Stellar classification 28927 226054137 2008-07-16T16:56:46Z Secret Squïrrel 1868769 /* Class T: methane dwarfs */ corrected temp range for T class In [[astronomy]], '''stellar classification''' is a classification of [[star]]s based initially on [[photosphere|photospheric temperature]] and its associated [[spectral]] characteristics, and subsequently refined in terms of other characteristics. Stellar temperatures can be classified by using [[Wien's displacement law]], but this poses difficulties for distant stars. [[Stellar spectroscopy]] offers a way to classify stars according to their [[absorption line]]s; particular absorption lines can be observed only for a certain range of temperatures because only in that range are the involved [[atomic energy level]]s populated. An early scheme (from the 19th century) ranked stars from ''A'' to ''Q'', which is the origin of the currently used spectral classes. == Secchi classes == During the 1860s and 1870s, pioneering stellar spectroscopist Father [[Angelo Secchi]] created the '''Secchi classes''' in order to classify observed spectra. By 1868, he had developed four classes of stars:<ref name=pp>p. 376, {{cite journal | title = Pioneering Women in the Spectral Classification of Stars | author = E. Dorrit Hoffleit | journal = Physics in Perspective | volume=4 | year=2002 | pages=370&ndash;398 | doi=10.1007/s000160200001 | bibcode= 2002PhP.....4..370H}}</ref><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&ndash;December 1866), pp. 364&ndash;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&ndash;December 1866), pp. 621&ndash;628.</ref> * '''Class I:''' white and blue stars with broad heavy [[hydrogen]] lines (modern class A) * '''Class II:''' yellow stars&mdash;hydrogen less strong, but evident metallic lines (modern classes G and K) * '''Class III:''' orange to red stars with complex band spectra (modern class M) * '''Class IV:''' red stars with significant [[carbon]] bands and lines ([[carbon star]]s) In 1878, he added a fifth class:<ref name=pp /> * '''Class V:''' [[emission line]]s (e.g., Be, Bf, etc.) In the late 1890s, this classification was superseded by the Harvard classification, which is discussed in the remainder of this article.<ref>[http://www.astro.ufl.edu/~gott/AST1002/Additional_Notes/Add_notes.week5 Classification of Stellar Spectra: Some History]</ref><ref>pp. 62&ndash;63, ''Stars and Their Spectra: An Introduction to the Spectral Sequence'', James B. Kaler, Cambridge: Cambridge University Press, 1997, ISBN 0521585708.</ref> == Harvard spectral classification == '''Harvard''' one-dimensional (temperature) classification scheme (based on hydrogen Balmer line strengths) was developed at [[Harvard College Observatory]] in about 1912 by [[Annie Jump Cannon]] and [[Edward C. Pickering]].<ref>Cannon, Annie Jump; Pickering, Edward Charles (1912), Annals of the Astronomical Observatory of Harvard College; vol. 56, no. 4, Cambridge, Mass.: The Observatory </ref> The common classes are normally listed from hottest to coldest (with mass, radius and luminosity compared to the Sun) and are given in the following table. {| class="wikitable" ! Class ! Temperature ! abbr="color" | Conventional color ! abbr="color" | Apparent color<ref name="möre">The Guinness book of astronomy facts & feats, Patrick Moore, 1992, 0-900424-76-1</ref><ref>{{cite web | date=[[2004-12-21]] | url=http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html | title=The Colour of Stars | publisher=Australia Telescope Outreach and Education | accessdate=2007-09-26 }} &mdash; Explains the reason for the difference in color perception.</ref> ! Mass<br />([[solar masses]]) ! Radius<br />([[solar radii]]) ! Luminosity ! Hydrogen lines ! % of all [[Main sequence star|Main Sequence Stars]]<ref name="LeDrew2001"/> |- style="background:#9bb0ff;" !style="background:#9bb0ff;"| [[#Class O|O]] | 30,000–60,000&nbsp;K |style="background:#9aafff;"| blue |style="background:#aabfff;"| blue | 64 [[solar mass|M<sub>&#9737;</sub>]] | 16 [[solar radius|R<sub>&#9737;</sub>]] | 1,400,000 [[solar luminosity|L<sub>&#9737;</sub>]] | Weak | ~0.00003% |- style="background:#aabfff;" !style="background:#abbfff;"| [[#Class B|B]] | 10,000–30,000&nbsp;K |style="background:#cad7ff;"| blue to blue white |style="background:#cad7ff;"| blue white | 18 [[solar mass|M<sub>&#9737;</sub>]] | 7 [[solar radius|R<sub>&#9737;</sub>]] | 20,000 [[solar luminosity|L<sub>&#9737;</sub>]] | Medium | 0.13% |- style="background:#d3ddff;" !style="background:#d3ddff;"| [[#Class A|A]] | 7,500–10,000&nbsp;K |style="background:#f8f7ff;"| white |style="background:#f8f7ff;"| white | 3.1 [[solar mass|M<sub>&#9737;</sub>]] | 2.1 [[solar radius|R<sub>&#9737;</sub>]] | 40 [[solar luminosity|L<sub>&#9737;</sub>]] | Strong | 0.6% |- style="background:#f8f7ff;" !style="background:#f8f7ff;"| [[#Class F|F]] | 6,000–7,500&nbsp;K |style="background:#fff4ea;"| yellowish white |style="background:#f8f7ff;"| white | 1.7 [[solar mass|M<sub>&#9737;</sub>]] | 1.4 [[solar radius|R<sub>&#9737;</sub>]] | 6 [[solar luminosity|L<sub>&#9737;</sub>]] | Medium | 3% |- style="background:#fff4ea;" !style="background:#fff4ea;"| [[#Class G|G]] | 5,000–6,000&nbsp;K |style="background:#fff2a1;"| yellow |style="background:#fff4ea;"| yellowish white | 1.1 [[solar mass|M<sub>&#9737;</sub>]] | 1.1 [[solar radius|R<sub>&#9737;</sub>]] | 1.2 [[solar luminosity|L<sub>&#9737;</sub>]] | Weak | 7.6% |- style="background:#ffd2a1;" !style="background:#ffd2a1;"| [[#Class K|K]] | 3,500–5,000&nbsp;K |style="background:#ffc46f;"| orange |style="background:#ffe46f;"| yellow orange | 0.8 [[solar mass|M<sub>&#9737;</sub>]] | 0.9 [[solar radius|R<sub>&#9737;</sub>]] | 0.4 [[solar luminosity|L<sub>&#9737;</sub>]] | Very weak | 12.1% |- style="background:#ffcc6f;" !style="background:#ffcc6f;"| [[#Class M|M]] | 2,000–3,500&nbsp;K |style="background:#ff6060;"| red |style="background:#ffa040;"| orange red | 0.4 [[solar mass|M<sub>&#9737;</sub>]] | 0.5 [[solar radius|R<sub>&#9737;</sub>]] | 0.04 [[solar luminosity|L<sub>&#9737;</sub>]] | Very weak | 76.45% |} [[Image:H-R diagram.svg|thumb|right|270px|[[Hertzsprung-Russell diagram]]]] The mass, radius, and luminosity listed for each class are appropriate only for stars on the [[main sequence]] portion of their lives and so are not appropriate for [[red giants]]. A popular [[mnemonic]] for remembering the order is "'''O'''h '''B'''e '''A''' '''F'''ine '''G'''irl/Guy, '''K'''iss '''M'''e" (there are many variants of this mnemonic). The [[Hertzsprung-Russell diagram]] relates stellar classification with [[absolute magnitude]], [[luminosity]], and surface [[temperature]]. The reason for the odd arrangement of letters is historical. When people first started taking [[power spectrum|spectra]] of stars, they noticed that stars had very different [[hydrogen]] [[spectral line]]s strengths, and so they classified stars based on the strength of the hydrogen [[Balmer series]] lines from A (strongest) to Q (weakest). Other lines of neutral and ionized species then came into play (H and K lines of [[calcium]], [[sodium]] D lines, etc). Later it was found that some of the classes were actually duplicates and those classes were removed. It was only much later that it was discovered that the strength of the hydrogen line was connected with the surface [[temperature]] of the star. The basic work was done by the "girls" of [[Harvard College Observatory]], primarily [[Annie Jump Cannon]], [[Henrietta Swan Leavitt]] and [[Antonia Maury]], based on the work of [[Williamina Fleming]]. In the 1920s, the Indian physicist [[Megh Nad 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 applied to the solar chromosphere, he then applied it to stellar spectra.<ref>Saha, M. N.; [http://adsabs.harvard.edu/abs/1921RSPSA..99..135S ''On a Physical Theory of Stellar Spectra''], Proceedings of the Royal Society of London, Series A, Volume 99, Issue 697 (May 1921), pp. 135–153</ref> The Harvard astronomer Cecilia Helena Payne (later to become [[Cecilia Payne-Gaposchkin]]) then demonstrated that the OBAFGKM spectral sequence is actually a sequence in temperature.<ref>Payne, C. H.; [http://adsabs.harvard.edu/abs/1925PhDT.........1P ''Stellar Atmospheres; A Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars''], Ph. D. Thesis, Radcliffe College, 1925</ref> Spectral classes are further subdivided by [[Arabic numerals]] (0&ndash;9). For example, A0 denotes the hottest stars in the A class and A9 denotes the coolest ones. Because the classification sequence predates our understanding that it is a temperature sequence, the precise values of these digits depend upon (largely subjective) estimates of the strengths of absorption features in stellar spectra. As a result, the subclasses are not evenly divided into any sort of mathematically representable intervals. The Sun is classified as G2. O, B, and A stars are sometimes misleadingly called "early type", while K and M stars are said to be "late type". This stems from a early 20th century model of stellar evaluation in which stars were powered by gravitational contraction via the [[Kelvin–Helmholtz mechanism]] in which stars start their lives as very hot "early type" stars, and then gradually cool down, thereby evolving into "late type" stars. This mechanism provided ages of the sun that were much smaller than what is observed, and was rendered obsolete by the discovery that stars are powered by [[nuclear fusion]]. However, brown dwarfs, whose energy comes from [[gravitational]] attraction alone, cool as they age and so progress to later spectral types. The highest mass brown dwarfs start their lives with M-type spectra and will cool through the L, T, and Y spectral classes. ===Conventional and apparent colors=== The ''Conventional color'' descriptions are traditional in astronomy, and represent colors ''relative to [[Vega]]'', a star that is perceived as white under naked eye observational conditions, but which magnified appears as blue. The ''Apparent color''<ref name="möre">Möremöre</ref> descriptions is what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars. The table colors used, are D65 standard colors, which are what you would see if the star light would be magnified to be filling non-dazzlingly bright areas. <ref name="Charity"> {{cite web | author=Charity, Mitchell | title=What color are the stars? | url=http://www.vendian.org/mncharity/dir3/starcolor/ | accessdate=2006-05-13 }} </ref> Most stars in the sky, except the brightest ones, appear white or bluish white to the unaided eye because they are too dim for color vision to work. Our Sun itself is white. It is sometimes called a yellow star (spectroscopically, relative to Vega), and may appear yellow or red (viewed through the atmosphere), or appear white (viewed when too bright for the eye to see any color). Astronomy images often use a variety of exaggerated colors (partially founded in faint light conditions observations, partially in conventions). But the Sun's own intrinsic color is white (aside from sunspots), with no trace of color, and closely approximates a [[black body]] of 5780&nbsp;[[Kelvin|K]] (see [[color temperature]]). This is a natural consequence of the evolution of our optical senses: the response curve that maximizes the overall efficiency against solar illumination will by definition perceive the Sun as white. The sun is known as a G type star. == Yerkes spectral classification == <span id="Luminosity class" /><span id="Luminosity classes" /> The '''Yerkes spectral classification''', also called the '''MKK''' system from the authors' initials, is a system of stellar spectral classification introduced in 1943 by [[William Wilson Morgan]], [[Phillip C. Keenan]] and [[Edith Kellman]] from [[Yerkes Observatory]].<ref>Morgan, William Wilson; Keenan, Philip Childs; Kellman, Edith (1943), "An atlas of stellar spectra, with an outline of spectral classification", Chicago, Ill., The University of Chicago press</ref> This classification is based on [[spectral line]]s sensitive to stellar surface gravity which is related to luminosity, as opposed to the Harvard classification which is based on surface temperature. Later, in 1953, after some revisions of list of standard stars and classification criteria, the scheme was named '''MK''' (by William Wilson Morgan and Phillip C. Keenan initials).<ref name="ref_MK">{{cite journal | last = Phillip C. Keenan | first = William Wilson Morgan | title = Spectral Classification | journal = [[Annual Reviews]] of Astronomy and Astrophysics | volume = 11 | pages = 29–50 | date = 1973 | publisher = [[Annual Reviews]] | url = http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?bibcode=1973ARA%26A..11...29M&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=449aa1cc7c02014 | id = | doi = 10.1146/annurev.aa.11.090173.000333 }} </ref> Since the radius of a [[giant star]] is much larger than a [[dwarf star]] while their masses are roughly comparable, the gravity and thus the gas density and pressure on the surface of a giant star are much lower than for a dwarf. These differences manifest themselves in the form of ''luminosity effects'' which affect both the width and the intensity of spectral lines which can then be measured. Denser stars with higher surface gravity will exhibit greater ''pressure broadening'' of spectral lines. <!-- description of the effects: --> A number of different '''luminosity classes''' are distinguished: {{star nav}} * '''I''' [[supergiant]]s ** '''Ia-0''' ([[hypergiant]]s or extremely luminous [[supergiant]]s (later addition), Example: [[Eta Carinae]] (spectrum-peculiar) ** '''Ia''' (luminous supergiants), Example: [[Deneb]] (spectrum is A2Ia) ** '''Iab''' (intermediate luminous supergiants) ** '''Ib''' (less luminous supergiants), Example: [[Betelgeuse]] (spectrum is M2Ib) * '''II''' [[bright giant]]s ** '''IIa''', Example: [[Beta Scuti|β Scuti]] (HD 173764) (spectrum is G4 IIa) ** '''IIab''' Example: HR 8752 (spectrum is G0Iab:) ** '''IIb''', Example: HR 6902 (spectrum is G9 IIb) * '''III''' normal [[giant star|giant]]s ** '''IIIa''', Example: [[Rho Persei|ρ Persei]] (spectrum is M4 IIIa) ** '''IIIab''' Example: δ Reticuli (spectrum is M2 IIIab) ** '''IIIb''', Example: Pollux (spectrum is K2 IIIb) * '''IV''' [[subgiant star|subgiant]]s ** '''IVa''', Example: [[Epsilon Reticuli|ε Reticuli]] (spectrum is K1-2 IVa-III) <!--** '''IVab'''--> ** '''IVb''', Example: HR 672 A (spectrum is G0.5 IVb) * '''V''' [[main sequence]] stars (dwarfs) ** '''Va''', Example: AD Leonis (spectrum M4Vae) <!--** '''Vab'''--> ** '''Vb''', Example: 85 Pegasi A (spectrum G5 Vb) * '''VI''' [[subdwarf star|subdwarf]]s (rarely used) * '''VII''' [[white dwarf]]s (rarely used) Marginal cases are allowed; for instance a star classified as Ia0-Ia would be a very luminous supergiant, verging on hypergiant. Examples are below. The spectral type of the star are not a factor. {| class="wikitable" ! Marginal Symbols ! Example ! Explanation |- !- |G2 '''I-II''' |The star is between super giant and bright giant. |- !+ |O9.5 '''Ia+''' |The star is a hypergiant star. |- !/ |M2 '''IV/V''' |The star is either a subgiant or a dwarf star. |- |} == Spectral types == The following illustration represents star classes with the colors very close to those actually perceived by the human eye. The relative sizes are for [[main sequence]] or "dwarf" stars. [[Image:Morgan-Keenan spectral classification.png|thumb|left|550px|The Morgan-Keenan spectral classification]] {{clear}} === Class O === Class '''O''' stars are very hot and very luminous, being bluish in color; in fact, most of their output is in the [[ultraviolet]] range. These are the rarest of all main sequence stars, constituting as few as 1 in 3,000,000 in the solar neighborhood (''Note:'' these proportions are fractions of stars brighter than absolute magnitude 16; lowering this limit will render earlier types even rarer while generally adding only to the M class).<ref name="LeDrew2001">[[Glenn LeDrew|LeDrew, G.]]; ''[http://adsabs.harvard.edu/abs/2001JRASC..95...32L The Real Starry Sky]'', Journal of the Royal Astronomical Society of Canada, Vol. 95, No. 1 (whole No. 686, February 2001), pp. 32–33 - Note Table 2 has an error and so this article will use 824 as the assumed correct total of main sequence stars</ref> O-stars shine with a power over a million times our Sun's output. These stars have dominant lines of absorption and sometimes emission for [[helium|He]] II lines, prominent ionized ([[silicon|Si]] IV, [[oxygen|O]] III, [[nitrogen|N]] III, and [[carbon|C]] III) and neutral [[helium]] lines, strengthening from O5 to O9, and prominent hydrogen [[Balmer lines]], although not as strong as in later types. Because they are so huge, class O stars burn through their hydrogen fuel very quickly, and are the first stars to leave the [[main sequence]]. Recent observations by the [[Spitzer Space Telescope]] indicate that planetary formation does not occur around other stars in the vicinity of an O class star due to the [[photoevaporation]] effect.<ref>[http://www.spitzer.caltech.edu/Media/happenings/20061003/ Planets Prefer Safe Neighborhoods]</ref> :'''Examples:''' [[Zeta Orionis]], [[Zeta Puppis]], [[Lambda Orionis]], [[Delta Orionis]] === Class B === [[Image:Pleiades Lanoue.png|thumb|right|The [[Pleiades (star cluster)|Pleiades]] [[open cluster|open star cluster]] with many bright B stars]] Class '''B''' stars are extremely luminous and blue. Their spectra have neutral helium, which are most prominent at the B2 subclass, and moderate hydrogen lines. Ionized metal lines include [[magnesium|Mg]] II and [[silicon|Si]] II. As [[OB star|O and B stars]] are so powerful, they only live for a very short time, and thus they do not stray far from the area in which they were formed. These stars tend to cluster together in what are called [[OB association]]s, which are associated with giant [[molecular cloud]]s. The Orion OB1 association occupies a large portion of a [[spiral arm]] of our [[Milky Way|galaxy]] and contains many of the brighter stars of the [[Orion constellation|constellation Orion]]. They constitute about 1 in 800 main sequence stars in the solar neighborhood<ref name="LeDrew2001"/> —rare, but much more common than those of class O. :'''Examples:''' [[Rigel]], [[Spica]], the brighter [[Pleiades (star cluster)|Pleiades]] === Class A === Class '''A''' stars are amongst 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]] II, [[magnesium|Mg]] II, [[silicon|Si]] II) at a maximum at A5. The presence of [[calcium|Ca]] II lines is notably strengthening by this point. They comprise about 1 in 160 of the main sequence stars in the solar neighborhood.<ref name="LeDrew2001"/> :'''Examples:''' [[Vega]], [[Sirius]], [[Deneb]] === Class F ===<!-- This section is linked from [[V838 Monocerotis]] --> Class '''F''' stars have strengthening ''H'' and ''K'' lines of [[calcium|Ca]] II. Neutral metals ([[iron|Fe]] I, [[chromium|Cr]] 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 with a slight tinge of yellow. These represent about 1 in 33 of the main sequence stars in the solar neighborhood.<ref name="LeDrew2001"/> :'''Examples:''' [[Mu Draconis|Arrakis]], [[Canopus]], [[Procyon]] === Class G === [[Image:Sun920607.jpg|thumb|right|The most important class G star to humanity: our [[Sun]]. The dark area visible northwest of the South pole is a large [[sunspot]].]] Class '''G''' stars are probably the best known, if only for the reason that our [[Sun]] is of this class. Most notable are the ''H'' and ''K'' lines of [[calcium|Ca]] II, which are most prominent 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 CH molecules. G is host to the "Yellow Evolutionary Void".<ref>[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2000A%26A...353..163N&db_key=AST&data_type=HTML&format= ''Checking the yellow evolutionary void. Three evolutionary critical Hypergiants: HD 33579, HR 8752 & IRC +10420'']</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 G classification as this is an extremely unstable place for a supergiant to be. G stars represent about 1 in 13 of the main sequence stars in the solar neighborhood.<ref name="LeDrew2001"/> :'''Examples:''' [[Sun]], [[Alpha Centauri A]], [[Capella (star)|Capella]], [[Tau Ceti]] === Class K === Class '''K''' are orangish stars which are slightly cooler than our Sun. Some K stars are [[giant (star)|giants]] and [[supergiant star|supergiants]], such as [[Arcturus]], while others, like [[Alpha Centauri]] B, are main sequence stars. They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals ([[manganese|Mn]] I, [[iron|Fe]] I, [[silicon|Si]] I). By late K, molecular bands of [[titanium oxide]] become present. These make up 1 in 8 of the main sequence stars in the solar neighborhood.<ref name="LeDrew2001"/> :'''Examples:''' [[Alpha Centauri B]], [[Epsilon Eridani]], [[Arcturus]], [[Aldebaran]] === Class M === [[Image:Betelgeuse star (Hubble).jpg|thumb|right|[[Betelgeuse]] is a [[red supergiant]], one of the [[List of largest known stars|largest stars]] known. Image from the [[Hubble Space Telescope]].]] Class '''M''' is by far the most common class. About 76% of the main sequence stars in the solar neighborhood are [[red dwarf]]s (78.6% if we include all stars: see the note under [[#Class O|Class O]]),<ref name="LeDrew2001"/> such as [[Proxima Centauri]]. M is also host to most giants and some supergiants such as [[Antares]] and [[Betelgeuse]], as well as [[Mira]] [[variable star|variables]]. The late-M group holds hotter [[brown dwarf]]s that are above the L spectrum. This is usually in the range of M6.5 to M9.5. The spectrum of an M star shows lines belonging to [[molecule]]s and all neutral metals but hydrogen lines are usually absent. [[Titanium oxide]] can be strong in M stars, usually dominating by about M5. [[Vanadium oxide]] bands become present by late M. :'''Example:''' [[Betelgeuse]] ([[supergiant]]) :'''Examples:''' [[Proxima Centauri]], [[Barnard's star]], [[Gliese 581]] (red dwarf) :'''Example:''' LEHPM 2-59 <ref>[http://xxx.lanl.gov/abs/astro-ph/0610096 Optical Spectroscopy of 2MASS Color-Selected Ultracool Subdwarfs], Adam J. Burgasser et al., 2006</ref> (subdwarf) :'''Examples:''' [[Teide 1]] (field brown dwarf), GSC 08047-00232 B <ref>[http://xxx.lanl.gov/abs/astro-ph/0412548 Astrometric and Spectroscopic Confirmation of a Brown Dwarf Companion to GSC 08047-00232], G. Chauvin et al., 2004</ref> (companion brown dwarf) == Extended spectral types == A number of new spectral types have been taken into use from newly discovered types of stars. === Hot blue emission star classes === Spectra of some very hot and bluish stars exhibit marked emission lines from carbon or nitrogen, or sometimes oxygen. ==== Class W: Wolf-Rayet ==== {{main|Wolf-Rayet stars}} [[Image:Wolf-rayet.jpg|thumb|right|Artist's impression of a Wolf-Rayet star]] Class '''W''' or '''WR''' represents the superluminous Wolf-Rayet stars, notably unusual since they have mostly helium in their atmospheres instead of hydrogen. They are thought to be dying supergiants with their hydrogen layer blown away by hot [[stellar wind]]s caused by their high temperatures, thereby directly exposing their hot helium shell. Class W is subdivided into subclasses '''WC''' ('''WCE''' early-type, '''WCL''' late-type), '''WN''' ('''WNE''' early-type, '''WNL''' late-type), and '''WO''' according to the dominance of carbon, nitrogen, or oxygen emission in their spectra (and outer layers). *W: Up to 70,000&nbsp;K :'''Example:''' [[Gamma Velorum|Gamma Velorum A]] (WC) :'''Example:''' [[WR124]] (WN) :'''Example:''' [[WR93B]] (WO) ==== Classes OC, ON, BC, BN: Wolf-Rayet related O and B stars ==== Intermediary between the genuine Wolf-Rayet's and ordinary hot stars of classes O and early B, there are OC, ON, BC and BN stars. They seem to constitute a short continuum from the Wolf-Rayet's into the ordinary OB:s. :'''Example:''' HD 152249 (OC) :'''Example:''' HD 105056 (ON) :'''Example:''' HD 2905 (BC) :'''Example:''' HD 163181 (BN) ==== The "class" OB ==== {{main|OB star}} In lists of spectra, the ''"spectrum OB"'' may occur. This is in fact not a spectrum, but a marker which means that ''"the spectrum of this star is unknown, but it belongs to an [[Stellar association|OB association]], so probably either a class '''O''' or class '''B''' star, or perhaps a fairly hot class '''A''' star."'' === Cool red and brown dwarf classes === The novel spectral types L and T were created to classify infrared spectra of cool stars and [[brown dwarfs]] which were very faint in the [[visual spectrum]]. The hypothetical spectral type Y has been reserved for objects cooler than T dwarfs having spectra which are qualitatively distinct from T dwarfs.<ref>[http://adsabs.harvard.edu/abs/2007arXiv0704.1522K Outstanding Issues in Our Understanding of L, T, and Y Dwarfs], J. D. Kirkpatrick, April 2007, arXiv:0704.1522. Accessed on line [[September 18]], [[2007]].</ref> ==== Class L ==== [[Image:L-dwarf-nasa-hurt.png|thumb|right|Artists vision of an L-dwarf]] Class '''L''' dwarfs get their designation because they are cooler than M stars and '''L''' is the remaining letter alphabetically closest to '''M'''. '''L''' does not mean lithium dwarf; a large fraction of these stars do not have [[lithium]] in their spectra. Some of these objects have mass large enough to support [[hydrogen fusion]], but some are of [[substellar object|substellar]] mass and do not, so collectively these objects should be referred to as ''L dwarfs'', not ''L stars''. They are a very dark red in color and brightest in [[infrared]]. Their [[atmosphere]] is cool enough to allow [[metal hydride]]s and [[alkali metal]]s to be prominent in their spectra.<ref name="kirk_ARAA">{{cite journal | last = Kirkpatrick ''et al'' | first = J. Davy | title = Dwarfs Cooler than M: the Definition of Spectral Type L Using Discovery from the 2-µ ALL-SKY Survey (2MASS) | journal = [[Astrophysical Journal]] | volume = 519 | issue = 2 | pages = 802–833 | date = [[July 10]], [[1999]] | publisher = [[The University of Chicago Press]] | url = http://www.journals.uchicago.edu/ApJ/front.html | id = ISSN: 0004-637X | doi = 10.1086/307414 | format = {{dead link|date=June 2008}} &ndash; <sup>[http://scholar.google.co.uk/scholar?hl=en&lr=&q=author%3AKirkpatrick+%27%27et+al%27%27+intitle%3ADwarfs+Cooler+than+M%3A+the+Definition+of+Spectral+Type+L+Using+Discovery+from+the+2-%C2%B5+ALL-SKY+Survey+%282MASS%29&as_publication=%5B%5BAstrophysical+Journal%5D%5D&as_ylo=&as_yhi=&btnG=Search Scholar search]</sup> }} </ref><ref name="kirk_ApJ">{{cite journal | last = Kirkpatrick | first = J. Davy | title = New Spectral Types L and T | journal = [[Annual Reviews]] of Astronomy and Astrophysics | volume = 43 | issue = 1 | pages = 195–246 | date = 2005 | publisher = [[Annual Reviews]] | doi = 10.1146/annurev.astro.42.053102.134017 | id = ISSN: 0066-4146 }} </ref> Due to low gravities in giant stars, TiO- and VO-bearing condensates never form. Thus, larger L-type stars can never form in an isolated environment. It may be possible for these L-type supergiants to form through stellar collisions, however, an example of which is [[V838 Monocerotis]]. * L: 1,300–2,000&nbsp;K, dwarfs (some stellar, some substellar) with [[metal hydride]]s and [[alkali metal]]s prominent in their spectra. :'''Example:''' VW Hyi :'''Example:''' 2MASSW J0746425+2000321 binary<ref>[http://hubblesite.org/newscenter/archive/releases/2004/51/image/a Ultra-cool Diminutive Star Weighs In]</ref> ::Component '''A''' is an L dwarf star ::Component '''B''' is an L brown dwarf :'''Example:''' [[V838 Monocerotis]] (supergiants) <span id="Class T"/> ==== Class T: methane dwarfs ==== [[Image:T-dwarf-nasa-hurt.png|thumb|right|Artists vision of a T-dwarf]] Class '''T''' dwarfs are cool [[brown dwarfs]] with surface temperatures of between approximately 700 and 1,300 K. Their emission peaks in the [[infrared]]. [[Methane]] is prominent in their spectra.<ref name="kirk_ARAA"/><ref name="kirk_ApJ"/> *T: ~700-1,300&nbsp;K, cooler [[brown dwarf]]s with [[methane]] in the spectrum :'''Examples:''' [[SIMP J013656.5+093347|SIMP 0136]] (the brightest T dwarf discovered in northern hemisphere)<ref>[http://xxx.lanl.gov/abs/astro-ph/0609419 Discovery of the brightest T dwarf in the northern hemisphere, 2007]</ref> :'''Examples:''' [[Epsilon Indi]] Ba & Epsilon Indi Bb Class T and L could be more common than all the other classes combined if recent research is accurate. From studying the number of [[protoplanetary disk|proplyd]]s (protoplanetary discs, clumps of gas in [[nebula]]e from which stars and solar systems are formed) then the number of stars in the [[galaxy]] should be several [[order of magnitude|orders of magnitude]] higher than what we know about. It is theorized that these proplyds are in a race with each other. The first one to form will become a [[proto-star]], which are very violent objects and will disrupt other proplyds in the vicinity, stripping them of their gas. The victim proplyds will then probably go on to become main sequence stars or brown dwarf stars of the L and T classes, but quite invisible to us. Since they live so long, these smaller stars will accumulate over time. ==== Class Y ==== Class '''Y''' dwarfs are expected to be much cooler than T-dwarfs. They have been modelled<ref>[http://xxx.lanl.gov/abs/astro-ph/0607305 Y-Spectral class for Ultra-Cool Dwarfs, N.R.Deacon and N.C.Hambly, 2006]</ref>, though there is no well-defined spectral sequence yet with prototypes. In March [[2008]], a 620 kelvin brown dwarf named CFBDS J005910.90-011401.3 was discovered, displaying wide ammonia absorption in the near-infrared. It is believed to be the first prototype of a Y0 dwarf. <ref>[http://arxiv.org/abs/0802.4387 CFBDS J005910.90-011401.3: reaching the T-Y Brown Dwarf transition?, Philippe Delorme et al. 2008]</ref> * Y: < 700&nbsp;K, ultra-cool [[brown dwarf]]s (theoretical) === Carbon related late giant star classes === Carbon related stars are stars whose spectra indicate production of carbon by helium [[Triple-alpha process|triple-alpha]] fusion. With increased carbon abundance, and some parallel [[s-process]] heavy element production, the spectra of these stars are becoming increasingly deviant from the usual late spectral classes G, K and M. The giants among those stars are presumed to produce this carbon themselves, but not too few of this class of stars are believed to be double stars whose odd atmosphere once was transferred from a former carbon star companion that is now a white dwarf. ==== Class C: carbon stars ==== {{main|Carbon star}} Originally classified as '''R''' and '''N''' stars, these are also known as 'carbon stars'. These are red giants, near the end of their lives, in which there is an excess of carbon in the atmosphere. The old R and N classes ran parallel to the normal classification system from roughly mid G to late M. These have more recently been remapped into a unified carbon classifier '''C''', with N0 starting at roughly C6. Another subset of cool carbon stars are the '''J'''-type stars, which are characterized by the strong presence of molecules of <sup>13</sup>CN in addition to those of <sup>12</sup>CN.<ref>Bouigue, R. 1954, Annales d'Astrophysique, Vol. 17, p.104</ref> A few dwarf (that is, main sequence) carbon stars are known, but the overwhelming majority of known carbon stars are giants or supergiants. * C: Carbon stars, e.g. ''R CMi'' ** C-R: Formerly a class on its own representing the carbon star equivalent of late G to early K stars. Example: S Camelopardalis ** C-N: Formerly a class on its own representing the carbon star equivalent of late K to M stars. Example: [[R Leporis]] ** C-J: A subtype of cool C stars with a high content of <sup>13</sup>C. Example: [[La Superba|Y Canum Venaticorum]] ** C-H: Population II analogues of the C-R stars. Examples: V Ari, TT CVn<ref>[http://adc.astro.umd.edu/adc-cgi/cat.pl?/journal_tables/ApJS/105/419/ Spectral Atlas of Carbon Stars (Barnbaum+ 1996)]</ref> ** C-Hd: Hydrogen-Deficient Carbon Stars, similar to late G supergiants with CH and C<sub>2</sub> bands added. Example: HD 137613 ==== Class S ==== Class '''S''' stars have [[zirconium oxide]] lines in addition to (or, rarely, instead of) those of [[titanium oxide]], and are in between the Class M stars and the carbon stars.<ref>Keenan, P. C. 1954 Astrophysical Journal, vol. 120, p.484</ref> S stars have excess amounts of [[zirconium]] and other elements produced by the [[s-process]], and have their carbon and oxygen abundances closer to equal than is the case for M stars. The latter condition results in both [[carbon]] and [[oxygen]] being locked up almost entirely in [[carbon monoxide]] molecules. For stars cool enough for carbon monoxide to form that molecule tends to "eat up" all of whichever element is less abundant, resulting in "leftover oxygen" (which becomes available to form titanium oxide) in stars of normal composition, "leftover carbon" (which becomes available to form the [[diatomic carbon]] molecules) in carbon stars, and "leftover nothing" in the S stars. The relation between these stars and the ordinary M stars indicates a continuum of carbon abundance. Like carbon stars, nearly all known S stars are giants or supergiants. :'''Examples:''' ''S Ursae Majoris'', ''HR 1105'' ==== Classes MS and SC: intermediary carbon related classes ==== In between the M class and the S class, border cases are named MS stars. In a similar way border cases between the S class and the C-N class are named SC or CS. The sequence M → MS → S → SC → C-N is believed to be a sequence of increased carbon abundance with age for [[carbon stars]] in the [[asymptotic giant branch]]. :'''Examples:''' R Serpentis, ST Monocerotis (MS) :'''Examples:''' CY Cygni, BH Crucis (SC) === White dwarf classifications === {{main|White dwarf spectroscopy}} [[Image:Sirius A and B Hubble photo.jpg|thumb|right|[[Sirius]] A and B (a [[white dwarf]] of type DA2) resolved by [[Hubble Space Telescope|HST]]]] The class '''D''' is the modern classification used for white dwarfs, low-mass stars that are no longer undergoing [[nuclear fusion]] and have shrunk to planetary size, slowly cooling down. Class D is further divided into spectral types DA, DB, DC, DO, DQ, DX, and DZ. The letters are not related to the letters used in the classification of other stars, but instead indicate the composition of the white dwarf's visible outer layer or atmosphere. :'''Examples:''' [[Sirius B]] (DA2), [[Procyon B]] (DA4), [[Van Maanen's star]] (DZ7)<ref>[http://adsabs.harvard.edu/abs/2002ApJ...571..512H A Determination of the Local Density of White Dwarf Stars], J. B. Holberg, Terry D. Oswalt and E. M. Sion, ''The Astrophysical Journal'' '''571''', #1 (May 2002), pp. 512&ndash;518.</ref><sup>, Table 1</sup> The white dwarf types are as follows:<ref name="sionspectra">[http://adsabs.harvard.edu/abs/1983ApJ...269..253S A proposed new white dwarf spectral classification system], E. M. Sion, J. L. Greenstein, J. D. Landstreet, J. Liebert, H. L. Shipman, and G. A. Wegner, ''The Astrophysical Journal'' '''269''', #1 ([[June 1]], [[1983]]), pp. 253&ndash;257.</ref> * '''DA''': a [[hydrogen]]-rich atmosphere or outer layer, indicated by strong Balmer hydrogen [[spectral lines]]. * '''DB''': a [[helium]]-rich atmosphere, indicated by neutral helium, [[spectroscopic notation|He I]], spectral lines. * '''DO''': a helium-rich atmosphere, indicated by ionized helium, [[spectroscopic notation|He II]], spectral lines. * '''DQ''': a [[carbon]]-rich atmosphere, indicated by atomic or molecular carbon lines. * '''DZ''': a [[metal (astronomy)|metal]]-rich atmosphere, indicated by metal spectral lines. * '''DC''': no strong spectral lines indicating one of the above categories. * '''DX''': spectral lines are insufficiently clear to classify into one of the above categories. The type is followed by a number giving the white dwarf's surface temperature. This number is a rounded form of 50400/''T''<sub>eff</sub>, where ''T''<sub>eff</sub> is the [[effective temperature|effective surface temperature]], measured in [[kelvins]]. Originally, this number was rounded to one of the digits 1 through 9, but more recently fractional values have started to be used, as well as values below 1 and above 9.<ref name="sionspectra" /><ref name="villanovar4">[http://adsabs.harvard.edu/abs/1999ApJS..121....1M A Catalog of Spectroscopically Identified White Dwarfs], George P. McCook and Edward M. Sion, ''The Astrophysical Journal Supplement Series'' '''121''', #1 (March 1999), pp. 1&ndash;130.</ref> Two or more of the type letters may be used to indicate a white dwarf which displays more than one of the spectral features above. Also, the letter ''V'' is used to indicate a [[pulsating white dwarf|variable white dwarf]].<ref name="sionspectra" /> '''Extended white dwarf spectral types:'''<ref name="sionspectra" /> * '''DAB''': a hydrogen- and helium-rich white dwarf displaying neutral helium lines. * '''DAO''': a hydrogen- and helium-rich white dwarf displaying ionized helium lines. * '''DAZ''': a hydrogen-rich metallic white dwarf. * '''DBZ''': a helium-rich metallic white dwarf. '''Variable star designations:''' * '''[[DAV star|DAV]]''' or '''ZZ Ceti''': a hydrogen-rich [[pulsating white dwarf]].<ref name="physrev">[http://adsabs.harvard.edu/abs/1990RPPh...53..837K Physics of white dwarf stars], D. Koester and G. Chanmugam, ''Reports on Progress in Physics'' '''53''' (1990), pp. 837&ndash;915.</ref><sup>, pp. 891, 895</sup> * '''[[DBV star|DBV]]''' or '''V777 Her''': a helium-rich pulsating white dwarf.<ref name="wden">White dwarfs, Gilles Fontaine and Fran&ccedil;ois Wesemael, in ''Encyclopedia of Astronomy and Astrophysics'', ed. Paul Murdin, Bristol and Philadelphia: Institute of Physics Publishing and London, New York and Tokyo: Nature Publishing Group, 2001. ISBN 0333750888.</ref><sup>, p. 3525</sup> * '''[[GW Vir star|GW Vir]]''', '''DOV''' or '''PNNV''': a hot helium-rich pulsating white dwarf (or pre-white dwarf.)<ref name="quirion">[http://adsabs.harvard.edu/abs/2007ApJS..171..219Q Mapping the Instability Domains of GW Vir Stars in the Effective Temperature-Surface Gravity Diagram], Quirion, P.-O., Fontaine, G., Brassard, P., ''Astrophysical Journal Supplement Series'' '''171''' (2007), pp. 219&ndash;248.</ref><sup>, §1.1, 1.2;</sup><ref>&sect;1, [http://adsabs.harvard.edu/abs/2004A%26A...426L..45N Detection of non-radial g-mode pulsations in the newly discovered PG 1159 star HE 1429-1209], T. Nagel and K. Werner, ''Astronomy and Astrophysics'' '''426''' (2004), pp. L45&ndash;L48.</ref><ref name="obrien">[http://adsabs.harvard.edu/abs/2000ApJ...532.1078O The Extent and Cause of the Pre-White Dwarf Instability Strip], M. S. O'Brien, ''Astrophysical Journal'' '''532''', #2 (April 2000), pp. 1078&ndash;1088.</ref> === Non-stellar spectral types: Class P & Q === Finally, the classes '''P''' and '''Q''' are occasionally used for certain non-stellar objects. Type P objects are [[planetary nebula]]e and type Q objects are [[nova]]e. == Spectral peculiarities == Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum.<ref>[http://skytonight.com/howto/basics/3305876.html?showAll=y SkyTonight: The Spectral Types of Stars]</ref> {| class="wikitable" ! Code ! Spectral peculiarities for stars |- ! : | Blending and/or uncertain spectral value |- ! … | Undescribed spectral peculiarities exist |- ! ! | Special peculiarity |- ! comp | Composite spectrum |- ! e | Emission lines present |- ! [e] | "Forbidden" emission lines present |- ! er | "Reversed" center of emission lines weaker than edges |- ! ep | Emission lines with peculiarity |- ! eq | Emission lines with [[P Cygni]] profile |- ! ev | Spectral emission that exhibits variability |- ! f | NIII and HeII emission |- ! f+ | Si IV emission additional to HeII and NIII emission |- ! f* | NIV emission stronger than NIII emission |- ! (f) | Weak emission lines of He |- ! ((f)) | No emission of He |- ! He wk | Weak He lines |- ! k | Spectra with interstellar absorption features |- ! m | Enhanced metal features |- ! n | Broad ("nebulous") absorption due to spinning |- ! nn | Very broad absorption features due to spinning very fast |- ! neb | A nebula's spectrum mixed in |- ! p | Unspecified peculiarity, [[peculiar star]]. |- ! pq | Peculiar spectrum, similar to the spectra of novae |- ! q | Red & blue shifts line present |- ! s | Narrowly "sharp" absorption lines |- ! ss | Very narrow lines |- ! sh | Shell star |- ! v | Variable spectral feature (also "var") |- ! w | Weak lines (also "wl" & "wk") |- ! d Del | Type A and F giants with weak calcium H and K lines, as in prototype [[Delta Delphini]] |- ! d Sct | Type A and F stars with spectra similar to that of short-period variable [[Delta Scuti]] |- ! Code ! If spectrum shows enhanced metal features |- ! Ba | Abnormally strong [[Barium]] |- ! Ca | Abnormally strong [[Calcium]] |- ! Cr | Abnormally strong [[Chromium]] |- ! Eu | Abnormally strong [[Europium]] |- ! He | Abnormally strong [[Helium]] |- ! Hg | Abnormally strong [[Mercury (element)|Mercury]] |- ! Mn | Abnormally strong [[Manganese]] |- ! Si | Abnormally strong [[Silicon]] |- ! Sr | Abnormally strong [[Strontium]] |- ! Code ! Spectral peculiarities for white dwarfs |- ! : | Uncertain assigned classification |- ! P | Magnetic white dwarf with detectable polarization |- ! E | Emission lines present |- ! H | Magnetic white dwarf without detectable polarization |- ! V | Variable |- ! PEC | Spectral peculiarities exist |- |} For example, [[Epsilon Ursae Majoris]] is listed as spectral type A0pCr, indicating general classification A0 with a strong emission lines of the element [[chromium]]. There are several common classes of chemically [[peculiar star]]s, where the spectral lines of a number of elements appear abnormally strong. == Photometric classification == Stars can also be classified using photometric data from any [[photometric system]]. For example, we can calibrate [[color index]] diagrams of U&minus;B and B&minus;V in the [[UBV system]] according to [[Spectral class|spectral]] and [[Luminosity class|luminosity]] classes. Nevertheless, this calibration is not straightforward, because many effects are superimposed in such diagrams: [[interstellar reddening]], color changes due to [[metallicity]], and the blending of light from [[Binary star|binary]] and [[Multiple star|multiple]] stars. Photometric systems with more colors and narrower passbands allow a star's class, and hence physical parameters, to be determined more precisely. The most accurate determination comes of course from spectral measurements, but there is not always enough time to get qualitative spectra with high [[signal-to-noise ratio]]. == See also == * [[Stellar evolution]] * [[Stellar association]]s * [[Metallicity]] * [[Astrograph]] * [[H-R diagram]] == References == <div style="column-count:2;-moz-column-count:2; font-size: 90%"> <references/> </div> == External links == * [http://www.ucm.es/info/Astrof/invest/actividad/spectra.html Libraries of stellar spectra, D. Montes, UCM] * [http://www.seattleastro.org/webfoot/feb00/pg2.htm Webfooted Astronomer] * [http://xxx.lanl.gov/abs/astro-ph/0408237 The rate of period change in pulsating DB white dwarf stars, A. H. Corsico, L. G. Althaus], has the DAV, DBV, & DOV explanation. * [http://www.ssl.berkeley.edu/~euve/sci/Resources_pubs_abstracts_j0720_burle.html The Close DAO+dM Binary RE J0720-318: A Stratified White Dwarf with a Thin H Layer and a Possible Circumbinary Disk] DAO type White Dwarf. * [http://www.aas.org/publications/baas/v31n3/aas194/688.htm The Spatial Distribution and Kinematics of Cool Metallic Line White Dwarfs], has DAZ and DBZ spectrums * [http://adsabs.harvard.edu/abs/1997ApJ...486..420F A K-Band Spectral Atlas of Wolf-Rayet Stars], has WC, WN, and WO spectrums * [http://adsabs.harvard.edu/abs/1995A&A...295...75K Properties of the WO Wolf-Rayet stars], has WO spectrum ranging from WO1 to WO5 * [http://vizier.u-strasbg.fr/ftp/cats/more/HIP/cdroms/docs/vol11sp.pdf Spectral Types for Hipparcos Catalogue Entries] * [http://personal.tcu.edu/~mfanelli/imastro/The%20Hertsprung-Russell%20(HR)%20Diagram.htm], has the luminous subclasses. * [http://xxx.lanl.gov/abs/astro-ph/0511462 Discovery of a Very Young Field L Dwarf, 2MASS J01415823-4633574], J. Davy Kirkpatrick et al. 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