Editing Stellar classification (section)

===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.