Editing Stellar classification (section)

==Extended spectral types==
A number of new spectral types have been taken into use from newly discovered types of stars.<ref name=nasa1010/>

===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"/>

===Cool red and brown dwarf classes===
{{main|Brown dwarf|Red dwarf}}
The new spectral types L, T, and Y were created to classify infrared spectra of cool stars. This includes both [[red dwarf]]s and [[brown dwarf]]s that are very faint in the [[visible spectrum]].<ref>{{cite journal |bibcode=2008ASPC..384...85K |arxiv=0704.1522 |title=Outstanding Issues in Our Understanding of L, T, and Y Dwarfs |journal=14th Cambridge Workshop on Cool Stars |volume=384 |pages=85 |last1=Kirkpatrick |first1=J. D. |year=2008 }}</ref>

[[Brown dwarf]]s, stars that do not undergo [[hydrogen fusion]], cool as they age and so progress to later spectral types. Brown dwarfs start their lives with M-type spectra and will cool through the L, T, and Y spectral classes, faster the less massive they are; the highest-mass brown dwarfs cannot have cooled to Y or even T dwarfs within the age of the universe. Because this leads to an unresolvable overlap between spectral types{{'}} [[effective temperature]] and [[luminosity]] for some masses and ages of different L-T-Y types, no distinct [[effective temperature|temperature]] or [[luminosity]] values can be given.<ref name=bdevol>{{cite journal |title=Evolutionary models for cool brown dwarfs and extrasolar giant planets. The case of HD 209458 |journal=Astronomy and Astrophysics |first1=I. |last1=Baraffe |first2=G. |last2=Chabrier |first3=T. S. |last3=Barman |first4=F. |last4=Allard |first5=P. H. |last5=Hauschildt |volume=402 |issue=2 |pages=701–712 |date=May 2003 |doi=10.1051/0004-6361:20030252 |bibcode=2003A&A...402..701B |arxiv=astro-ph/0302293|s2cid=15838318 }}</ref>

====Class L====
{{see also|L dwarf}}
[[File:L-dwarf-nasa-hurt.png|thumb|Artist's impression of an L-dwarf]]

Class L dwarfs get their designation because they are cooler than M stars and L is the remaining letter alphabetically closest to M. Some of these objects have masses large enough to support hydrogen fusion and are therefore stars, but most are of [[substellar object|substellar]] mass and are therefore brown dwarfs. They are a very dark red in color and brightest in [[infrared]]. Their [[atmosphere]] is cool enough to allow [[metal hydride]]s and [[alkali metal]]s to be prominent in their spectra.<ref name="kirk_ARAA">{{cite journal |first1=J. Davy |last1=Kirkpatrick |first2=I. Neill |last2=Reid |first3=James |last3=Liebert |first4=Roc M. |last4=Cutri |first5=Brant |last5=Nelson |first6=Charles A. |last6=Beichman |first7=Conard C. |last7=Dahn |first8=David G. |last8=Monet |first9=John E. |last9=Gizis| first10 = Michael F. | last10 = Skrutskie |title=Dwarfs Cooler than M: the Definition of Spectral Type L Using Discovery from the 2-µ ALL-SKY Survey (2MASS) |journal=The Astrophysical Journal |volume=519 |issue=2 |pages=802–833 |date=10 July 1999 |doi=10.1086/307414 |bibcode=1999ApJ...519..802K|doi-access=free }}
</ref><ref name="kirk_ApJ">{{cite journal |last=Kirkpatrick |first=J. Davy |title=New Spectral Types L and T |journal=[[Annual Review of Astronomy and Astrophysics]] |volume=43 |issue=1 |pages=195–246 |date=2005 |doi=10.1146/annurev.astro.42.053102.134017 |bibcode=2005ARA&A..43..195K|s2cid=122318616 |url=https://authors.library.caltech.edu/868/1/KIRaraa05.pdf }}
</ref><ref>{{Cite journal |doi=10.1086/499622 |title=Discovery of a Very Young Field L Dwarf, 2MASS J01415823−4633574 |year=2006 |last1=Kirkpatrick |first1=J. Davy |last2=Barman |first2=Travis S. |last3=Burgasser |first3=Adam J. |last4=McGovern |first4=Mark R. |last5=McLean |first5=Ian S. |last6=Tinney |first6=Christopher G. |last7=Lowrance |first7=Patrick J. |journal=The Astrophysical Journal |volume=639 |issue=2 |pages=1120–1128 |arxiv=astro-ph/0511462 |bibcode=2006ApJ...639.1120K|s2cid=13075577 }}</ref>

Due to low surface gravity in giant stars, [[Titanium monoxide|TiO]]- and [[Vanadium monoxide|VO]]-bearing condensates never form. Thus, L-type stars larger than dwarfs can never form in an isolated environment. However, it may be possible for these [[L-type supergiant]]s to form through stellar collisions, an example of which is [[V838 Monocerotis]] while in the height of its [[luminous red nova]] eruption.

====Class T{{anchor|Class T}}====
{{see also|T dwarf}}
[[File:T-dwarf-nasa-hurt.png|thumb|Artist's impression of a T-dwarf]]

Class&nbsp;T dwarfs are cool [[brown dwarf]]s with surface temperatures between approximately {{cvt|550|and|1300|K|C F}}. Their emission peaks in the [[infrared]]. [[Methane]] is prominent in their spectra.<ref name="kirk_ARAA"/><ref name="kirk_ApJ"/>

Study of the number of [[protoplanetary disk|proplyds]] (protoplanetary disks, clumps of gas in [[nebula]]e from which stars and planetary systems are formed) indicates that the number of stars in the [[galaxy]] should be several [[order of magnitude|orders of magnitude]] higher than what was previously conjectured. It is theorized that these proplyds are in a race with each other. The first one to form will become a [[protostar]], which are very violent objects and will disrupt other proplyds in the vicinity, stripping them of their gas. The victim proplyds will then probably go on to become main-sequence stars or brown dwarfs of the L and T classes, which are quite invisible to us.<ref>{{Cite journal|last=Camenzind|first=Max|date=September 27, 2006|title=Classification of Stellar Spectra and its Physical Interpretation|url=http://www.lsw.uni-heidelberg.de/users/jheidt/praktikum/Astrolab_WS2020/Task3/Task3.pdf|journal=Astro Lab Landessternwarte Königstuhl|pages=6|via=Heidelberg University}}</ref>

====Class Y====
{{See also|Y dwarf}}
[[File:WISE 1828+2650 Brown dwarf.jpg|thumb|Artist's impression of a Y-dwarf]]

Brown dwarfs of spectral class&nbsp;Y are cooler than those of spectral class&nbsp;T and have qualitatively different spectra from them. A total of 17&nbsp;objects have been placed in class&nbsp;Y as of August 2013.<ref>{{cite journal |bibcode=2013ApJ...776..128K |arxiv=1308.5372 |title=Discovery of the Y1 Dwarf WISE J064723.23-623235.5 |journal=The Astrophysical Journal |volume=776 |issue=2 |pages=128 |last1=Kirkpatrick |first1=J. Davy |last2=Cushing |first2=Michael C. |last3=Gelino |first3=Christopher R. |last4=Beichman |first4=Charles A. |last5=Tinney |first5=C. G. |last6=Faherty |first6=Jacqueline K. |author6-link=Jackie Faherty|last7=Schneider |first7=Adam |last8=Mace |first8=Gregory N. |year=2013 |doi=10.1088/0004-637X/776/2/128|s2cid=6230841 }}</ref> Although such dwarfs have been modelled<ref>{{cite journal |arxiv=astro-ph/0607305 |title=Y-Spectral class for Ultra-Cool Dwarfs |journal=Monthly Notices of the Royal Astronomical Society|volume=371 |pages=1722–1730 |first1=N. R. |last1=Deacon |first2=N. C. |last2=Hambly |year=2006 |doi=10.1111/j.1365-2966.2006.10795.x |doi-access=free |s2cid=14081778 }}</ref> and detected within forty light-years by the [[Wide-field Infrared Survey Explorer]] (WISE)<ref name=nasa1010>{{Cite web|url=https://science.nasa.gov/science-news/science-at-nasa/2011/23aug_coldeststars/|title=Discovered: Stars as Cool as the Human Body &#124; Science Mission Directorate|website=science.nasa.gov|access-date=12 July 2017|archive-date=7 October 2011|archive-url=https://web.archive.org/web/20111007124027/http://science.nasa.gov/science-news/science-at-nasa/2011/23aug_coldeststars/|url-status=dead}}</ref><ref name="stars-cooler-than-body">{{cite web |last=Wehner |first=Mike |url=http://ca.news.yahoo.com/blogs/technology-blog/nasa-spots-chilled-stars-cooler-human-body-004551421.html |title=NASA spots chilled-out stars cooler than the human body &#124; Technology News Blog – Yahoo! News Canada |publisher=Ca.news.yahoo.com |date=24 August 2011 |access-date=22 May 2012}}</ref><ref name="cool-y-dwarf">{{Cite magazine|url=https://www.wired.com/2011/08/y-dwarf-stars/|title=NASA Satellite Finds Coldest, Darkest Stars Yet|magazine=Wired|first=Danielle|last=Venton|date=23 August 2011|via=www.wired.com}}</ref><ref>{{Cite web|url=https://www.nasa.gov/mission_pages/WISE/news/wise20110823.html|title=NASA - NASA'S Wise Mission Discovers Coolest Class of Stars|website=www.nasa.gov|access-date=1 November 2019|archive-date=14 February 2021|archive-url=https://web.archive.org/web/20210214132907/https://www.nasa.gov/mission_pages/WISE/news/wise20110823.html|url-status=dead}}</ref><ref>{{cite journal |doi=10.1051/0004-6361:200810038 |bibcode=2009A&A...493.1149Z |title=The minimum Jeans mass, brown dwarf companion IMF, and predictions for detection of Y-type dwarfs |journal=Astronomy and Astrophysics |volume=493 |issue=3 |pages=1149–1154 |year=2009 |last1=Zuckerman |first1=B. |last2=Song |first2=I. |arxiv=0811.0429|s2cid=18147550 }}</ref> there is no well-defined spectral sequence yet and no prototypes. Nevertheless, several objects have been proposed as spectral classes Y0, Y1, and Y2.<ref name=ydwarfsurvey>{{cite journal |arxiv=1309.1422 |doi=10.1126/science.1241917 |pmid=24009359 |title=Distances, Luminosities, and Temperatures of the Coldest Known Substellar Objects |journal=Science |volume=341 |issue=6153 |pages=1492–5 |year=2013 |last1=Dupuy |first1=T. J. |last2=Kraus |first2=A. L. |bibcode=2013Sci...341.1492D|s2cid=30379513 }}</ref>

The spectra of these prospective Y objects display absorption around 1.55&nbsp;[[micrometers]].<ref name=four600k>{{cite journal |doi=10.1088/0004-637X/695/2/1517 |last1=Leggett |first1=Sandy K. |last2=Cushing |first2=Michael C. |last3=Saumon |first3=Didier |last4=Marley |first4=Mark S. |last5=Roellig |first5=Thomas L. |last6=Warren |first6=Stephen J. |last7=Burningham |first7=Ben |last8=Jones |first8=Hugh R. A. |last9=Kirkpatrick |first9=J. Davy |last10=Lodieu |first10=Nicolas |last11=Lucas |first11=Philip W. |last12=Mainzer |first12=Amy K. |last13=Martín |first13=Eduardo L. |last14=McCaughrean |first14=Mark J. |last15=Pinfield |first15=David J. |last16=Sloan |first16=Gregory C. |last17=Smart |first17=Richard L. |last18=Tamura |first18=Motohide |last19=Van Cleve |first19=Jeffrey E. |title=The Physical Properties of Four ~600 K T Dwarfs |date=2009 |journal=[[The Astrophysical Journal]] |volume=695 |issue=2 |pages=1517–1526 |arxiv=0901.4093 |bibcode=2009ApJ...695.1517L |s2cid=44050900 }}</ref> Delorme et al. have suggested that this feature is due to absorption from [[ammonia]], and that this should be taken as the indicative feature for the T-Y transition.<ref name=four600k/><ref name=tytrans>{{cite journal |doi=10.1051/0004-6361:20079317 |bibcode=2008A&A...482..961D |title=CFBDS J005910.90-011401.3: Reaching the T-Y brown dwarf transition? |journal=Astronomy and Astrophysics |volume=482 |issue=3 |pages=961–971 |year=2008 |last1=Delorme |first1=Philippe |last2=Delfosse |first2=Xavier |last3=Albert |first3=Loïc |last4=Artigau |first4=Étienne |last5=Forveille |first5=Thierry |last6=Reylé |first6=Céline |last7=Allard |first7=France |last8=Homeier |first8=Derek |last9=Robin |first9=Annie C. |last10=Willott |first10=Chris J. |last11=Liu |first11=Michael C. |last12=Dupuy |first12=Trent J. |arxiv=0802.4387 |s2cid=847552 }}</ref> In fact, this ammonia-absorption feature is the main criterion that has been adopted to define this class.<ref name=ydwarfsurvey/> However, this feature is difficult to distinguish from absorption by [[water]] and [[methane]],<ref name=four600k/> and other authors have stated that the assignment of class Y0 is premature.<ref>{{cite journal |doi=10.1111/j.1365-2966.2008.13885.x |bibcode=2008MNRAS.391..320B |title=Exploring the substellar temperature regime down to ~550 K |journal=Monthly Notices of the Royal Astronomical Society |volume=391 |issue=1 |pages=320–333 |year=2008 |last1=Burningham |first1=Ben |last2=Pinfield |first2=D. J. |last3=Leggett |first3=S. K. |last4=Tamura |first4=M. |last5=Lucas |first5=P. W. |last6=Homeier |first6=D. |last7=Day-Jones |first7=A. |last8=Jones |first8=H. R. A. |last9=Clarke |first9=J. R. A.|last10=Ishii|first10=M. |last11=Kuzuhara |first11=M. |last12=Lodieu |first12=N. |last13=Zapatero-Osorio |first13=María Rosa |last14=Venemans |first14=B. P. |last15=Mortlock |first15=D. J. |last16=Barrado y Navascués |first16=D. |last17=Martin |first17=Eduardo L. |last18=Magazzù |first18=Antonio |doi-access=free |arxiv=0806.0067|s2cid=1438322 }}</ref>

The latest brown dwarf proposed for the Y spectral type, [[WISE 1828+2650]], is a >&nbsp;Y2&nbsp;dwarf with an effective temperature originally estimated around 300&nbsp;[[kelvin|K]], the temperature of the human body.<ref name="stars-cooler-than-body"/><ref name="cool-y-dwarf"/><ref name=eso1110>[[European Southern Observatory]]. [http://www.eso.org/public/news/eso1110/ "A Very Cool Pair of Brown Dwarfs"], 23 March 2011</ref> [[Parallax]] measurements have, however, since shown that its luminosity is inconsistent with it being colder than ~400&nbsp;K. The coolest Y dwarf currently known is [[WISE 0855−0714]] with an approximate temperature of 250&nbsp;K, and a mass just seven times that of Jupiter.<ref name="Luhman2016">{{cite journal |title=The Spectral Energy Distribution of the Coldest Known Brown Dwarf |arxiv=1605.06655 |first1=Kevin L. |last1=Luhman |first2=Taran L. |last2=Esplin |date=May 2016 |doi=10.3847/0004-6256/152/3/78 |volume=152 |issue=3 |journal=The Astronomical Journal |page=78 |bibcode=2016AJ....152...78L|s2cid=118577918 |doi-access=free }}</ref>

The mass range for Y&nbsp;dwarfs is 9–25&nbsp;[[Jupiter]] masses, but young objects might reach below one Jupiter mass (although they cool to become planets), which means that Y class objects straddle the 13&nbsp;Jupiter mass [[deuterium]]-fusion limit that marks the current [[International Astronomical Union|IAU]] division between brown dwarfs and planets.<ref name=ydwarfsurvey/>

==== Peculiar brown dwarfs ====
{| class="wikitable" style="float:right; width: 370px"
 ! colspan="2" |Symbols used for peculiar brown dwarfs
 |-
 |style="text-align:center;"| pec
 | This suffix stands for "peculiar" (e.g. L2pec).<ref>
  {{cite web
   |title=Spectral type codes
   |website=simbad.u-strasbg.fr
   |url=http://simbad.u-strasbg.fr/simbad/sim-display?data=sptypes
   |access-date=2020-03-06
}}
</ref>
 |-
 |style="text-align:center;"| sd
 | This prefix (e.g. sdL0) stands for [[subdwarf]] and indicates a low metallicity and blue color<ref name=":5">
  {{cite journal
   |last1=Burningham |first1=Ben           |last2=Smith  |first2=L.
   |last3=Cardoso    |first3=C.V.          |last4=Lucas  |first4=P.W.
   |last5=Burgasser  |first5=Adam J.       |last6=Jones  |first6=H.R.A.
   |last7=Smart      |first7=R.L.
   |date=May 2014
   |title=The discovery of a T6.5 subdwarf |language=en
   |journal=Monthly Notices of the Royal Astronomical Society
   |volume=440 |issue=1 |pages=359–364
   |doi=10.1093/mnras/stu184 |doi-access=free |arxiv=1401.5982
   |bibcode=2014MNRAS.440..359B |s2cid=119283917
   |issn=0035-8711
}}
</ref>
 |-
 |style="text-align:center;"| {{mvar|β}}
 | Objects with the beta ({{mvar|β}}) suffix (e.g. L4{{mvar|β}}) have an intermediate surface gravity.<ref name=":6">
  {{cite journal
   |last1=Cruz |first1=Kelle L.
   |last2=Kirkpatrick |first2=J. Davy
   |last3=Burgasser |first3=Adam J.
   |date=February 2009
   |title=Young L dwarfs identified in the field: A preliminary low-gravity, optical spectral Sequence from L0 to L5 |language=en
   |journal=The Astronomical Journal
   |volume=137 |issue=2 |pages=3345–3357
   |doi=10.1088/0004-6256/137/2/3345 |arxiv=0812.0364
   |bibcode=2009AJ....137.3345C |s2cid=15376964
   |issn=0004-6256
}}
</ref>
 |-
 |style="text-align:center;"| {{mvar|γ}}
 | Objects with the gamma ({{mvar|γ}}) suffix (e.g. L5{{mvar|γ}}) have a low surface gravity.<ref name=":6"/>
 |-
 |style="text-align:center;"| red
 | The red suffix (e.g. L0red) indicates objects without signs of youth, but high dust content.<ref name=":7">
  {{cite journal
   |last1=Looper    |first1=Dagny L.        |last2=Kirkpatrick |first2=J. Davy
   |last3=Cutri     |first3=Roc M.          |last4=Barman    |first4=Travis
   |last5=Burgasser |first5=Adam J.         |last6=Cushing   |first6=Michael C.
   |last7=Roellig   |first7=Thomas          |last8=McGovern  |first8=Mark R.
   |last9=McLean    |first9=Ian S.          |last10=Rice     |first10=Emily
   |last11=Swift |first11=Brandon J.
   |date=October 2008
   |title=Discovery of two nearby peculiar L dwarfs from the 2MASS Proper-Motion Survey: Young or metal-rich?  |language=en
   |journal=Astrophysical Journal
   |volume=686  |issue=1  |pages=528–541
   |doi=10.1086/591025 |arxiv=0806.1059
   |bibcode=2008ApJ...686..528L |s2cid=18381182
   |issn=0004-637X
}}
</ref>
 |-
 |style="text-align:center;"| blue
 | The blue suffix (e.g. L3blue) indicates unusual blue near-infrared colors for L-dwarfs without obvious low metallicity.<ref name=":8">
  {{cite journal
   |last1=Kirkpatrick |first1=J. Davy           |last2=Looper  |first2=Dagny L.
   |last3=Burgasser   |first3=Adam J.           |last4=Schurr  |first4=Steven D.
   |last5=Cutri       |first5=Roc M.            |last6=Cushing |first6=Michael C.
   |last7=Cruz        |first7=Kelle L.          |last8=Sweet   |first8=Anne C.
   |last9=Knapp       |first9=Gillian R.        |last10=Barman |first10=Travis S.
   |last11=Bochanski  |first11=John J.
   |date=September 2010
   |title=Discoveries from a near-infrared proper motion survey using multi-epoch Two Micron All-Sky Survey data |language=en
   |journal=Astrophysical Journal Supplement Series
   |volume=190 |issue=1 |pages=100–146
   |doi=10.1088/0067-0049/190/1/100 |arxiv=1008.3591
   |bibcode=2010ApJS..190..100K |s2cid=118435904
   |issn=0067-0049
}}
</ref>
 |}
Young brown dwarfs have low [[Surface gravity|surface gravities]] because they have larger radii and lower masses compared to the field stars of similar spectral type. These sources are marked by a letter beta ({{mvar|β}}) for intermediate surface gravity and gamma ({{mvar|γ}}) for low surface gravity. Indication for low surface gravity are weak CaH, K{{sup|I}} and Na{{sup|I}} lines, as well as strong VO line.<ref name=":6"/> Alpha ({{mvar|α}}) stands for normal surface gravity and is usually dropped. Sometimes an extremely low surface gravity is denoted by a delta ({{mvar|δ}}).<ref name=":8"/> The suffix "pec" stands for peculiar. The peculiar suffix is still used for other features that are unusual and summarizes different properties, indicative of low surface gravity, subdwarfs and unresolved binaries.<ref>
{{cite journal
 |last1=Faherty     |first1=Jacqueline K.     |last2=Riedel     |first2=Adric R.
 |last3=Cruz        |first3=Kelle L.          |last4=Gagne      |first4=Jonathan
 |last5=Filippazzo  |first5=Joseph C.         |last6=Lambrides  |first6=Erini
 |last7=Fica        |first7=Haley             |last8=Weinberger |first8=Alycia
 |last9=Thorstensen |first9=John R.           |last10=Tinney    |first10=C.G.
 |last11=Baldassare |first11=Vivienne
 |date=July 2016
 |title=Population properties of brown dwarf analogs to exoplanets |language=en
 |journal=Astrophysical Journal Supplement Series
 |volume=225 |issue=1 |pages=10
 |doi=10.3847/0067-0049/225/1/10 |arxiv=1605.07927
 |bibcode=2016ApJS..225...10F |s2cid=118446190 |issn=0067-0049
|doi-access=free }}
</ref>
The prefix sd stands for [[subdwarf]] and only includes cool subdwarfs. This prefix indicates a low [[metallicity]] and kinematic properties that are more similar to [[Galactic halo|halo]] stars than to [[Thin disk|disk]] stars.<ref name=":5"/> Subdwarfs appear bluer than disk objects.<ref>
{{cite web
 |title=Colour-magnitude data
 |website=[[Space Telescope Science Institute]] (www.stsci.edu)
 |url=http://www.stsci.edu/~inr/cmd.html
 |access-date=2020-03-06
}}
</ref>
The red suffix describes objects with red color, but an older age. This is not interpreted as low surface gravity, but as a high dust content.<ref name=":7"/><ref name=":8"/> The blue suffix describes objects with blue [[near-infrared]] colors that cannot be explained with low metallicity. Some are explained as L+T binaries, others are not binaries, such as [[2MASS J11263991−5003550]] and are explained with thin and/or large-grained clouds.<ref name=":8"/>

===Late giant carbon-star classes===
Carbon-stars are stars whose spectra indicate production of carbon – a byproduct of [[Triple-alpha process|triple-alpha]] helium fusion. With increased carbon abundance, and some parallel [[s-process]] heavy element production, the spectra of these stars become increasingly deviant from the usual late spectral classes&nbsp;G, K, and M. Equivalent classes for carbon-rich stars are S and C.

The giants among those stars are presumed to produce this carbon themselves, but some stars in this class are double stars, whose odd atmosphere is suspected of having been transferred from a companion that is now a white dwarf, when the companion was a carbon-star.

====Class C====
{{Main|Carbon star}}
[[File:Curious spiral spotted by ALMA around red giant star R Sculptoris (data visualisation).jpg|thumb|Image of the carbon star [[R Sculptoris]] and its striking spiral structure]]
Originally classified as R and N&nbsp;stars, these are also known as ''carbon stars''. These are red giants, near the end of their lives, in which there is an excess of carbon in the atmosphere. The old R and N classes ran parallel to the normal classification system from roughly mid-G to late M. These have more recently been remapped into a unified carbon classifier C with N0 starting at roughly C6. Another subset of cool carbon stars are the C–J-type stars, which are characterized by the strong presence of molecules of [[Carbon-13|<sup>13</sup>]] [[cyanide|CN]] in addition to those of [[Carbon-12|<sup>12</sup>]] [[cyanide|CN]].<ref>Bouigue, R. (1954). Annales d'Astrophysique, Vol.&nbsp;17, p.&nbsp;104</ref> A few main-sequence carbon stars are known, but the overwhelming majority of known carbon stars are giants or supergiants. There are several subclasses:
* C-R – Formerly its own class (''R'') representing the carbon star equivalent of late G- to early K-type stars.
* C-N – Formerly its own class representing the carbon star equivalent of late K- to M-type stars.
* C-J – A subtype of cool C stars with a high content of [[Carbon-13|<sup>13</sup>C]].
* C-H – [[Population II]] analogues of the C-R stars.
* C-Hd – Hydrogen-deficient carbon stars, similar to late G supergiants with [[Methylidyne radical|CH]] and [[diatomic carbon|C<sub>2</sub>]] bands added.

====Class S====
{{Main|S-type star}}
Class&nbsp;S stars form a continuum between class&nbsp;M stars and carbon stars. Those most similar to class&nbsp;M stars have strong [[Zirconium monoxide|ZrO]] [[spectral bands|absorption bands]] analogous to the [[Titanium monoxide|TiO]] bands of class&nbsp;M stars, whereas those most similar to carbon stars have strong [[sodium]] D lines and weak [[Diatomic carbon|C<sub>2</sub>]] bands.<ref>{{cite journal |last=Keenan |first=P. C. |year=1954 |title=Classification of the S-Type Stars |journal=Astrophysical Journal |volume=120 |page=484 |doi=10.1086/145937 |bibcode=1954ApJ...120..484K}}</ref> Class&nbsp;S stars have excess amounts of [[zirconium]] and other elements produced by the [[s-process]], and have more similar carbon and oxygen abundances to class&nbsp;M or carbon stars. Like carbon stars, nearly all known class&nbsp;S stars are [[asymptotic giant branch|asymptotic-giant-branch]] stars.

The spectral type is formed by the letter&nbsp;S and a number between zero and ten. This number corresponds to the temperature of the star and approximately follows the temperature scale used for class&nbsp;M giants. The most common types are S3 to S5. The non-standard designation S10 has only been used for the star [[Chi Cygni]] when at an extreme minimum.

The basic classification is usually followed by an abundance indication, following one of several schemes: S2,5; S2/5; S2 Zr4 Ti2; or S2*5. A number following a comma is a scale between 1 and 9 based on the ratio of ZrO and TiO. A number following a slash is a more-recent but less-common scheme designed to represent the ratio of carbon to oxygen on a scale of 1 to 10, where a 0 would be an MS star. Intensities of zirconium and [[titanium]] may be indicated explicitly. Also occasionally seen is a number following an asterisk, which represents the strength of the ZrO bands on a scale from 1 to 5.

====Classes MS and SC: Intermediate carbon-related classes====
In between the M and S classes, border cases are named MS stars. In a similar way, border cases between the S and C-N classes are named SC or CS. The sequence M → MS → S → SC → C-N is hypothesized to be a sequence of increased carbon abundance with age for [[carbon star]]s in the [[asymptotic giant branch]].

===White dwarf classifications{{anchor|Class D}}===
The class&nbsp;D (for [[Electron-degenerate matter|Degenerate]]) is the modern classification used for white dwarfs—low-mass stars that are no longer undergoing [[nuclear fusion]] and have shrunk to planetary size, slowly cooling down. Class&nbsp;D is further divided into spectral types DA, DB, DC, DO, DQ, DX, and DZ. The letters are not related to the letters used in the classification of other stars, but instead indicate the composition of the white dwarf's visible outer layer or atmosphere.

The white dwarf types are as follows:<ref name="sionspectra">{{cite journal |bibcode=1983ApJ...269..253S |title=A proposed new white dwarf spectral classification system |journal=Astrophysical Journal |volume=269 |pages=253 |last1=Sion |first1=E. M. |last2=Greenstein |first2=J. L. |last3=Landstreet |first3=J. D. |last4=Liebert |first4=James |last5=Shipman |first5=H. L. |last6=Wegner |first6=G. A. |year=1983 |doi=10.1086/161036|doi-access=free }}</ref><ref>{{Cite journal |doi=10.1051/0004-6361:20041372 |title=The rate of period change in pulsating DB-white dwarf stars |year=2004 |last1=Córsico |first1=A. H. |last2=Althaus |first2=L. G. |journal=Astronomy and Astrophysics |volume=428 |pages=159–170 |arxiv=astro-ph/0408237 |bibcode=2004A&A...428..159C|s2cid=14653913 }}</ref>
* DA – a [[hydrogen]]-rich atmosphere or outer layer, indicated by strong Balmer hydrogen [[spectral line]]s.
* DB – a [[helium]]-rich atmosphere, indicated by neutral helium, [[spectroscopic notation|He&nbsp;I]], spectral lines.
* DO – a helium-rich atmosphere, indicated by ionized helium, [[spectroscopic notation|He&nbsp;II]], spectral lines.
* DQ – a [[carbon]]-rich atmosphere, indicated by atomic or molecular carbon lines.
* DZ – a [[metal (astronomy)|metal]]-rich atmosphere, indicated by metal spectral lines (a merger of the obsolete white dwarf spectral types, DG, DK, and DM).
* DC – no strong spectral lines indicating one of the above categories.
* DX – spectral lines are insufficiently clear to classify into one of the above categories.

The type is followed by a number giving the white dwarf's surface temperature. This number is a rounded form of 50400/''T''<sub>eff</sub>, where ''T''<sub>eff</sub> is the [[effective temperature|effective surface temperature]], measured in [[kelvin]]s. Originally, this number was rounded to one of the digits 1 through 9, but more recently fractional values have started to be used, as well as values below 1 and above 9.(For example DA1.5 for IK Pegasi B)<ref name="sionspectra"/><ref name="villanovar4">{{cite journal |bibcode=1999ApJS..121....1M |title=A Catalog of Spectroscopically Identified White Dwarfs |journal=The Astrophysical Journal Supplement Series |volume=121 |issue=1 |pages=1–130 |last1=McCook |first1=George P. |last2=Sion |first2=Edward M. |year=1999 |doi=10.1086/313186 |citeseerx=10.1.1.565.5507|s2cid=122286998 }}</ref>

Two or more of the type letters may be used to indicate a white dwarf that displays more than one of the spectral features above.<ref name="sionspectra"/>

==== Extended white dwarf spectral types ====
[[File:Sirius A and B Hubble photo.jpg|thumb|[[Sirius]] A and B (a [[white dwarf]] of type DA2) resolved by [[Hubble Space Telescope|Hubble]]]]
* DAB – a hydrogen- and helium-rich white dwarf displaying neutral helium lines
* DAO – a hydrogen- and helium-rich white dwarf displaying ionized helium lines
* DAZ – a hydrogen-rich metallic white dwarf
* DBZ – a helium-rich metallic white dwarf

A different set of spectral peculiarity symbols are used for white dwarfs than for other types of stars:<ref name="sionspectra"/>

{| class="wikitable"
|-
! Code
! Spectral peculiarities for stars
|-
! P
| Magnetic white dwarf with detectable polarization
|-
! E
| Emission lines present
|-
! H
| Magnetic white dwarf without detectable polarization
|-
! V
| Variable
|-
! PEC
| Spectral peculiarities exist
|}

=== Luminous blue variables ===
{{Main articles|Luminous blue variable}}
Luminous blue variables (LBVs) are rare, massive and evolved stars that show unpredictable and sometimes dramatic variations in their spectra and brightness. During their "quiescent" states, they are usually similar to B-type stars, although with unusual spectral lines. During outbursts, they are more similar to F-type stars, with significantly lower temperatures. Many papers treat LBV as its own spectral type.<ref>{{Cite journal |last1=Apellániz |first1=J. Maíz |last2=Barbá |first2=R. H. |last3=Aranda |first3=R. Fernández |last4=González |first4=M. Pantaleoni |last5=Bellido |first5=P. Crespo |last6=Sota |first6=A. |last7=Alfaro |first7=E. J. |date=2022-01-01 |title=The Villafranca catalog of Galactic OB groups – II. From Gaia DR2 to EDR3 and ten new systems with O stars |url=https://www.aanda.org/articles/aa/full_html/2022/01/aa42364-21/aa42364-21.html |journal=Astronomy & Astrophysics |language=en |volume=657 |pages=A131 |doi=10.1051/0004-6361/202142364 |arxiv=2110.01464 |bibcode=2022A&A...657A.131M |issn=0004-6361}}</ref><ref>{{Cite journal |last1=Massey |first1=Philip |last2=Neugent |first2=Kathryn F. |last3=Smart |first3=Brianna M. |date=2016-09-01 |title=A spectroscopic survey of massive stars in M31 AND M33* |journal=The Astronomical Journal |volume=152 |issue=3 |pages=62 |doi=10.3847/0004-6256/152/3/62 |doi-access=free |arxiv=1604.00112 |bibcode=2016AJ....152...62M |issn=0004-6256}}</ref>

===Spectral types of non-single objects: Classes P and Q===
Finally, the classes '''P''' and '''Q''' are left over from the system developed by [[Annie Jump Cannon|Cannon]] for the ''[[Henry Draper Catalogue]]''. They are occasionally used for certain objects, not associated with a single star: Type&nbsp;P objects are stars within [[planetary nebula]]e (typically young white dwarfs or hydrogen-poor M giants); type&nbsp;Q objects are [[nova]]e.{{citation needed|date=April 2019}}