/WN5-7, which are even hotter than the original "slash" stars.<ref>{{Cite journal |last1=Crowther |first1=P. A. |last2=Walborn |first2=N. R. |doi=10.1111/j.1365-2966.2011.19129.x |title=Spectral classification of O2-3.5 If*/WN5-7 stars |journal=Monthly Notices of the Royal Astronomical Society |volume=416 |issue=2 |pages=1311–1323 |year=2011 |doi-access=free |arxiv=1105.4757 |bibcode=2011MNRAS.416.1311C|s2cid=118455138 }}</ref>

====Magnetic O stars====
They are O stars with strong magnetic fields. Designation is Of?p.<ref name="Walborn"/>