View source for Stellar classification

{{Short description|Classification of stars based on spectral properties}}
{{Redirect|Star classification|the ranking system, e.g. "four out of five stars"|Star (classification)}}
{{Use dmy dates|date=December 2017}}
In [[astronomy]], '''stellar classification''' is the classification of [[star]]s based on their [[stellar spectrum|spectral]] characteristics. [[Electromagnetic radiation]] from the star is analyzed by splitting it with a [[Prism (optics)|prism]] or [[diffraction grating]] into a [[spectrum]] exhibiting the [[Continuum (spectrum)|rainbow]] of colors interspersed with [[spectral line]]s. Each line indicates a particular [[chemical element]] or [[molecule]], with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary mainly due to the temperature of the [[photosphere]], although in some cases there are true abundance differences. The ''spectral class'' of a star is a short code primarily summarizing the [[ionization]] state, giving an objective measure of the photosphere's temperature.

Most stars are currently classified under the Morgan–Keenan (MK) system using the letters ''O'', ''B'', ''A'', ''F'', ''G'', ''K'', and ''M'', a sequence from the hottest (''O'' type) to the coolest (''M'' type). Each letter class is then subdivided using a numeric digit with ''0'' being hottest and ''9'' being coolest (e.g., A8, A9, F0, and F1 form a sequence from hotter to cooler). The sequence has been expanded with three classes for other stars that do not fit in the classical system: ''[[W-type star|W]]'', ''[[S-type star|S]]'' and ''[[C-type star|C]]''. Some [[Compact object|stellar remnants]] or objects of deviating mass have also been assigned letters: ''[[Stellar classification#White dwarf classifications|D]]'' for [[white dwarf]]s and ''[[L dwarf|L]]'', ''[[T dwarf|T]]'' and ''[[Y Dwarf|Y]]'' for [[brown dwarfs]] (and [[exoplanet]]s).

In the MK system, a [[luminosity class]] is added to the spectral class using [[Roman numerals]]. This is based on the width of certain absorption lines in the star's spectrum, which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. Luminosity class&nbsp;''0'' or ''Ia+'' is used for ''[[hypergiant]]s'', class&nbsp;''I'' for ''[[supergiant]]s'', class&nbsp;''II'' for bright ''[[Giant star|giants]]'', class&nbsp;''III'' for regular ''giants'', class&nbsp;''IV'' for ''[[subgiant]]s'', class&nbsp;''V'' for ''[[main-sequence]] stars'', class&nbsp;''sd'' (or ''VI'') for ''[[subdwarf]]s'', and class&nbsp;''D'' (or ''VII'') for ''[[white dwarf]]s''. The full spectral class for the [[Sun]] is then G2V, indicating a main-sequence star with a surface temperature around 5,800&nbsp;K.

==Conventional colour description==
The conventional colour description takes into account only the peak of the stellar spectrum. In actuality, however, stars radiate in all parts of the spectrum. Because all spectral colours combined appear white, the actual apparent colours the human eye would observe are far lighter than the conventional colour descriptions would suggest. This characteristic of 'lightness' indicates that the simplified assignment of colours within the spectrum can be misleading. Excluding colour-contrast effects in dim light, in typical viewing conditions there are no green, cyan, indigo, or violet stars. [[Yellow dwarf star|"Yellow" dwarfs]] such as the [[Sun]] are white, [[red dwarf|"red" dwarf]]s are a deep shade of yellow/orange, and [[brown dwarf|"brown" dwarf]]s do not literally appear brown, but hypothetically would appear dim red or grey/black to a nearby observer.

==Modern classification{{anchor|MK}}==
The modern classification system is known as the ''Morgan–Keenan'' (MK) classification. Each star is assigned a spectral class (from the older Harvard spectral classification, which did not include luminosity<ref>{{Cite web |title=Morgan-Keenan Luminosity Class {{!}} COSMOS |url=https://astronomy.swin.edu.au/cosmos/m/morgan-keenan+luminosity+class |access-date=2022-08-31 |website=astronomy.swin.edu.au}}</ref>) and a luminosity class using Roman numerals as explained below, forming the star's spectral type.

Other modern [[photometric system|stellar classification systems]], such as the [[UBV photometric system|UBV system]], are based on [[color index|color indices]]—the measured differences in three or more [[Magnitude (astronomy)|color magnitude]]s.<ref>{{Cite web |last=O'Connell |date=March 27, 2023 |title=MAGNITUDE AND COLOR SYSTEMS |url=https://web.ipac.caltech.edu/staff/fmasci/home/astro_refs/magsystems.pdf |url-status=live |archive-url=https://web.archive.org/web/20230328003032/https://web.ipac.caltech.edu/staff/fmasci/home/astro_refs/magsystems.pdf |archive-date=28 March 2023 |access-date=March 27, 2023 |website=Caltech ASTR 511}}</ref> Those numbers are given labels such as "U−V" or "B−V", which represent the colors passed by two standard filters (e.g. ''U''ltraviolet, ''B''lue and ''V''isual).

===Harvard spectral classification===
The ''Harvard system'' is a one-dimensional classification scheme by astronomer [[Annie Jump Cannon]], who re-ordered and simplified the prior alphabetical system by Draper (see [[#History|History]]). Stars are grouped according to their spectral characteristics by single letters of the alphabet, optionally with numeric subdivisions. Main-sequence stars vary in surface temperature from approximately 2,000 to 50,000&nbsp;[[kelvin|K]], whereas more-evolved stars – in particular, newly-formed white dwarfs – can have surface temperatures above 100,000&nbsp;K.<ref name=Jeffery2023>{{cite journal |first1=C. S. |last1=Jeffery |first2=K. |last2=Werner |first3=D. |last3=Kilkenny |first4=B. |last4=Miszalski |first5=I. |last5=Monageng |first6=E. J. |last6=Snowdon |title=Hot white dwarfs and pre-white dwarfs discovered with SALT |journal=Monthly Notices of the Royal Astronomical Society |volume=519 |issue=2 |date=2023 |pages=2321–2330 |doi=10.1093/mnras/stac3531|doi-access=free |arxiv=2301.03550 }}</ref> Physically, the classes indicate the temperature of the star's atmosphere and are normally listed from hottest to coldest.
{| class="wikitable"
! Class
! [[Effective temperature]]{{wbr}}<ref name=calib/>{{zwj}}<ref name=weidner>{{Cite journal |title=The masses, and the mass discrepancy of O-type stars |journal=Astronomy and Astrophysics |first1=Carsten |last1=Weidner |first2=Jorick S. |last2=Vink |volume=524 |at=A98 |date=December 2010 |doi=10.1051/0004-6361/201014491 |bibcode=2010A&A...524A..98W |arxiv=1010.2204|s2cid=118836634 }}</ref>
! abbr="Chromaticity" | Vega-relative [[chromaticity]]{{wbr}}<ref name="Charity"/>{{zwj}}<ref name="ATNFcolor">{{cite web |url=http://www.atnf.csiro.au/outreach/education/senior/astrophysics/photometry_colour.html |title=The Colour of Stars |publisher=Australia Telescope National Facility|date=2018-10-17 }}</ref>{{zwj}}<ref group=lower-alpha>This is the relative color of the star if [[Vega]], generally considered a bluish star, is used as a standard for "white".</ref>
! abbr="chromaticity" | Chromaticity ([[Illuminant D65|D65]]){{wbr}}<ref name="möre">{{cite book |title=The Guinness Book of Astronomy: Facts & Feats |publisher=Guinness |first=Patrick |last=Moore |edition= 4th |date=1992 |isbn=978-0-85112-940-2}}</ref>{{zwj}}<ref>{{cite web |url=http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html |title=The Colour of Stars |publisher=Australia Telescope Outreach and Education |date=21 December 2004 |access-date=26 September 2007 |archive-date=3 December 2013 |archive-url=https://web.archive.org/web/20131203222826/http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html |url-status=dead }} — Explains the reason for the difference in color perception.</ref>{{zwj}}<ref name="Charity">{{cite web |url=http://www.vendian.org/mncharity/dir3/starcolor/ |title=What color are the stars? |website=Vendian.org |last=Charity |first=Mitchell |access-date=13 May 2006}}</ref>{{zwj}}<ref group=lower-alpha>Chromaticity can vary significantly within a class; for example, the [[Sun]] (a G2 star) is white, while a G9 star is yellow.</ref>
! Main{{nbh}}sequence mass<ref name=calib>{{cite journal |title=Empirical bolometric corrections for the main-sequence |journal=Astronomy and Astrophysics Supplement Series |first1=G. M. H. J. |last1=Habets |first2=J. R. W. |last2=Heinze |volume=46 |pages=193–237 (Tables VII and VIII) |date=November 1981 |bibcode=1981A&AS...46..193H}}&nbsp;– Luminosities are derived from M<sub>bol</sub> figures, using M<sub>bol</sub>(☉)=4.75.</ref>{{zwj}}<ref name=bdevol/>{{nowrap|([[solar mass]]es)}}
! Main{{nbh}}sequence radius<ref name=calib/>{{zwj}}<ref name=bdevol/>{{nowrap|([[solar radii]])}}
! Main{{nbh}}sequence luminosity<ref name=calib/>{{zwj}}<ref name=bdevol/>([[Bolometric magnitude#Bolometric magnitude|bolometric]])
! Hydrogen lines
! Percentage of all [[main sequence|main{{nbh}}sequence star]]s<ref group=lower-alpha name="proportions"/>{{zwj}}<ref name="LeDrew2001"/>
|-
![[#Class O|O]]
| ≥&nbsp;33,000&nbsp;K
| style="background:#9bb0ff;" class="mw-no-invert"|blue
| style="background:#{{Color temperature|50000|hexval}};" class="mw-no-invert"|blue
| ≥&nbsp;{{Solar mass|link=y|16}}
| ≥&nbsp;{{Solar radius|link=y|6.6}}
| ≥&nbsp;{{Solar luminosity|link=y|30,000}}
| Weak
| 0.00003%
|-
![[#Class B|B]]
| 10,000–33,000&nbsp;K
| style="background:#aabfff;" class="mw-no-invert"|bluish white
| style="background:#{{Color temperature|20000|hexval}};" class="mw-no-invert"|deep bluish white
| {{Solar mass|link=y|2.1–16}}
| {{Solar radius|link=y|1.8–6.6}}
| {{Solar luminosity|link=y|25–30,000}}
| Medium
| 0.12%
|-
![[#Class A|A]]
| 7,300–10,000&nbsp;K
| style="background:#cad7ff;" class="mw-no-invert"|white
| style="background:#{{Color temperature|8750|hexval}};" class="mw-no-invert"|bluish white
| {{Solar mass|link=y|1.4–2.1}}
| {{Solar radius|link=y|1.4–1.8}}
| {{Solar luminosity|link=y|5–25}}
| Strong
| 0.61%
|-
![[#Class F|F]]
| 6,000–7,300&nbsp;K
| style="background:#f8f7ff;" class="mw-no-invert"|yellowish white
| style="background:#{{Color temperature|6750|hexval}};" class="mw-no-invert"|white
| {{Solar mass|link=y|1.04–1.4}}
| {{Solar radius|link=y|1.15–1.4}}
| {{Solar luminosity|link=y|1.5–5}}
| Medium
| 3.0%
|-
![[#Class G|G]]
| 5,300–6,000&nbsp;K
| style="background:#fff4ea;" class="mw-no-invert"|yellow
| style="background:#{{Color temperature|5600|hexval}};" class="mw-no-invert"|yellowish white
| {{Solar mass|link=y|0.8–1.04}}
| {{Solar radius|link=y|0.96–1.15}}
| {{Solar luminosity|link=y|0.6–1.5}}
| Weak
| 7.6%
|-
![[#Class K|K]]
| 3,900–5,300&nbsp;K
| style="background:#ffd2a1;" class="mw-no-invert"|light orange
| style="background:#{{Color temperature|4450|hexval}};" class="mw-no-invert"|pale yellowish orange
| {{Solar mass|link=y|0.45–0.8}}
| {{Solar radius|link=y|0.7–0.96}}
| {{Solar luminosity|link=y|0.08–0.6}}
| Very weak
| 12%
|-
![[#Class M|M]]
| 2,300–3,900&nbsp;K
| style="background:#ffcc6f;" class="mw-no-invert"|Light orangish red
| style="background:#{{Color temperature|3050|hexval}};" class="mw-no-invert"|orangish red
| {{Solar mass|link=y|0.08–0.45}}
| ≤&nbsp;{{Solar radius|link=y|0.7}}
| ≤&nbsp;{{Solar luminosity|link=y|0.08}}
| Very weak
| 76%
|}

The traditional [[mnemonic]] for remembering the order of the spectral type letters, from hottest to coolest, is "<u>O</u>h, <u>B</u>e <u>A</u> <u>F</u>ine <u>G</u>uy/<u>G</u>irl: <u>K</u>iss <u>M</u>e!".<ref name="mnem">{{cite web |title=Spectral classification of stars (OBAFGKM) |url=http://www.eudesign.com/mnems/startemp.htm |website=www.eudesign.com |access-date=2019-04-06}}</ref> Many alternative mnemonics have been proposed, in contests held by astronomy courses and organizations, but the traditional mnemonic remains the most popular.<ref>{{cite web |title=Mnemonics for the Harvard Spectral Classification Scheme |url=http://www.star.ucl.ac.uk/~pac/obafgkmrns.html |access-date=2025-06-10}}</ref><ref>{{cite web|title=AST 101: The Great Mnemonic Contest|url=https://www.astro.sunysb.edu/fwalter/AST101/mnemonic.html|access-date=2025-06-10}}</ref>

The spectral classes O through M, as well as other more specialized classes discussed later, are subdivided by [[Arabic numerals]] (0–9), where 0 denotes the hottest stars of a given class. For example, A0 denotes the hottest stars in class&nbsp;A and A9 denotes the coolest ones. Fractional numbers are allowed; for example, the star [[Mu Normae]] is classified as O9.7.<ref name="UpsOriType">{{cite journal |title=The Galactic O-Star Spectroscopic Survey (GOSSS). II. Bright Southern Stars |journal=The Astrophysical Journal Supplement Series |last1=Sota |first1=A. |last2=Maíz Apellániz |first2=J. |last3=Morrell |first3=N. I. |last4=Barbá |first4=R. H. |last5=Walborn |first5=N. R. | author5-link=Nolan R. Walborn |last6=Gamen |first6=R. C. |last7=Arias |first7=J. I. |last8=Alfaro |first8=E. J. |display-authors=5 |volume=211 |issue=1 |at=10 |date=March 2014 |doi=10.1088/0067-0049/211/1/10 |bibcode=2014ApJS..211...10S |arxiv=1312.6222|s2cid=118847528 }}</ref> The [[Sun]] is classified as G2.<ref name="SunSpectrum">{{Cite book |title=Guide to the Sun |publisher=[[Cambridge University Press]] |last=Phillips |first=Kenneth J. H. |pages=47–53 |date=1995 |isbn=978-0-521-39788-9}}</ref>

The fact that the Harvard classification of a star indicated its surface or [[Photosphere|photospheric]] [[temperature]] (or more precisely, its [[effective temperature]]) was not fully understood until after its development, though by the time the first [[Hertzsprung–Russell diagram]] was formulated (by 1914), this was generally suspected to be true.<ref>{{cite magazine |title=Relations Between the Spectra and Other Characteristics of the Stars |magazine=Popular Astronomy |last=Russell |first=Henry Norris |volume=22 |pages=275–294 |date=March 1914 |bibcode=1914PA.....22..275R}}</ref> In the 1920s, the Indian physicist [[Meghnad Saha]] derived a theory of ionization by extending well-known ideas in physical chemistry pertaining to the dissociation of molecules to the ionization of atoms. First he applied it to the solar chromosphere, then to stellar spectra.<ref>{{cite journal |title=On a Physical Theory of Stellar Spectra |journal=Proceedings of the Royal Society of London. Series A |last=Saha |first=M. N. |volume=99 |issue=697 |pages=135–153 |date=May 1921 |doi=10.1098/rspa.1921.0029 |bibcode=1921RSPSA..99..135S|doi-access=free }}</ref>

Harvard astronomer [[Cecilia Payne-Gaposchkin|Cecilia Payne]] then demonstrated that the ''O-B-A-F-G-K-M'' spectral sequence is actually a sequence in temperature.<ref>{{cite thesis |title=Stellar Atmospheres; a Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars |publisher=Radcliffe College |first=Cecilia Helena |last=Payne |type=Ph.D |date=1925 |bibcode=1925PhDT.........1P}}</ref> Because the classification sequence predates our understanding that it is a temperature sequence, the placement of a spectrum into a given subtype, such as B3 or A7, depends upon (largely subjective) estimates of the strengths of absorption features in stellar spectra. As a result, these subtypes are not evenly divided into any sort of mathematically representable intervals.

=== Morgan–Keenan classification{{anchor|Luminosity class|Luminosity classes}} ===
The ''Yerkes spectral classification'', also called the ''MK,'' or Morgan-Keenan (alternatively referred to as the MKK, or Morgan-Keenan-Kellman)<ref>{{Cite web |last=Universe |first=Physics And |date=2013-06-14 |title=The Yerkes spectral classification |url=https://physicsanduniverse.com/the-yerkes-spectral-classification/ |access-date=2022-08-31 |website=Physics and Universe |language=en-US}}</ref><ref>{{Cite web |last=UCL |date=2018-11-30 |title=The MKK and Revised MK Atlas |url=https://www.ucl.ac.uk/ucl-observatory/resources/mkk-and-revised-mk-atlas |access-date=2022-08-31 |website=UCL Observatory (UCLO) |language=en}}</ref> system from the authors' initials, is a system of stellar spectral classification introduced in 1943 by [[William Wilson Morgan]], [[Philip Childs Keenan|Philip C. Keenan]], and [[Edith Kellman]] from [[Yerkes Observatory]].<ref>{{cite book |title=An atlas of stellar spectra, with an outline of spectral classification |publisher=The University of Chicago Press |first1=William Wilson |last1=Morgan |first2=Philip Childs |last2=Keenan |first3=Edith |last3=Kellman |date=1943 |bibcode=1943assw.book.....M |oclc=1806249}}</ref> This two-dimensional ([[temperature]] and [[luminosity]]) classification scheme is based on [[spectral line]]s sensitive to stellar temperature and [[surface gravity]], which is related to luminosity (whilst the ''Harvard classification'' is based on just surface temperature). Later, in 1953, after some revisions to the list of standard stars and classification criteria, the scheme was named the ''Morgan–Keenan classification'', or ''MK'',<ref name="ref_MK">{{cite journal |title=Spectral Classification |journal=Annual Review of Astronomy and Astrophysics |first1=William Wilson |last1=Morgan |first2=Philip Childs |last2=Keenan |volume=11 |pages=29–50 |date=1973 |doi=10.1146/annurev.aa.11.090173.000333 |bibcode=1973ARA&A..11...29M}}</ref> which remains in use today.

Denser stars with higher surface gravity exhibit greater [[pressure broadening]] of spectral lines. The gravity, and hence the pressure, on the surface of a [[giant star]] is much lower than for a [[dwarf star]] because the radius of the giant is much greater than a dwarf of similar mass. Therefore, differences in the spectrum can be interpreted as ''luminosity effects'' and a luminosity class can be assigned purely from examination of the spectrum.

A number of different ''luminosity classes'' are distinguished, as listed in the table below.<ref name="CDS">{{cite web |url=https://www.cfa.harvard.edu/~pberlind/atlas/htmls/note.html |title=A note on the spectral atlas and spectral classification |publisher=[[Centre de données astronomiques de Strasbourg]] |access-date=2 January 2015}}</ref>

{| class="wikitable"
|+ Yerkes luminosity classes
! Luminosity class !! Description
! Examples
|-
| 0 ''or'' Ia<sup>+</sup> || [[hypergiant]]s or extremely luminous supergiants
| [[Cygnus OB2-12|Cygnus OB2#12]] – B3-4Ia+<ref name="Caballero-Nieves">{{Cite journal |title=A High Angular Resolution Survey of Massive Stars in Cygnus OB2: Results from the Hubble Space Telescope Fine Guidance Sensors |journal=The Astronomical Journal |last1=Caballero-Nieves |first1=S. M. |last2=Nelan |first2=E. P. |last3=Gies |first3=D. R. |last4=Wallace |first4=D. J. |last5=DeGioia-Eastwood |first5=K. |author5-link=Kathy Eastwood|last6=Herrero |first6=A. |last7=Jao |first7=W.-C. |last8=Mason |first8=B. D. |last9=Massey |first9=P. |last10=Moffat |first10=A. F. J. |last11=Walborn |first11=N. R. |display-authors=5 |volume=147 |issue=2 |at=40 |date=February 2014 |doi=10.1088/0004-6256/147/2/40 |bibcode=2014AJ....147...40C |arxiv=1311.5087|s2cid=22036552 }}</ref>
|-
| Ia || luminous [[supergiants]]
| [[Eta Canis Majoris]] – B5Ia<ref name="Prinja">{{Cite journal |title=Signature of wide-spread clumping in B supergiant winds |journal=Astronomy and Astrophysics |last1=Prinja |first1=R. K. |last2=Massa |first2=D. L. |volume=521 |at=L55 |date=October 2010 |doi=10.1051/0004-6361/201015252 |bibcode=2010A&A...521L..55P |arxiv=1007.2744|s2cid=59151633 }}</ref>
|-
| Iab || intermediate-size luminous [[supergiants]]
| [[Gamma Cygni]] – F8Iab<ref name="GrayDF">{{Cite journal |title=Photospheric Variations of the Supergiant γ Cyg |journal=The Astronomical Journal |last1=Gray |first1=David F. |volume=140 |issue=5 |pages=1329–1336 |date=November 2010 |doi=10.1088/0004-6256/140/5/1329 |bibcode=2010AJ....140.1329G|doi-access=free }}</ref>
|-
| Ib || less luminous [[supergiants]]
| [[Zeta Persei]] – B1Ib<ref name="Nazé"/>
|-
| II || [[bright giant]]s
| [[Beta Leporis]] – G5II<ref name="Lyubimkov">{{Cite journal |title=Accurate fundamental parameters for A-, F- and G-type Supergiants in the solar neighbourhood |journal=Monthly Notices of the Royal Astronomical Society |last1=Lyubimkov |first1=Leonid S. |last2=Lambert |first2=David L. |last3=Rostopchin |first3=Sergey I. |last4=Rachkovskaya |first4=Tamara M. |last5=Poklad |first5=Dmitry B. |volume=402 |issue=2 |pages=1369–1379 |date=February 2010 |doi=10.1111/j.1365-2966.2009.15979.x |doi-access=free |bibcode=2010MNRAS.402.1369L |arxiv=0911.1335|s2cid=119096173 }}</ref>
|-
| III || normal [[giant star|giants]]
| [[Arcturus]] – K0III<ref name="GrayRO">{{Cite journal |title=Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I |journal=The Astronomical Journal |last1=Gray |first1=R. O. |last2=Corbally |first2=C. J. |last3=Garrison |first3=R. F. |last4=McFadden |first4=M. T. |last5=Robinson |first5=P. E. |volume=126 |issue=4 |pages=2048–2059 |date=October 2003 |bibcode=2003AJ....126.2048G |doi=10.1086/378365 |arxiv=astro-ph/0308182|s2cid=119417105 }}</ref>
|-
| IV || [[subgiant]]s
| [[Gamma Cassiopeiae]] – B0.5IVpe<ref name="Shenavrin">{{Cite journal |title=Search for and study of hot circumstellar dust envelopes |journal=Astronomy Reports |last1=Shenavrin |first1=V. I. |last2=Taranova |first2=O. G. |last3=Nadzhip |first3=A. E. |volume=55 |issue=1 |pages=31–81 |date=January 2011 |doi=10.1134/S1063772911010070 |bibcode=2011ARep...55...31S|s2cid=122700080 }}</ref>
|-
| V || [[main-sequence star]]s (dwarfs)
| [[Achernar]] – B6Vep<ref name="Nazé">{{Cite journal |title=Hot stars observed by XMM-Newton. I. The catalog and the properties of OB stars |journal=Astronomy and Astrophysics |last1=Nazé |first1=Y. |volume=506 |issue=2 |pages=1055–1064 |date=November 2009 |doi=10.1051/0004-6361/200912659 |bibcode=2009A&A...506.1055N |arxiv=0908.1461|s2cid=17317459 }}</ref>
|-
| sd (''prefix'') ''or'' VI || [[subdwarf]]s
| [[HD 149382]] – sdB5 ''or'' B5VI<ref name="Cenarro">{{Cite journal |title=Medium-resolution Isaac Newton Telescope library of empirical spectra - II. The stellar atmospheric parameters |journal=Monthly Notices of the Royal Astronomical Society |last1=Cenarro |first1=A. J. |last2=Peletier |first2=R. F. |last3=Sanchez-Blazquez |first3=P. |last4=Selam |first4=S. O. |last5=Toloba |first5=E. |last6=Cardiel |first6=N. |last7=Falcon-Barroso |first7=J. |last8=Gorgas |first8=J. |last9=Jimenez-Vicente |first9=J. |last10=Vazdekis |first10=A. |volume=374 |issue=2 |pages=664–690 |date=January 2007 |doi=10.1111/j.1365-2966.2006.11196.x |doi-access=free |bibcode=2007MNRAS.374..664C |arxiv=astro-ph/0611618|s2cid=119428437 }}</ref>
|-
| D (''prefix'') ''or'' VII || [[white dwarf]]s<ref group=lower-alpha>Technically, white dwarfs are no longer "live" stars but, rather, the "dead" remains of extinguished stars. Their classification uses a different set of spectral types from element-burning "live" stars.</ref>
| [[van Maanen 2]] – DZ8<ref name="Sion">{{cite journal |title=The White Dwarfs Within 20&nbsp;Parsecs of the Sun: Kinematics and Statistics |journal=[[The Astronomical Journal]] |first1=Edward M. |last1=Sion |first2=J. B. |last2=Holberg |first3=Terry D. |last3=Oswalt |first4=George P. |last4=McCook |first5=Richard |last5=Wasatonic |volume=138 |issue=6 |pages=1681–1689 |date=December 2009 |doi=10.1088/0004-6256/138/6/1681 |bibcode=2009AJ....138.1681S |arxiv=0910.1288|s2cid=119284418 }}</ref>
|}

Marginal cases are allowed; for example, a star may be either a supergiant or a bright giant, or may be in between the subgiant and main-sequence classifications.
In these cases, two special symbols are used between the two luminosity classes:
* A slash ('''/''') means that a star is either one class or the other.
* A hyphen ('''-''') means that the star is in between the two classes.

For example, a star classified as A3-4III/IV would be in between spectral types A3 and A4, while being either a giant star or a subgiant.

Sub-dwarf classes have also been used: VI for sub-dwarfs (stars slightly less luminous than the main sequence).

Nominal luminosity class&nbsp;VII (and sometimes higher numerals) is now rarely used for white dwarf or "hot sub-dwarf" classes, since the temperature-letters of the main sequence and giant stars no longer apply to white dwarfs.

Occasionally, letters ''a'' and ''b'' are applied to luminosity classes other than supergiants; for example, a giant star slightly less luminous than typical may be given a luminosity class of IIIb, while a luminosity class IIIa indicates a star slightly brighter than a typical giant.<ref name="HayesPasinetti2012">{{cite book|author1=D.S. Hayes|author2=L.E. Pasinetti|author3=A.G. Davis Philip|title=Calibration of Fundamental Stellar Quantities: Proceedings of the 111th Symposium of the International Astronomical Union held at Villa Olmo, Como, Italy, May 24–29, 1984|url=https://books.google.com/books?id=1LL1CAAAQBAJ&pg=PA129|date=6 December 2012|publisher=Springer Science & Business Media|isbn=978-94-009-5456-4|pages=129–}}</ref>

A sample of extreme V stars with strong absorption in He II λ4686 spectral lines have been given the ''Vz'' designation. An example star is [[HD 93129|HD 93129 B]].<ref name=arias>{{cite journal |title=Spectral Classification and Properties of the OVz Stars in the Galactic O Star Spectroscopic Survey (GOSSS) |journal=The Astronomical Journal |last1=Arias |first1=Julia I. |last2=Walborn |first2=Nolan R. |last3=Simon-Diaz |first3=Sergio |last4=Barba |first4=Rodolfo |last5=Maiz Apellaniz |first5=Jesus |last6=Sabin-Sanjulian |first6=Carolina |last7=Gamen |first7=Roberto C. |last8=Morrell |first8=Nidia I. |last9=Sota |first9=Alfredo |last10=Marco |first10=Amparo |last11=Negueruela |first11=Ignacio |last12=Leao |first12=Joao R. S. |last13=Herrero |first13=Artemio |last14=Alfaro |first14=Emilio |display-authors=1 |volume=152 |issue=2 |pages=31 |date=August 2016 |doi=10.3847/0004-6256/152/2/31 |bibcode=2016AJ....152...31A |arxiv=1604.03842|s2cid=119259952 |doi-access=free }}</ref>

===Spectral peculiarities===
Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum.<ref>{{cite web |url=http://www.skyandtelescope.com/astronomy-equipment/the-spectral-types-of-stars/?showAll=y |title=The Spectral Types of Stars |work=Sky & Telescope |first=Alan |last=MacRobert |date=1 August 2006}}</ref>

{| class="wikitable"
|-
! Code
! Spectral peculiarities for stars
|-
! :
| uncertain spectral value<ref name="CDS"/>
|-
! ...
| Undescribed spectral peculiarities exist
|-
! !
| Special peculiarity
|-
! comp
| Composite spectrum<ref name="Allen">{{cite web |last1=Allen |first1=J. S. |title=The Classification of Stellar Spectra |website=[[University College London|UCL]] Department of Physics and Astronomy: Astrophysics Group |url=http://www.star.ucl.ac.uk/~pac/spectral_classification.html |access-date=1 January 2014}}</ref>
|-
! e
| Emission lines present<ref name="Allen"/>
|-
! [e]
| [[Forbidden mechanism|"Forbidden" emission lines]] present
|-
! er
| "Reversed" center of emission lines weaker than edges
|-
! eq
| Emission lines with [[P Cygni profile]]
|-
! f
| N&nbsp;III and He&nbsp;II emission<ref name="CDS"/>
|-
! f*
| [[nitrogen|N]]&nbsp;IV 4058Å is stronger than the [[Nitrogen|N]]&nbsp;III 4634Å, 4640Å, & 4642Å lines<ref name="JMA">{{cite journal |bibcode=2007ApJ...660.1480M |arxiv=astro-ph/0612012 |title=Pismis 24-1: The Stellar Upper Mass Limit Preserved |journal=The Astrophysical Journal |volume=660 |issue=2 |pages=1480–1485 |last1=Maíz Apellániz |first1=J. |last2=Walborn |first2=Nolan R. |last3=Morrell |first3=N. I. |last4=Niemela |first4=V. S. |last5=Nelan |first5=E. P. |year=2007 |doi=10.1086/513098|s2cid=15936535 }}</ref>
|-
! f+
| Si&nbsp;IV 4089Å & 4116Å are emitted, in addition to the N&nbsp;III line<ref name="JMA"/>
|-
! f?
| C&nbsp;III 4647–4650–4652Å emission lines with comparable strength to the N&nbsp;III line<ref name=walborn2010>{{cite journal|bibcode=2010ApJ...711L.143W|title=Early Results from the Galactic O-Star Spectroscopic Survey: C III Emission Lines in of Spectra|journal=The Astrophysical Journal Letters|volume=711|issue=2|pages=L143|last1=Walborn|first1=Nolan R.|last2=Sota|first2=Alfredo|last3=Maíz Apellániz|first3=Jesús|last4=Alfaro|first4=Emilio J.|last5=Morrell|first5=Nidia I.|last6=Barbá|first6=Rodolfo H.|last7=Arias|first7=Julia I.|last8=Gamen|first8=Roberto C.|year=2010|doi=10.1088/2041-8205/711/2/L143|arxiv = 1002.3293 |s2cid=119122481}}</ref>
|-
! (f)
| N&nbsp;III emission, absence or weak absorption of He&nbsp;II
|-
! (f+)
|<ref name="Cecilia">{{cite journal |bibcode=2009AJ....138..510F |arxiv=0907.1033 |title=Spectroscopic Study of the N159/N160 Complex in the Large Magellanic Cloud |journal=The Astronomical Journal |volume=138 |issue=2 |pages=510–516 |last1=Fariña |first1=Cecilia |last2=Bosch |first2=Guillermo L. |last3=Morrell |first3=Nidia I. |last4=Barbá |first4=Rodolfo H. |last5=Walborn |first5=Nolan R. |year=2009 |doi=10.1088/0004-6256/138/2/510|s2cid=18844754 }}</ref>
|-
! ((f))
| Displays strong He II absorption accompanied by weak N III emissions<ref name="GRauw">{{cite journal |bibcode=2007A&A...463..981R |arxiv=astro-ph/0612622 |title=Early-type stars in the core of the young open cluster Westerlund 2 |journal=Astronomy and Astrophysics |volume=463 |issue=3 |pages=981–991 |last1=Rauw |first1=G. |last2=Manfroid |first2=J. |last3=Gosset |first3=E. |last4=Nazé |first4=Y. |last5=Sana |first5=H. |last6=De Becker |first6=M. |last7=Foellmi |first7=C. |last8=Moffat |first8=A. F. J. |year=2007 |doi=10.1051/0004-6361:20066495|s2cid=17776145 }}</ref>
|-
! ((f*))
|<ref name=Cecilia/>
|-
! h
| WR stars with hydrogen emission lines.<ref name=WR/>
|-
! ha
| WR stars with hydrogen seen in both absorption and emission.<ref name=WR/>
|-
! He wk
| Weak Helium lines
|-
! k
| Spectra with interstellar absorption features
|-
! m
| Enhanced metal features<ref name="Allen"/>
|-
! n
| Broad ("nebulous") absorption due to spinning<ref name="Allen"/>
|-
! nn
| Very broad absorption features<ref name="CDS"/>
|-
! neb
| A nebula's spectrum mixed in<ref name="Allen"/>
|-
! p
| Unspecified peculiarity, [[peculiar star]].<ref group=lower-alpha>When [[Ap and Bp stars|used with A-type stars]], this instead refers to abnormally strong metallic spectral lines</ref><ref name="Allen"/>
|-
! pq
| Peculiar spectrum, similar to the spectra of novae
|-
! q
| [[P Cygni profile]]s
|-
! s
| Narrow ("sharp") absorption lines<ref name="Allen"/>
|-
! ss
| Very narrow lines
|-
! sh
| [[Shell star]] features<ref name="Allen"/>
|-
! var
| Variable spectral feature<ref name="Allen"/> (sometimes abbreviated to "v")
|-
! wl
| Weak lines<ref name="Allen"/> (also "w" & "wk")
|-
! Element<br />symbol
| Abnormally strong spectral lines of the specified element(s)<ref name="Allen"/>
|-
! z
| indicating an abnormally strong ionised helium line at {{val|468.6|ul=nm}}<ref name=arias/>
|}

For example, [[59 Cygni]] is listed as spectral type&nbsp;B1.5Vnne,<ref name="Lesh">{{Cite journal |doi=10.1086/190179 |title=The Kinematics of the Gould Belt: An Expanding Group? |journal=The Astrophysical Journal Supplement Series |volume=17 |pages=371 |year=1968 |last1=Rountree Lesh |first1=J. |bibcode=1968ApJS...17..371L|doi-access=free }}</ref> indicating a spectrum with the general classification B1.5V, as well as very broad absorption lines and certain emission lines.

==History==
The reason for the odd arrangement of letters in the Harvard classification is historical, having evolved from the earlier Secchi classes and been progressively modified as understanding improved.

===Secchi classes===
During the 1860s and 1870s, pioneering stellar spectroscopist [[Angelo Secchi]] created the ''Secchi classes'' in order to classify observed spectra. By 1866, he had developed three classes of stellar spectra, shown in the table below.<ref>[http://gallica.bnf.fr/ark:/12148/bpt6k30204/f364.table Analyse spectrale de la lumière de quelques étoiles, et nouvelles observations sur les taches solaires], P. Secchi, ''Comptes Rendus des Séances de l'Académie des Sciences'' '''63''' (July–December 1866), pp. 364–368.</ref><ref>[http://gallica.bnf.fr/ark:/12148/bpt6k30204/f623.table Nouvelles recherches sur l'analyse spectrale de la lumière des étoiles], P. Secchi, ''Comptes Rendus des Séances de l'Académie des Sciences'' '''63''' (July–December 1866), pp. 621–628.</ref><ref>{{cite book |last=Hearnshaw |first=J. B. |date=1986 |title=The Analysis of Starlight: One Hundred and Fifty Years of Astronomical Spectroscopy |location=Cambridge, UK |publisher=Cambridge University Press |pages=60, 134 |isbn=978-0-521-25548-6}}</ref>

In the late 1890s, this classification began to be superseded by the Harvard classification, which is discussed in the remainder of this article.<ref>{{cite web| url = http://www.astro.ufl.edu/~gott/AST1002/Additional_Notes/Add_notes.week5| title = Classification of Stellar Spectra: Some History}}</ref><ref>{{cite book |last=Kaler |first=James B. |date=1997 |title=Stars and Their Spectra: An Introduction to the Spectral Sequence |location=Cambridge |publisher=Cambridge University Press |pages=[https://archive.org/details/starstheirspectr0000kale/page/62 62–63] |isbn=978-0-521-58570-5 |url=https://archive.org/details/starstheirspectr0000kale/page/62 }}</ref><ref>p. 60–63, Hearnshaw 1986; pp. 623–625, Secchi 1866.</ref>

{| class="wikitable"
|-
! Class&nbsp;number !! Secchi&nbsp;class&nbsp;description
|-
| Secchi class&nbsp;I || White and blue stars with broad heavy [[hydrogen line]]s, such as [[Vega]] and [[Altair]]. This includes the modern class&nbsp;A and early class&nbsp;F.
|-
| Secchi class&nbsp;I<br />(Orion&nbsp;subtype) || A subtype of Secchi class&nbsp;I with narrow lines in place of wide bands, such as [[Rigel]] and [[Bellatrix]]. In modern terms, this corresponds to early B-type stars
|-
| Secchi class&nbsp;II || Yellow stars – hydrogen less strong, but evident metallic lines, such as the [[Sun]], [[Arcturus]], and [[Capella]]. This includes the modern classes G and K as well as late class&nbsp;F.
|-
| Secchi class&nbsp;III || Orange to red stars with complex band spectra, such as [[Betelgeuse]] and [[Antares]].<br />This corresponds to the modern class&nbsp;M.
|-
| Secchi class&nbsp;IV || In 1868, he discovered [[carbon star]]s, which he put into a distinct group:<ref>pp. 62–63, Hearnshaw 1986.</ref><br />Red stars with significant [[carbon]] bands and lines, corresponding to modern classes C and S.
|-
| Secchi class&nbsp;V || In 1877, he added a fifth class:<ref>p. 60, Hearnshaw 1986.</ref><br />[[Emission-line]] stars, such as [[Gamma Cassiopeiae]] and [[Beta Lyrae|Sheliak]], which are in modern class&nbsp;Be. In 1891, Edward Charles Pickering proposed that class V should correspond to the modern class O (which then included [[Wolf–Rayet star]]s) and stars within planetary nebulae.<ref>''Catchers of the Light: The Forgotten Lives of the Men and Women Who First Photographed the Heavens'' by Stefan Hughes.</ref>
|}

The [[Roman numerals]] used for Secchi classes should not be confused with the completely unrelated Roman numerals used for Yerkes luminosity classes and the proposed neutron star classes.

===Draper system===
{| class="wikitable" style="float: right"
|+ Classifications in the Draper Catalogue of Stellar Spectra<ref name="drapera"/><ref name="draperb"/>
|-
! Secchi || Draper || Comment
|-
|style="text-align:center;"| I
| '''A''', '''B''', C, D
| Hydrogen lines dominant
|-
|style="text-align:center;"| II
| E, '''F''', '''G''', H, I, '''K''', L
|
|-
|style="text-align:center;"| III
| '''M'''
|
|-
|style="text-align:center;"| IV
| N || Did not appear in the catalogue
|-
|style="text-align:center;"| V
| '''O'''
|style="max-width:25em;" | Included [[Wolf–Rayet star|Wolf–Rayet]] spectra with bright lines, sometimes classified separately as type W<ref>{{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>
|-
|style="text-align:center;"| V
| P || Planetary nebulae
|-
| &nbsp;
| Q || Other spectra
|-
|colspan="3" style="text-align:center;" | ''Classes carried through into the MK system are in '''bold'''.''
|}

After the death of [[Henry Draper|her husband]], [[Mary Anna Draper]] began to fund the creation of the [[Harvard Plate Stacks]] and the study of these plates at the [[Harvard College Observatory]]. The director of the Observatory, [[Edward C. Pickering]] began to hire pioneering female astronomers collectively known as the [[Harvard Computers]]. Thought they would study many different astronomical subjects, an early result of this work was the first edition of ''[[Draper Catalogue of Stellar Spectra|The Henry Draper Memorial Catalogue of Stellar Spectra]]'', first published in 1890. [[Williamina Fleming]] classified most of the spectra in the first edition of the catalogue and is credited with classifying over 10,000 featured stars and discovering 10 novae and more than 200 variable stars.<ref name="Williamina Fleming">{{Cite web|title=Williamina Fleming|url=https://www.oxfordreference.com/view/10.1093/oi/authority.20110803095823407|access-date=2020-06-10|website=Oxford Reference|language=en}}</ref> With the help of the [[Harvard Computers]], especially [[Williamina Fleming]], the first iteration of the Henry Draper catalogue was devised to replace the Roman-numeral scheme established by Angelo Secchi.<ref>{{Cite web|title=Williamina Paton Fleming -|url=http://www.projectcontinua.org/williamina-paton-fleming/|access-date=2020-06-10|website=www.projectcontinua.org|language=en-US}}</ref>

The catalogue used a scheme in which the previously used Secchi classes (I to V) were subdivided into more specific classes, given letters from A to P. Also, the letter Q was used for stars not fitting into any other class.<ref name="drapera">{{cite journal |bibcode=1890AnHar..27....1P |title=The Draper Catalogue of stellar spectra photographed with the 8-inch Bache telescope as a part of the Henry Draper memorial |journal=Annals of Harvard College Observatory |volume=27 |pages=1 |last1=Pickering |first1=Edward C. |year=1890}}</ref><ref name="draperb">pp. 106–108, Hearnshaw 1986.</ref> Fleming worked with Pickering to differentiate 17 different classes based on the intensity of hydrogen spectral lines, which causes variation in the wavelengths emanated from stars and results in variation in color appearance. The spectra in class A tended to produce the strongest hydrogen absorption lines while spectra in class O produced virtually no visible lines. The lettering system displayed the gradual decrease in hydrogen absorption in the spectral classes when moving down the alphabet. This classification system was later modified by Annie Jump Cannon and Antonia Maury to produce the Harvard spectral classification scheme.<ref name="Williamina Fleming"/><ref>{{Cite web|url=http://spiff.rit.edu/classes/phys301/lectures/class/class.html|title=Classification of stellar spectra | access-date=2020-06-10|website=spiff.rit.edu}}</ref>

=== The old Harvard system (1897) ===
In 1897, another astronomer at Harvard, [[Antonia Maury]], placed the Orion subtype of Secchi class&nbsp;I ahead of the remainder of Secchi class&nbsp;I, thus placing the modern type&nbsp;B ahead of the modern type&nbsp;A. She was the first to do so, although she did not use lettered spectral types, but rather a series of twenty-two types numbered from I–XXII.<ref>Hearnshaw (1986) pp.&nbsp;111–112</ref><ref>{{cite journal |last1=Maury |first1=Antonia C. |last2=Pickering |first2=Edward C. |year=1897 |title=Spectra of bright stars photographed with the 11&nbsp;inch Draper Telescope as part of the Henry Draper Memorial |journal=Annals of Harvard College Observatory |volume=28 |pages=1 |bibcode=1897AnHar..28....1M}}</ref>
{| class="wikitable"
|+ Summary of 1897 Harvard system<ref name=spectroscopists/>
|-
! Groups !! Summary
|-
|align=center| I−V || included 'Orion type' stars that displayed an increasing strength in hydrogen absorption lines from group&nbsp;I to group&nbsp;V
|-
|align=center| VI || acted as an intermediate between the 'Orion type' and Secchi type&nbsp;I group
|-
|align=center| VII−XI || were Secchi's type&nbsp;1 stars, with decreasing strength in hydrogen absorption lines from groups&nbsp;VII−XI
|-
|align=center| XIII−XVI || included Secchi type&nbsp;2 stars with decreasing hydrogen absorption lines and increasing solar-type metallic lines
|-
|align=center| XVII−XX || included Secchi type&nbsp;3 stars with increasing spectral lines
|-
|align=center| XXI || included Secchi type&nbsp;4 stars
|-
|align=center| XXII || included Wolf–Rayet stars
|}

Because the 22&nbsp;Roman numeral groupings did not account for additional variations in spectra, three additional divisions were made to further specify differences: Lowercase letters were added to differentiate relative line appearance in spectra; the lines were defined as:<ref name=spectroscopists>{{cite web |title=Antonia Maury |website=www.projectcontinua.org |language=en-US |url=http://www.projectcontinua.org/antonia-maury/ |access-date=2020-06-10}}<br />
{{cite book |last=Hearnshaw |first=J.B. |date=17 March 2014 |title=The analysis of starlight: Two centuries of astronomical spectroscopy |edition=2nd |isbn=978-1-107-03174-6 |location=New York, NY |oclc=855909920 }}<br />
{{cite book |last1=Gray |first1=Richard O. |last2=Corbally |first2=Christopher J. |last3=Burgasser |first3=Adam J. |title=Stellar spectral classification |year=2009 |publisher=Princeton University Press |isbn=978-0-691-12510-7 |location=Princeton, NJ |oclc=276340686 }}</ref>
* (a): average width
* (b): hazy
* (c): sharp

Antonia Maury published her own stellar classification catalogue in 1897 called "Spectra of Bright Stars Photographed with the 11&nbsp;inch Draper Telescope as Part of the Henry Draper Memorial", which included 4,800&nbsp;photographs and Maury's analyses of 681&nbsp;bright northern stars. This was the first instance in which a woman was credited for an observatory publication.<ref>{{cite book |last1=Jones |first1=Bessie Zaban |last2=Boyd |first2=Lyle Gifford |year=1971 |title=The Harvard College Observatory: The first four directorships, 1839-1919 |edition=1st |place=Cambridge |publisher=M.A. Belknap Press of Harvard University Press |isbn=978-0-674-41880-6 |oclc=1013948519}}</ref>

===The current Harvard system (1912)===
In 1901, [[Annie Jump Cannon]] returned to the lettered types, but dropped all letters except O, B, A, F, G, K, M, and N<!--class N now absorbed into class C --> used in that order, as well as P for planetary nebulae and Q for some peculiar spectra. She also used types such as B5A for stars halfway between types B and A, F2G for stars one fifth of the way from F to G, and so on.<ref>{{cite journal |last1=Cannon |first1=Annie J. |last2=Pickering |first2=Edward C. |year=1901 |title=Spectra of bright southern stars photographed with the 13&nbsp;inch Boyden telescope as part of the Henry Draper Memorial |journal=Annals of Harvard College Observatory |volume=28 |pages=129 |bibcode=1901AnHar..28..129C}}</ref><ref>Hearnshaw (1986) pp.&nbsp;117–119,</ref>

Finally, by 1912, Cannon had changed the types B, A, B5A, F2G, etc. to B0, A0, B5, F2, etc.<ref>{{cite journal |last1=Cannon |first1=Annie Jump |last2=Pickering |first2=Edward Charles |year=1912 |title=Classification of 1,688&nbsp;southern stars by means of their spectra |journal=Annals of the Astronomical Observatory of Harvard College |volume=56 |issue=5 |pages=115 |bibcode=1912AnHar..56..115C}}</ref><ref>Hearnshaw (1986) pp. 121–122</ref> This is essentially the modern form of the Harvard classification system. This system was developed through the analysis of spectra on photographic plates, which could convert light emanated from stars into a readable spectrum.<ref>{{cite web |title=Annie Jump Cannon |website=www.projectcontinua.org |language=en-US |url=http://www.projectcontinua.org/annie-jump-cannon/ |access-date=2020-06-10}}</ref>

===Mount Wilson classes===
A luminosity classification known as the Mount Wilson system was used to distinguish between stars of different luminosities.<ref name="Nassau1946">{{cite journal |title=Spectra of BD Stars Within Five Degrees of the North Pole |journal=Astrophysical Journal |first1=J. J. |last1=Nassau |first2=Carl K. |last2=Seyfert |volume=103 |page=117 |date=March 1946 |doi=10.1086/144796 |bibcode=1946ApJ...103..117N}}</ref><ref name="FitzGerald1969">{{cite journal |title=Comparison Between Spectral-Luminosity Classes on the Mount Wilson and Morgan–Keenan Systems of Classification |journal=Journal of the Royal Astronomical Society of Canada |first=M. Pim |last=FitzGerald |volume=63 |page=251 |date=October 1969 |bibcode=1969JRASC..63..251P}}</ref><ref name="Sandage1969">{{cite journal |title=New subdwarfs. II. Radial velocities, photometry, and preliminary space motions for 112 stars with large proper motion |journal=Astrophysical Journal |first=A. |last=Sandage |volume=158 |page=1115 |date=December 1969 |doi=10.1086/150271 |bibcode=1969ApJ...158.1115S|doi-access=free }}</ref> This notation system is still sometimes seen on modern spectra.<ref name="Norris2011">{{cite journal |title=Non-detection of the Putative Substellar Companion to HD 149382 |journal=The Astrophysical Journal |first1=Jackson M. |last1=Norris |first2=Jason T. |last2=Wright |first3=Richard A. |last3=Wade |first4=Suvrath |last4=Mahadevan |author4-link=Suvrath Mahadevan |first5=Sara |last5=Gettel |volume=743 |issue=1 |at=88 |date=December 2011 |doi=10.1088/0004-637X/743/1/88 |bibcode=2011ApJ...743...88N |arxiv=1110.1384|s2cid=118337277 }}</ref>
* sd: subdwarf
* d: dwarf
* sg: subgiant
* g: giant
* c: supergiant

==Spectral types==
{{redirect|Spectral type|spectral types of asteroids|Asteroid spectral types}}
The stellar classification system is [[taxonomy (general)|taxonomic]], based on [[Type (biology)|type specimens]], similar to classification of species in [[biology]]: The categories are defined by one or more standard stars for each category and sub-category, with an associated description of the distinguishing features.<ref name="Garrison"/>

==="Early" and "late" nomenclature<span class="anchor" id="Early and late nomenclature"></span>===
Stars are often referred to as ''early'' or ''late'' types. "Early" is a synonym for ''hotter'', while "late" is a synonym for ''cooler''.

Depending on the context, "early" and "late" may be absolute or relative terms. "Early" as an absolute term would therefore refer to O or B, and possibly A stars. As a relative reference it relates to stars hotter than others, such as "early K" being perhaps K0, K1, K2 and K3.

"Late" is used in the same way, with an unqualified use of the term indicating stars with spectral types such as K and M, but it can also be used for stars that are cool relative to other stars, as in using "late G" to refer to G7, G8, and G9.

In the relative sense, "early" means a lower Arabic numeral following the class letter, and "late" means a higher number.

This obscure terminology is a hold-over from a late nineteenth century model of [[stellar evolution]], which supposed that stars were powered by gravitational contraction via the [[Kelvin–Helmholtz mechanism]], which is now known to not apply to [[main-sequence star]]s. If that were true, then stars would start their lives as very hot "early-type" stars and then gradually cool down into "late-type" stars. This mechanism provided ages of the [[Sun]] that were much smaller than what is observed in the [[geologic record]], and was rendered obsolete by the discovery that stars are powered by [[nuclear fusion]].<ref>{{cite encyclopedia |url=http://www.daviddarling.info/encyclopedia/L/latetype.html |title=late-type star |first=David |last=Darling |encyclopedia=The Internet Encyclopedia of Science |access-date=14 October 2007}}</ref> The terms "early" and "late" were carried over, beyond the demise of the model they were based on.

===Class O===
{{Main|O-type star}}
{{See also|O-type main-sequence star|Blue giant|Blue supergiant}}
[[File:O5V star spectrum.png|thumb|Spectrum of a hypothetical O5V star]]
O-type stars are very hot and extremely luminous, with most of their radiated output in the [[ultraviolet]] range. These are the rarest of all main-sequence stars. About 1 in 3,000,000 (0.00003%) of the main-sequence stars in the [[solar neighborhood]] are O-type stars.<ref group=lower-alpha name="proportions">These proportions are fractions of stars brighter than absolute magnitude&nbsp;16; lowering this limit will render earlier types even rarer, whereas generally adding only to the M class.  The proportions are calculated ignoring the value of 800 in the total column since the actual numbers add up to 824.</ref><ref name="LeDrew2001">{{cite journal |title=The Real Starry Sky |journal=Journal of the Royal Astronomical Society of Canada |last=Ledrew |first=Glenn |volume=95 |page=32 |date=February 2001 |bibcode=2001JRASC..95...32L}}</ref> Some of the [[List of most massive stars|most massive stars]] lie within this spectral class. O-type stars frequently have complicated surroundings that make measurement of their spectra difficult.

O-type spectra formerly were defined by the ratio of the strength of the [[Helium|He]]&nbsp;II λ4541 relative to that of He&nbsp;I λ4471, where λ is the radiation [[wavelength]]. Spectral type O7 was defined to be the point at which the two intensities are equal, with the He&nbsp;I line weakening towards earlier types. Type&nbsp;O3 was, by definition, the point at which said line disappears altogether, although it can be seen very faintly with modern technology. Due to this, the modern definition uses the ratio of the [[nitrogen]] line N&nbsp;IV λ4058 to N&nbsp;III λλ4634-40-42.<ref name="Walborn"/>

O-type stars have dominant lines of absorption and sometimes emission for [[helium|He]]&nbsp;II lines, prominent ionized ([[silicon|Si]]&nbsp;IV, [[oxygen|O]]&nbsp;III, [[nitrogen|N]]&nbsp;III, and [[carbon|C]]&nbsp;III) and neutral [[helium]] lines, strengthening from O5 to O9, and prominent hydrogen [[Balmer lines]], although not as strong as in later types. Higher-mass O-type stars do not retain extensive atmospheres due to the extreme velocity of their [[stellar wind]], which may reach 2,000&nbsp;km/s. Because they are so massive, O-type stars have very hot cores and burn through their hydrogen fuel very quickly, so they are the first stars to leave the [[main sequence]].

When the MKK classification scheme was first described in 1943, the only subtypes of class&nbsp;O used were O5 to O9.5.<ref>[http://nedwww.ipac.caltech.edu/level5/ASS_Atlas/frames.html ''An atlas of stellar spectra, with an outline of spectral classification''], W. W. Morgan, P. C. Keenan and E. Kellman, Chicago: The University of Chicago Press, 1943.</ref> The MKK scheme was extended to O9.7 in 1971<ref name=walborn>{{Cite journal |last1=Walborn |first1=N. R. |doi=10.1086/190239 |title=Some Spectroscopic Characteristics of the OB Stars: An Investigation of the Space Distribution of Certain OB Stars and the Reference Frame of the Classification |journal=The Astrophysical Journal Supplement Series |volume=23 |pages=257 |year=1971 |bibcode=1971ApJS...23..257W|doi-access=free }}</ref> and O4 in 1978,<ref>{{cite journal |bibcode=1978rmsa.book.....M |title=Revised MK Spectral Atlas for stars earlier than the sun |journal=Williams Bay: Yerkes Observatory |last1=Morgan |first1=W. W. |last2=Abt |first2=Helmut A. |last3=Tapscott |first3=J. W. |year=1978}}</ref> and new classification schemes that add types O2, O3, and O3.5 have subsequently been introduced.<ref>{{cite journal |doi=10.1086/339831 |bibcode=2002AJ....123.2754W |title=A New Spectral Classification System for the Earliest O Stars: Definition of Type O2 |journal=The Astronomical Journal |volume=123 |issue=5 |pages=2754–2771 |year=2002 |last1=Walborn |first1=Nolan R. |last2=Howarth |first2=Ian D. |last3=Lennon |first3=Daniel J. |last4=Massey |first4=Philip |last5=Oey |first5=M. S. |last6=Moffat |first6=Anthony F. J. |last7=Skalkowski |first7=Gwen |last8=Morrell |first8=Nidia I. |last9=Drissen |first9=Laurent|last10=Parker|first10=Joel Wm.|s2cid=122127697 |url=http://discovery.ucl.ac.uk/9632/1/9632.pdf }}</ref>

'''Example spectral standards:'''<ref name="Garrison">{{cite book |last=Garrison |first=R. F. |author-link=Robert F. Garrison |editor-last=Corbally |editor-first=C. J. |editor-last2=Gray |editor-first2=R. O. |editor-last3=Garrison |editor-first3=R. F. |chapter=A Hierarchy of Standards for the MK Process |chapter-url=http://aspbooks.org/publications/60/3.pdf |title=The MK Process at 50 Years: A Powerful Tool for Astrophysical Insight |url=http://aspbooks.org/a/volumes/table_of_contents/?book_id=172 |url-access=subscription |series=Astronomical Society of the Pacific conference series |volume=60 |publisher=Astronomical Society of the Pacific |publication-place=San Francisco |year=1994 |isbn=978-1-58381-396-6 |oclc=680222523 |pages=3–14}}</ref>
* O7V – [[S Monocerotis]]
* O9V – [[10 Lacertae]]

===Class B===
{{See also|B-type main-sequence star|Blue giant|Blue supergiant}}
[[File:B3V star spectrum.png|thumb|Spectrum of a hypothetical B3V star]]
B-type stars are very luminous and blue. Their spectra have neutral helium lines, which are most prominent at the B2 subclass, and moderate hydrogen lines. As [[OB star|O- and B-type stars]] are so energetic, they only live for a relatively short time. Thus, due to the low probability of kinematic interaction during their lifetime, they are unable to stray far from the area in which they formed, apart from [[runaway star]]s.

The transition from class&nbsp;O to class&nbsp;B was originally defined to be the point at which the [[Helium|He]]&nbsp;II λ4541 disappears. However, with modern equipment, the line is still apparent in the early B-type stars. Today for main-sequence stars, the B class is instead defined by the intensity of the He&nbsp;I violet spectrum, with the maximum intensity corresponding to class&nbsp;B2. For supergiants, lines of [[silicon]] are used instead; the Si&nbsp;IV λ4089 and Si&nbsp;III λ4552 lines are indicative of early B. At mid-B, the intensity of the latter relative to that of Si&nbsp;II λλ4128-30 is the defining characteristic, while for late B, it is the intensity of Mg&nbsp;II λ4481 relative to that of He&nbsp;I λ4471.<ref name="Walborn"/>

These stars tend to be found in their originating [[OB association]]s, which are associated with giant [[molecular cloud]]s. The Orion OB1 association occupies a large portion of a [[spiral arm]] of the [[Milky Way]] and contains many of the brighter stars of the [[Orion constellation|constellation Orion]]. About 1 in 800 (0.125%) of the main-sequence stars in the solar neighborhood are [[B-type main-sequence star]]s.<ref group=lower-alpha name="proportions"/><ref name="LeDrew2001"/> B-type stars are relatively uncommon and the closest is Regulus, at around 80 light years.<ref>{{Cite web |author1=Elizabeth Howell |date=2013-09-21 |title=Regulus: The Kingly Star |url=https://www.space.com/22890-regulus.html |access-date=2022-04-13 |website=Space.com |language=en}}</ref>

Massive yet non-[[supergiant]] stars known as [[Be&nbsp;star]]s have been observed to show one or more [[Balmer lines]] in emission, with the [[hydrogen]]-related [[electromagnetic radiation]] [[Hydrogen spectral series|series]] projected out by the stars being of particular interest. Be stars are generally thought to feature unusually strong [[stellar wind]]s, high surface temperatures, and significant attrition of [[stellar mass]] as the objects [[Stellar rotation|rotate]] at a curiously rapid rate.<ref name="Slettebak1988">{{cite journal |title=The Be Stars |journal=[[Publications of the Astronomical Society of the Pacific]] |first=Arne |last=Slettebak |volume=100 |pages=770–784 |date=July 1988 |doi=10.1086/132234 |bibcode=1988PASP..100..770S|doi-access=free }}</ref>

Objects known as [[B(e) star|B[e] stars]] – or B(e) stars for typographic reasons – possess distinctive neutral or low ionisation [[emission lines]] that are considered to have [[forbidden mechanism]]s, undergoing processes not normally allowed under current understandings of [[quantum mechanics]].

'''Example spectral standards:'''<ref name="Garrison"/>
* B0V – [[Upsilon Orionis]]
* B0Ia – [[Alnilam]]
* B2Ia – [[Chi2 Orionis|Chi<sup>2</sup> Orionis]]
* B2Ib – [[9 Cephei]]
* B3V – [[Alkaid]]
* B3V – [[Eta Aurigae|Haedus]]
* B3Ia – [[Omicron2 Canis Majoris|Omicron<sup>2</sup> Canis Majoris]]
* B5Ia – [[Eta Canis Majoris|Aludra]]
* B8Ia – [[Rigel]]

===Class A=== <!-- if this section is renamed, please add {{anchor|Class A}}-->
{{See also|A-type main-sequence star}}
[[File:A5V star spectrum.png|thumb|Spectrum of a hypothetical A5V star]]
A-type stars are among the more common naked eye stars, and are white or bluish-white. They have strong hydrogen lines, at a maximum by A0, and also lines of ionized metals ([[iron|Fe]]&nbsp;II, [[magnesium|Mg]]&nbsp;II, [[silicon|Si]]&nbsp;II) at a maximum at A5. The presence of [[calcium|Ca]]&nbsp;II lines is notably strengthening by this point. About 1 in 160 (0.625%) of the main-sequence stars in the solar neighborhood are A-type stars,<ref group=lower-alpha name="proportions"/><ref name="LeDrew2001"/> which includes 9 stars within 15 parsecs.<ref>{{Cite web |title=THE 100 NEAREST STAR SYSTEMS |url=http://www.astro.gsu.edu/RECONS/TOP100.posted.htm |access-date=2022-04-13 |website=www.astro.gsu.edu}}</ref>

'''Example spectral standards:'''<ref name="Garrison"/>
* A0Van – [[Phecda]]
* A0Va – [[Vega]]
* A0Ib – [[Eta Leonis]]
* A0Ia – [[HD 21389]]
* A1V – [[Sirius|Sirius A]]
* A2Ia – [[Deneb]]
* A3Va – [[Fomalhaut]]

===Class F===<!-- This section is linked from [[V838 Monocerotis]] -->
{{See also|F-type main-sequence star}}
[[File:F5V star spectrum.png|thumb|Spectrum of a hypothetical F5V star]]
F-type stars have strengthening spectral lines ''H'' and ''K'' of [[calcium|Ca]]&nbsp;II. Neutral metals ([[iron|Fe]]&nbsp;I, [[chromium|Cr]]&nbsp;I) beginning to gain on ionized metal lines by late F. Their spectra are characterized by the weaker hydrogen lines and ionized metals. Their color is white. About 1 in 33 (3.03%) of the main-sequence stars in the solar neighborhood are F-type stars,<ref group="lower-alpha" name="proportions" /><ref name="LeDrew2001" /> including 1 star [[Procyon A]] within 20 ly.<ref>{{cite web | url=http://solstation.com/stars/s20ly.htm | title=Stars within 20 light-years }}</ref>

'''Example spectral standards:'''<ref name="Garrison"/><ref name="MK73">{{cite journal | last1=Morgan | first1=W. W. | last2=Keenan | first2=P. C. | title=Spectral Classification | journal=Annual Review of Astronomy and Astrophysics | volume=11 | page=29 | date=1973 | doi=10.1146/annurev.aa.11.090173.000333 | bibcode=1973ARA&A..11...29M }}</ref><ref name="MK78">{{cite book|bibcode=1978rmsa.book.....M|year=1978|title=Revised MK Spectral Atlas for stars earlier than the sun|last1=Morgan|first1=W. W.|last2=Abt|first2=Helmut A.|last3=Tapscott|first3=J. W.|location=Yerkes Observatory, University of Chicago}}</ref><ref name="Gray89">{{cite journal|bibcode=1989ApJS...69..301G|title=The early F-type stars - Refined classification, confrontation with Stromgren photometry, and the effects of rotation|journal=Astrophysical Journal Supplement Series|volume=69|pages=301|last1=Gray|first1=R. O|last2=Garrison|first2=R. F|year=1989|doi=10.1086/191315}}</ref><ref name="Keenan89">{{cite journal
 | last1=Keenan | first1=Philip C. | last2=McNeil | first2=Raymond C.
 | title=The Perkins catalog of revised MK types for the cooler stars
 | journal=Astrophysical Journal Supplement Series
 | volume=71 | pages=245 | year=1989
 | bibcode=1989ApJS...71..245K | doi=10.1086/191373 | s2cid=123149047 }}</ref>
* F0IIIa – [[Zeta Leonis|Adhafera]]
* F0Ib – [[Alpha Leporis|Arneb]]
* F1V - [[37 Ursae Majoris]]
* F2V – [[78 Ursae Majoris]]
* F7V - [[Iota Piscium]]
* F9V - [[Beta Virginis|Zavijava]]
* F9V - [[HD 10647]]

===Class G===
{{Redirect|G star||G star (disambiguation)}}
{{See also|G-type main-sequence star|Yellow supergiant|Yellow hypergiant}}
[[File:G5V star spectrum.png|thumb|Spectrum of a hypothetical G5V star]]
G-type stars, including the [[Sun]],<ref name="SunSpectrum"/> have prominent spectral lines ''H'' and ''K'' of [[calcium|Ca]]&nbsp;II, which are most pronounced at G2.
They have even weaker hydrogen lines than F, but along with the ionized metals, they have neutral metals.
There is a prominent spike in the G band of [[cyanide|CN]] molecules.
Class&nbsp;G main-sequence stars make up about 7.5%, nearly one in thirteen, of the main-sequence stars in the solar neighborhood.
There are 21 G-type stars within 10pc.<ref group=lower-alpha name="proportions"/><ref name="LeDrew2001"/>

Class G contains the "Yellow Evolutionary Void".<ref>{{Cite journal |title=Checking the yellow evolutionary void. Three evolutionary critical Hypergiants: HD 33579, HR 8752 & IRC +10420 |journal=Astronomy and Astrophysics |volume=353 |pages=163 |bibcode=2000A&A...353..163N |last1=Nieuwenhuijzen |first1=H. |last2=De Jager |first2=C. |year=2000}}</ref> Supergiant stars often swing between O or B (blue) and K or M (red). While they do this, they do not stay for long in the unstable [[yellow supergiant]] class.

'''Example spectral standards:'''<ref name="Garrison"/>
* G0V – [[Beta Canum Venaticorum|Chara]]
* G0IV – [[Eta Boötis|Muphrid]]
* G0Ib – [[Beta Aquarii|Sadalsuud]]
* G2V – [[Sun]]
* G5V – [[Kappa1 Ceti]]
* G5IV – [[Mu Herculis]]
* G5Ib – [[9 Pegasi]]
* G8V – [[61 Ursae Majoris]]
* G8IV – [[Beta Aquilae|Alshain]]
* G8IIIa – [[Kappa Geminorum]]
* G8IIIab – [[Epsilon Virginis|Vindemiatrix]]
* G8Ib – [[Epsilon Geminorum|Mebsuta]]

===Class K===
{{See also|K-type main-sequence star}}
{{Redirect|K-type star|the Korean nuclear fusion project|KSTAR}}
[[File:K5V star spectrum.png|thumb|Spectrum of a hypothetical K5V star]]
K-type stars are orangish stars that are slightly cooler than the Sun. They make up about 12% of the main-sequence stars in the solar neighborhood.<ref group=lower-alpha name="proportions"/><ref name="LeDrew2001"/> There are also giant K-type stars, which range from [[hypergiant]]s like [[RW Cephei]], to [[Giant star|giants]] and [[supergiant]]s, such as [[Arcturus]], whereas [[K-type main-sequence star|orange dwarfs]], like [[Alpha Centauri]]&nbsp;B, are main-sequence stars.

They have extremely weak hydrogen lines, if those are present at all, and mostly neutral metals ([[manganese|Mn]]&nbsp;I, [[iron|Fe]]&nbsp;I, [[silicon|Si]]&nbsp;I). By late K, molecular bands of [[titanium monoxide|titanium oxide]] become present. Mainstream theories (those rooted in lower harmful radioactivity and star longevity) would thus suggest such stars have the optimal chances of heavily evolved life developing on orbiting planets (if such life is directly analogous to Earth's) due to a broad habitable zone yet much lower harmful periods of emission compared to those with the broadest such zones.<ref>{{cite web |url=http://spacefellowship.com/2009/08/10/on-a-cosmological-timescale-the-earths-period-of-habitability-is-nearly-over/ |title=On a cosmological timescale, The Earth's period of habitability is nearly over &#124; International Space Fellowship |publisher=Spacefellowship.com |access-date=22 May 2012}}</ref><ref>{{cite web |url=https://www.nasa.gov/feature/goddard/2019/k-star-advantage |title="Goldilocks" Stars May Be "Just Right" for Finding Habitable Worlds |publisher=NASA.com |date=7 March 2019 |access-date=26 August 2021 }}</ref>

'''Example spectral standards:'''<ref name="Garrison"/>
* K0V – [[Sigma Draconis|Alsafi]]
* K0III – [[Pollux (star)|Pollux]]
* K0III – [[Epsilon Cygni|Aljanah]]
* K2V – [[Epsilon Eridani|Ran]]
* K2III – [[Kappa Ophiuchi]]
* K3III – [[Rho Boötis]]
* K5V – [[61 Cygni|61 Cygni A]]
* K5III – [[Gamma Draconis|Eltanin]]

===Class M===
{{See also|Red dwarf|Red giant|Red supergiant}}
[[File:M5V star spectrum.png|thumb|Spectrum of a hypothetical M5V star]]
Class&nbsp;M stars are by far the most common. About 76% of the main-sequence stars in the solar neighborhood are class&nbsp;M stars.<ref group="lower-alpha" name="proportions" /><ref group="lower-alpha">This rises to 78.6% if we include all stars. (See the above note.)</ref><ref name="LeDrew2001" /> However, class&nbsp;M main-sequence stars ([[red dwarf]]s) have such low luminosities that none are bright enough to be seen with the unaided eye, unless under exceptional conditions. The brightest-known M&nbsp;class main-sequence star is [[Lacaille 8760]], class&nbsp;M0V, with [[stellar magnitude|magnitude]]&nbsp;6.7 (the limiting magnitude for typical naked-eye visibility under good conditions being typically quoted as 6.5), and it is extremely unlikely that any brighter examples will be found.

Although most class&nbsp;M stars are red dwarfs, most of the largest-known supergiant stars in the Milky Way are class M stars, such as [[VY Canis Majoris]], [[VV Cephei]], [[Antares]], and [[Betelgeuse]]. Furthermore, some larger, hotter [[brown dwarf]]s are late class&nbsp;M, usually in the range of M6.5 to M9.5.

The spectrum of a class&nbsp;M star contains lines from [[oxide]] [[molecule]]s (in the [[visible spectrum]], especially [[Titanium(II) oxide|TiO]]) and all neutral metals, but absorption lines of hydrogen are usually absent. TiO bands can be strong in class&nbsp;M stars, usually dominating their visible spectrum by about M5. [[Vanadium(II) oxide]] bands become present by late&nbsp;M.

'''Example spectral standards:'''<ref name="Garrison"/>
* M3V – [[Gliese 581]]
* M0IIIa – [[Mirach]]
* M2III – [[Chi Pegasi]]
* M1-M2Ia-Iab – [[Betelgeuse]]
* M2Ia – [[Mu Cephei]] ("[[William Herschel|Herschel]]'s garnet")

==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}}

==Stellar remnants==
{{Main|Neutron star|Black hole|Exotic star}}
Stellar remnants are objects associated with the death of stars. Included in the category are [[white dwarf]]s, and as can be seen from the radically different classification scheme for class&nbsp;D, stellar remnants are difficult to fit into the MK system.

The Hertzsprung–Russell diagram, which the MK system is based on, is observational in nature so these remnants cannot easily be plotted on the diagram, or cannot be placed at all. Old neutron stars are relatively small and cold, and would fall on the far right side of the diagram. [[Planetary nebula]]e are dynamic and tend to quickly fade in brightness as the progenitor star transitions to the white dwarf branch. If shown, a planetary nebula would be plotted to the right of the diagram's upper right quadrant. A black hole emits no visible light of its own, and therefore would not appear on the diagram.<ref name="chan-var-hr">{{cite web |url=http://chandra.harvard.edu/edu/formal/variable_stars/bg_info.html |title=Pulsating Variable Stars and the Hertzsprung-Russell (H-R) Diagram |publisher=Center for Astrophysics {{!}} Harvard & Smithsonian|date=9 March 2015 |access-date=23 July 2016}}</ref>

A classification system for neutron stars using Roman numerals has been proposed: type I for less massive neutron stars with low cooling rates, type II for more massive neutron stars with higher cooling rates, and a proposed type III for more massive neutron stars (possible exotic star candidates) with higher cooling rates.<ref>{{Cite journal |arxiv = astro-ph/0204233|doi = 10.1051/0004-6361:20020699|bibcode = 2002A&A...389L..24Y|title = The cooling neutron star in 3C 58|journal = Astronomy & Astrophysics|volume = 389|pages = L24–L27|year = 2002|last1 = Yakovlev|first1 = D. G.|last2 = Kaminker|first2 = A. D.|last3 = Haensel|first3 = P.|last4 = Gnedin|first4 = O. Y.|s2cid = 6247160}}</ref> The more massive a neutron star is, the higher [[neutrino]] flux it carries. These neutrinos carry away so much heat energy that after only a few years the temperature of an isolated neutron star falls from the order of billions to only around a million Kelvin. This proposed neutron star classification system is not to be confused with the earlier Secchi spectral classes and the Yerkes luminosity classes.

==Replaced spectral classes==
Several spectral types, all previously used for non-standard stars in the mid-20th century, have been replaced during revisions of the stellar classification system. They may still be found in old editions of star catalogs: R and N have been subsumed into the new C class as C-R and C-N.

==Stellar classification, habitability, and the search for life==
{{See also|Planetary habitability}}

While humans may eventually be able to [[space colonization|colonize]] any kind of stellar habitat, this section will address the probability of life arising around other stars.

Stability, luminosity, and lifespan are all factors in stellar habitability. Humans know of only one star that hosts life, the G-class Sun, a star with an abundance of heavy elements and low variability in brightness. The [[Solar System]] is also unlike many [[planetary system|stellar systems]] in that it only contains one star (see [[Habitability of binary star systems]]).

Working from these constraints and the problems of having an empirical sample set of only one, the range of stars that are predicted to be able to support life is limited by a few factors. Of the main-sequence star types, stars more massive than 1.5 times that of the Sun (spectral types O, B, and A) age too quickly for advanced life to develop (using Earth as a guideline). On the other extreme, dwarfs of less than half the mass of the Sun (spectral type M) are likely to tidally lock planets within their habitable zone, along with other problems (see [[Habitability of red dwarf systems]]).<ref name="auto">{{Cite web|url=http://www.solstation.com/habitable.htm|title=Stars and Habitable Planets|website=www.solstation.com}}</ref> While there are many problems facing life on red dwarfs, many astronomers continue to model these systems due to their sheer numbers and longevity.

For these reasons NASA's [[Kepler Mission]] is searching for habitable planets at nearby main-sequence stars that are less massive than spectral type A but more massive than type M—making the most probable stars to host life dwarf stars of types F, G, and K.<ref name="auto"/>

==See also==
* {{annotated link|Astrograph}}
* {{annotated link|Guest star (astronomy)|Guest star}}
* {{annotated link|Spectral signature}}
* {{annotated link|Star count}}, survey of stars
* {{annotated link|Stellar dynamics}}

== Notes ==
{{notelist}}

==References==
{{reflist}}

==Further reading==
* {{cite journal|arxiv=2101.06254 |doi=10.1002/asna.202113868 |title=Digital color codes of stars |date=2021 |last1=Harre |first1=Jan-Vincent |last2=Heller |first2=René |journal=Astronomische Nachrichten |volume=342 |issue=3 |pages=578–587 |bibcode=2021AN....342..578H |s2cid=231627588 }}

==External links==
{{Wiktionary|late-type star|early-type star}}
* [http://www.ucm.es/info/Astrof/invest/actividad/spectra.html Libraries of stellar spectra] by D. Montes, UCM
* [http://vizier.u-strasbg.fr/ftp/cats/more/HIP/cdroms/docs/vol11sp.pdf Spectral Types for Hipparcos Catalogue Entries]
* [http://www1.appstate.edu/dept/physics/MK/MKbook.html Stellar Spectral Classification] {{Webarchive|url=https://web.archive.org/web/20101031100445/http://www1.appstate.edu/dept/physics/MK/MKbook.html |date=31 October 2010 }} by Richard O. Gray and Christopher J. Corbally
* [http://specmodels.iag.usp.br Spectral models of stars] by P. Coelho
* {{cite web |url=http://www.sixtysymbols.com/videos/starclassification.htm |title=Star Classification |work=Sixty Symbols |publisher=[[Brady Haran]] for the [[University of Nottingham]] |last1=Merrifield |first1=Michael |last2=Bauer |first2=Amanda |last3=Häußler |first3=Boris |date=2010}}
* [http://www.isthe.com/chongo/tech/astro/HR-temp-mass-table-byhrclass.html Stellar classification table]

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