Editing Stellar classification (section)

==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In the late 1890s, this classification began to be superseded by the Harvard classification, which is discussed in the remainder of this article.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 |url=https://www.worldcat.org/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 |url=https://www.worldcat.org/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 |url=https://www.worldcat.org/oclc/1013948519 |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