In astronomy, stellar classification is a classification of stars based on its spectral characteristics. The spectral class of a star, is a designation of a class to a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure of the temperature in this chromosphere. Light from the star is analysed by splitting it up by a diffraction grating, subdividing the incoming photons into a spectrum exhibiting a rainbow of colors interspersed by absorption lines, each line indicating a certain ion of a certain chemical element. The presence of a certain chemical element in such an absorption spectrum primarily indicates that the temperature conditions is suitable for a certain excitation of this element. If the star temperature have been determined by a majority of absorption lines, unusual absences or strengths of lines for a certain element may indicate an unusual chemical composition of the chromosphere.

Most stars are currently classified using the letters O, B, A, F, G, K and M, where O stars are the hottest and the letter sequence indicates successively cooler stars up to the coolest M class. According to an informal tradition, O stars are "blue", B "blue-white", A stars "white", F stars "yellow-white", G stars "yellow", K stars "orange", and M stars "red", even though the actual star colors perceived by an observer may deviate from these colors depending on visual conditions and individual stars observed. This non-alphabetical scheme has been developed from an earlier scheme using all letters from A to O, but the star classes were reordered to the current one when the connection to the star's temperature became clarified, and a few star classes were omitted as duplicate of others. (The mnemonic "Oh, be a fine girl /guy, kiss me" is sometimes used.)

In the current star classification system, the Morgan-Keenan system, the spectrum letter is enhanced by a number from 0 to 9 indicating tenths of the range between two star classes, so that A5 is five tenths between A0 and F0, but A2 is two tenths of the full range from A0 to F0. Another dimension that is included in the Morgan-Keenan system is the luminosity class expressed by the Roman numbers I, II, III, IV and V, expressing the width of certain absorption lines in the star's spectrum. It has been shown that this feature is a general measure of the size of the star, and thus of the total luminosity output from the star. Class I are generally called supergiants, class III simply giants and class V either dwarfs or more properly main sequence stars. For example our Sun has the spectral type G2V, which might be interpreted as "a 'yellow' two tenths towards 'orange' main sequence star". The apparently brightest star Sirius has type A1V.

Secchi classes

During the 1860s and 1870s, pioneering stellar spectroscopist Father Angelo Secchi created the Secchi classes in order to classify observed spectra. By 1866, he had developed three classes of stellar spectra:

• Class I: white and blue stars with broad heavy hydrogen lines, such as Vega and Altair. This includes the modern class A and early class F.

• :Class I, Orion subtype: a subtype of class I with narrow lines in place of wide bands, such as Rigel and γ Orionis. In modern terms, this corresponds to early B-type stars.

• Class 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 F.

• Class III: orange to red stars with complex band spectra, such as Betelgeuse and Antares. This corresponds to the modern class M.

In 1868, he discovered carbon stars, which he put into a distinct group:

• Class IV: red stars with significant carbon bands and lines (carbon stars.)

In 1877, he added a fifth class:

• Class V: emission-line stars, such as γ Cassiopeiae and β Lyrae.

In the late 1890s, this classification began to be superseded by the Harvard classification, which is discussed in the remainder of this article.

Harvard spectral classification

The Harvard classification system is a one-dimensional classification scheme. Stars vary in surface temperature from about 2 to 40 kK (2,000 to 40,000 Kelvin). Physically, the classes indicate the temperature of the star's atmosphere and are normally listed from hottest to coldest, as is done in the following table:

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 "Oh Be A Fine Girl/Guy, Kiss Me" (there are many variants of this mnemonic). The spectral classes O through M are subdivided by Arabic numerals (0–9). For example, A0 denotes the hottest stars in the A class and A9 denotes the coolest ones. The Sun is classified as G2.

The reason for the odd arrangement of letters is historical. An early classification of spectra by Angelo Secchi in the 1860s divided stars into those with prominent lines from the hydrogen Balmer series (group I, with a subtype representing many of the stars in Orion); those with spectra which, like the Sun, showed calcium and sodium lines (group II); colored stars whose spectra showed wide bands (group III); and carbon stars (group IV). In the 1880s, the astronomer Edward C. Pickering began to make a survey of stellar spectra at the Harvard College Observatory, using the objective-prism method. A first result of this work was the Draper Catalogue of Stellar Spectra, published in 1890. Williamina Fleming classified most of the spectra in this catalogue. It used a scheme in which the previously used Secchi classes (I to IV) were divided into more specific classes, given letters from A to N. Also, the letters O, P and Q were used, O for stars whose spectra consisted mainly of bright lines, P for planetary nebulae, and Q for stars not fitting into any other class.The Draper Catalogue of stellar spectra photographed with the 8-inch Bache telescope as a part of the Henry Draper memorial, Edward C. Pickering, Annals of Harvard College Observatory 27 (1890), . See in particular pp. 1–4.pp. 106–108, Hearnshaw 1986.
In 1897, another worker at Harvard, Antonia Maury, placed the Orion subtype of Secchi class I ahead of the remainder of Secchi class I, thus placing the modern type B ahead of the modern type A. She was the first to do so, although she did not use lettered spectral types, but rather a series of 22 types numbered from I to XXII. In 1901, Annie Jump Cannon returned to the lettered types, but dropped all letters except O, B, A, F, G, K, and M, 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 forth. Finally, by 1912, Cannon had changed the types B, A, B5A, F2G, etc. to B0, A0, B5, F2, etc. This is essentially the modern form of the Harvard classification system.
The fact that the Harvard classification of a star indicated its surface temperature was not fully understood until after its development. 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. 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. 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.

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 evolution 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 the mean color of an A class star which is considered to be white. The Apparent color 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 is what you would see if the star light would be intensely magnified and projected onto a white paper, then observed in ordinary daylight.
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 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

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. This two-dimensional (temperature and luminosity) classification scheme is based on spectral lines sensitive to stellar temperature and surface gravity which is related to luminosity (whilst the Harvard classification is based on surface temperature only!). 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).
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.

A number of different luminosity classes are distinguished:

• 0 hypergiants

• I supergiants

• *Ia-0 (hypergiants or extremely luminous supergiants (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 giants

• *IIa, Example: 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 giants

• *IIIa, Example: ρ Persei (spectrum is M4 IIIa)

• *IIIab Example: δ Reticuli (spectrum is M2 IIIab)

• *IIIb, Example: Pollux (spectrum is K2 IIIb)

• IV subgiants

• *IVa, Example: ε Reticuli (spectrum is K1-2 IVa-III)

• *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)

• *"Vz", Example: LH10 : 3102 (spectrum O7 Vz), located in the Large Magellanic Cloud., Walborn, N. R., 2008

• VI subdwarfs (rarely used). Subdwarf are general represented with a prescript of sd or esd (extreme subdwarf) in front of the spectra.

• * sd, Example: SSSPM J1930-4311 (spectrum sdM7)

• * esd, Example: APMPM J0559-2903 (spectrum esdM7)

• VII white dwarfs (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 is not a factor.

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.

The Morgan-Keenan spectral classification

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. About 1 in 3,000,000 of the main sequence stars in the solar neighborhood are Class O stars.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.LeDrew, G.; , 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 Some of the most massive stars lie within this spectral class. Type-O stars are so hot as to have complicated surroundings which make measurement of their spectra difficult.

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 He II lines, prominent ionized (Si IV, O III, N III, and 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 massive, class O stars have very hot cores, thus burn through their hydrogen fuel very quickly, and so 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.

When the MKK classification scheme was first described in 1943, the only subtypes of class O used were O5 to O9.5. The MKK scheme was extended to O4 in 1978, and new classification schemes have subsequently been introduced which add types O2, O3 and O3.5.

Examples: Zeta Orionis, Zeta Puppis, Lambda Orionis, Delta Orionis, Theta¹ Orionis C, HD 93129A

Class B

The Pleiades 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 Mg II and Si II. As 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 associations, which are associated with giant molecular clouds. The Orion OB1 association occupies a large portion of a spiral arm of our galaxy and contains many of the brighter stars of the constellation Orion. About 1 in 800 of the main sequence stars in the solar neighborhood are Class B stars..

Examples: Rigel, Spica, the brighter 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 (Fe II, Mg II, Si II) at a maximum at A5. The presence of Ca II lines is notably strengthening by this point. About 1 in 160 of the main sequence stars in the solar neighborhood are Class A stars.

Examples: Vega, Sirius, Deneb, Altair

Class F

Class F stars have strengthening H and K lines of Ca II. Neutral metals (Fe I, 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. About 1 in 33 of the main sequence stars in the solar neighborhood are Class F stars.

Examples: Arrakis, Canopus, Procyon

Class G

The most important class G star to humanity: our Sun. The dark area visible in the lower left is a large sunspot.

Class G stars are probably the best known, if only for the reason that our Sun is of this class. About 1 in 13 of the main sequence stars in the solar neighborhood are Class G stars.
Most notable are the H and K lines of 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". 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.

Examples: Sun, Alpha Centauri A, Capella, Tau Ceti

Class K

Class K are orangish stars which are slightly cooler than our Sun. Some K stars are giants and supergiants, such as Arcturus, while orange dwarfs, like Alpha Centauri B, are main sequence stars. They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals (Mn I, Fe I, Si I). By late K, molecular bands of titanium oxide become present. About 1 in 8 of the main sequence stars in the solar neighborhood are Class K stars. There is a suggestion that K Spectrum stars are very well suited for life.

Examples: Alpha Centauri B, Epsilon Eridani, Arcturus, Aldebaran

Class M

Betelgeuse is a red supergiant, one of the 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 Class M stars. This rises to 78.6% if we include all stars. (See the above note.)
Although most Class M stars are red dwarfs, the class also hosts most giants and some supergiants such as Antares and Betelgeuse, as well as Mira variables. The late-M group holds hotter brown dwarfs 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 molecules 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.

Examples: Betelgeuse, Antares (supergiants)

Examples: Proxima Centauri, Barnard's star, Gliese 581 (red dwarf)

Examples: LEHPM 2-59 , SSSPM J1930-4311 (subdwarf)

Example: APMPM J0559-2903 (extreme subdwarf)

Examples: Teide 1 (field brown dwarf), GSC 08047-00232 B (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

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 winds caused by their high temperatures, thereby directly exposing their hot helium shell. Class W is subdivided into subclasses
WN (WNE early-type, WNL late-type) and WC (WCE early-type, WCL late-type, and extend class WO), according to the dominance of nitrogen and carbon emission lines in their spectra (and outer layers)., Crowther, Paul A., 2007

• WR spectra range is listed below:

WN

:WNE (WN2 to WN5 with some WN6)

:WNL (WN7 to WN9 with some WN6)

:Extended WN class (WN10 to WN11), was created to encompass the Ofpe/WN9 stars.

WN/C, and intermediate class between the nitrogen-rich and carbon-rich WR stars.

WC

:WCE (WC4 to WC6)

:WCL (WC7 to WC9)

:WO (WO1 to WO4)

• W: Up to 70,000 K

Example: WR124 (WN)

Example: Gamma Velorum A (WC)

Example: WR93B (WO)

Classes OC, ON, BC, BN: Wolf-Rayet related O and B stars

Intermediary between the genuine Wolf-Rayets 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-Rayets into the ordinary OBs.

Example: HD 152249 (OC)

Example: HD 105056 (ON)

Example: HD 2905 (BC)

Example: HD 163181 (BN)

The "Slash" stars

The slash stars are stars with O-type spectra and WN sequence in their spectra. The name slash comes from their spectra having a slash.

Example spectra: Of/WNL

There is a secondary group found with this spectra. A cooler, "intermediate" group. They are found in the Large Magellanic Cloud and have a designation of Ofpe/WN9.

The Magnetic O stars

They are O stars with strong magnetic fields. Designation is Of?p

The "class" OB

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 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. This included both red dwarfs and brown dwarfs which are very faint in the visual spectrum. The hypothetical spectral type Y has been reserved for objects cooler than T dwarfs which have spectra that are qualitatively distinct from T dwarfs.

Class L

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 masses large enough to support hydrogen fusion, but some are of 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 hydrides and alkali metals to be prominent in their spectra. 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 K, dwarfs (some stellar, some substellar) with metal hydrides and alkali metals prominent in their spectra.

Example: VW Hyi

Example: 2MASSW J0746425+2000321 binary

:Component A is an L dwarf star

:Component B is an L brown dwarf

Example: LSR 1610-0040 (subdwarf)

Example: V838 Monocerotis (supergiants)

Class T: methane dwarfs

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.

• T: ~700-1,300 K, cooler brown dwarfs with methane in the spectrum

Examples: SIMP 0136 (the brightest T dwarf discovered in northern hemisphere)

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 proplyds (protoplanetary discs, clumps of gas in nebulae from which stars and solar systems are formed) then the number of stars in the galaxy should be several 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

The spectral class Y has been proposed for brown dwarfs that are cooler than T dwarfs and have qualitatively different spectra from them. Although such dwarfs have been modelled, there is no well-defined spectral sequence yet with prototypes, and no certain example of class Y has yet been seen.

• Y: < 600 K, ultra-cool brown dwarfs (theoretical)

As of early 2009, the coolest known brown dwarfs have estimated effective temperatures between 500 and 600 K, and have been assigned the spectral class T9. Three examples are the brown dwarfs CFBDS J005910.90-011401.3, ULAS J133553.45+113005.2, and ULAS J003402.77−005206.7.The Physical Properties of Four ~600 K T Dwarfs, S. K. Leggett et al., The Astrophysical Journal 695, #2 (April 2009), pp. 1517–1526, , . The spectra of these objects display absorption around 1.55 micrometers. Delorme et al. has suggested that this feature is due to absorption from ammonia and that this should be taken as indicating the T-Y transition, making these objects of type Y0.CFBDS J005910.90-011401.3: reaching the T-Y brown dwarf transition?, P. Delorme et al., Astronomy and Astrophysics 482, #3 (May 2008), pp. 961–971, , . However, the feature is difficult to distinguish from absorption by water and methane, and other authors have stated that the assignment of class Y0 is premature.

Carbon related late giant star classes

Carbon related stars are stars whose spectra indicate production of carbon by helium 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

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 ¹³CN in addition to those of ¹²CN. 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 ¹³C. Example: Y Canum Venaticorum

• *C-H: Population II analogues of the C-R stars. Examples: V Ari, TT CVn

• *C-Hd: Hydrogen-Deficient Carbon Stars, similar to late G supergiants with CH and C₂ 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. 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

Sirius A and B (a white dwarf of type DA2) resolved by 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)^{, Table 1}

The white dwarf types are as follows:, 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. 253–257.

• DA: a hydrogen-rich atmosphere or outer layer, indicated by strong Balmer hydrogen spectral lines.

• DB: a helium-rich atmosphere, indicated by neutral helium, He I, spectral lines.

• DO: a helium-rich atmosphere, indicated by ionized helium, He II, spectral lines.

• DQ: a carbon-rich atmosphere, indicated by atomic or molecular carbon lines.

• DZ: a metal-rich atmosphere, indicated by metal spectral lines (a merger of the obsolete white dwarf spectral types, DG, DK and DM).

• 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_eff, where T_eff is the 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., George P. McCook and Edward M. Sion, The Astrophysical Journal Supplement Series 121, #1 (March 1999), pp. 1–130.
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 variable white dwarf.
Extended white dwarf spectral types:

• 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 or ZZ Ceti: a hydrogen-rich pulsating white dwarf., D. Koester and G. Chanmugam, Reports on Progress in Physics 53 (1990), pp. 837–915.^{, pp. 891, 895}

• DBV or V777 Her: a helium-rich pulsating white dwarf.White dwarfs, Gilles Fontaine and François 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.^{, p. 3525}

• GW Vir, sometimes divided into DOV and PNNV: a hot helium-rich pulsating white dwarf (or pre-white dwarf.), Quirion, P.-O., Fontaine, G., Brassard, P., Astrophysical Journal Supplement Series 171 (2007), pp. 219–248.^{, §1.1, 1.2;}, M. S. O'Brien, Astrophysical Journal 532, #2 (April 2000), pp. 1078–1088. These stars are generally PG 1159 stars, although some authors also include non-PG 1159 stars in this class.

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 nebulae and type Q objects are novae.

Spectral peculiarities

Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum.

For example, Epsilon Ursae Majoris is listed as spectral type A0pCr, indicating general classification A0 with strong emission lines of the element chromium. There are several common classes of chemically peculiar stars, 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−B and B−V in the UBV system according to spectral and 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 and 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

• Astrograph

• Hertzsprung–Russell diagram

• Metallicity

• Stellar evolution

• Stellar association