Editing Stellar classification (section)

==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 – [[Eta Ursae Majoris]]
* B3V – [[Eta Aurigae]]
* B3Ia – [[Omicron2 Canis Majoris|Omicron<sup>2</sup> Canis Majoris]]
* B5Ia – [[Eta Canis Majoris]]
* 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 – [[Gamma Ursae Majoris]]
* 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]]
* F0Ib – [[Alpha Leporis]]
* F1V - [[37 Ursae Majoris]]
* F2V – [[78 Ursae Majoris]]
* F7V - [[Iota Piscium]]
* F9V - [[Beta Virginis]]
* 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]]
* G0IV – [[Eta Boötis]]
* G0Ib – [[Beta Aquarii]]
* G2V – [[Sun]]
* G5V – [[Kappa1 Ceti]]
* G5IV – [[Mu Herculis]]
* G5Ib – [[9 Pegasi]]
* G8V – [[61 Ursae Majoris]]
* G8IV – [[Beta Aquilae]]
* G8IIIa – [[Kappa Geminorum]]
* G8IIIab – [[Epsilon Virginis]]
* G8Ib – [[Epsilon Geminorum]]

===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]]
* K0III – [[Pollux (star)|Pollux]]
* K0III – [[Epsilon Cygni]]
* K2V – [[Epsilon Eridani]]
* K2III – [[Kappa Ophiuchi]]
* K3III – [[Rho Boötis]]
* K5V – [[61 Cygni|61 Cygni A]]
* K5III – [[Gamma Draconis]]

===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 – [[Beta Andromedae]]
* M2III – [[Chi Pegasi]]
* M1-M2Ia-Iab – [[Betelgeuse]]
* M2Ia – [[Mu Cephei]] ("[[William Herschel|Herschel]]'s garnet")