Editing Stellar classification (section)

===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>