Editing Stellar classification (section)

===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"/>