Editing Stellar classification (section)

==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]]<ref name=calib/><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]]<ref name="Charity"/><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><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]])<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><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><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><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-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><ref name=bdevol/><br />([[solar mass]]es)
! Main-sequence radius<ref name=calib/><ref name=bdevol/><br />([[solar radii]])
! Main-sequence luminosity<ref name=calib/><ref name=bdevol/><br />([[Bolometric magnitude#Bolometric magnitude|bolometric]])
! Hydrogen<br />lines
! Percentage of all<br />[[main sequence|main-sequence star]]s<ref group=lower-alpha name="proportions"/><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%
|}

A common [[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!", or another one is "<u>O</u>ur <u>B</u>right <u>A</u>stronomers <u>F</u>requently <u>G</u>enerate <u>K</u>iller <u>M</u>nemonics!".<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>

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.

===Yerkes spectral 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:
* A slash ('''/''') means that a star is either one class or the other.
* A dash ('''-''') 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.