Stellar classification

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Hertzsprung–Russell diagram
Spectral type
Brown dwarfs
White dwarfs
Red dwarfs
Main sequence
Bright giants

In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Light from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with absorption lines. Each line indicates an ion of a certain chemical element, with the line strength indicating the abundance of that ion. The relative abundance of the different ions varies with the temperature of the photosphere. The spectral class of a star is a short code summarizing the ionization state, giving an objective measure of the photosphere's temperature and density.

Most stars are currently classified under the Morgan–Keenan (MK) system using the letters O, B, A, F, G, K, and M, a sequence from the hottest (O type) to the coolest (M type). Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. A8, A9, F0, F1 form a sequence from hotter to cooler). The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class D for white dwarfs and class C for carbon stars.

In the MK system a luminosity class is added to the spectral class using Roman numerals. This is based on the width of certain absorption lines in the star's spectrum which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. Luminosity class 0 or Ia+ stars for hypergiants, class I stars for supergiants, class II for bright giants, class III for regular giants, class IV for sub-giants, class V for main-sequence stars, class sd for sub-dwarfs, and class D for white dwarfs. The full spectral class for the Sun is then G2V, indicating a main-sequence star with a temperature around 5,800K.

Modern classification[edit]

The modern classification system is known as the Morgan–Keenan (MK) classification. Each star is assigned a spectral class from the older Harvard spectral classification and a luminosity class using Roman numerals as explained below, forming the star's spectral type.

Harvard spectral classification[edit]

The Harvard classification system is a one-dimensional classification scheme using single letters of the alphabet, optionally with numeric subdivisions, to group stars according to their spectral characteristics. Main-sequence stars vary in surface temperature from approximately 2,000 to 50,000 K, whereas more-evolved stars can have temperatures above 100,000 K. Physically, the classes indicate the temperature of the star's atmosphere and are normally listed from hottest to coldest.

Class Effective temperature[1][2][3] Conventional color description[4][nb 1] Actual apparent color[5][6][7] Main-sequence mass[1][8]
(solar masses)
Main-sequence radius[1][8]
(solar radii)
Main-sequence luminosity[1][8]
Fraction of all
main-sequence stars[9]
O ≥ 30,000 K blue blue ≥ 16 M ≥ 6.6 R ≥ 30,000 L Weak ~0.00003%
B 10,000–30,000 K blue white deep blue white 2.1–16 M 1.8–6.6 R 25–30,000 L Medium 0.13%
A 7,500–10,000 K white blue white 1.4–2.1 M 1.4–1.8 R 5–25 L Strong 0.6%
F 6,000–7,500 K yellow white white 1.04–1.4 M 1.15–1.4 R 1.5–5 L Medium 3%
G 5,200–6,000 K yellow yellowish white 0.8–1.04 M 0.96–1.15 R 0.6–1.5 L Weak 7.6%
K 3,700–5,200 K orange pale yellow orange 0.45–0.8 M 0.7–0.96 R 0.08–0.6 L Very weak 12.1%
M 2,400–3,700 K red light orange red 0.08–0.45 M ≤ 0.7 R ≤ 0.08 L Very weak 76.45%

The spectral classes O through M, as well as other more specialized classes discussed later, are subdivided by Arabic numerals (0–9), where 0 denotes the hottest stars of a given class. For example, A0 denotes the hottest stars in the A class and A9 denotes the coolest ones. Fractional numbers are allowed; for example, the star Mu Normae is classified as O9.7.[10] The Sun is classified as G2.[11]

Just-saturated RGB-camera discs

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[5] descriptions are what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars. However, 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. Red supergiants are cooler and redder than dwarfs of the same spectral type, and stars with particular spectral features such as carbon stars may be far redder than any black body.

O-, B-, and A-type stars are sometimes called "early type", whereas K and M stars are said to be "late type″. This stems from an early 20th-century model of stellar evolution in which stars were powered by gravitational contraction via the Kelvin–Helmholtz mechanism whereby stars start their lives as very hot "early-type" stars, and then gradually cool down, 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.

The Hertzsprung–Russell diagram relates stellar classification with absolute magnitude, luminosity, and surface temperature.

The fact that the Harvard classification of a star indicated its surface or photospheric temperature (or more precisely, its effective temperature) was not fully understood until after its development, though by the time the first Hertzsprung–Russell diagram was formulated (by 1914), this was generally suspected to be true.[12] In the 1920s, the Indian physicist Meghnad Saha derived a theory of ionization by extending well-known ideas in physical chemistry pertaining to the dissociation of molecules to the ionization of atoms. First he applied it to the solar chromosphere, then to stellar spectra.[13] 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.[14] 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.

Yerkes spectral classification[edit]

The Morgan–Keenan 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, Philip C. Keenan, and Edith Kellman from Yerkes Observatory.[15] 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 the Morgan–Keenan classification, or MK (by William Wilson Morgan and Philip C. Keenan's initials),[16] and this system remains the system in modern use today.

Denser stars with higher surface gravity exhibit greater pressure broadening of spectral lines. The gravity, and hence the pressure, on the surface of a giant star is much lower than for a dwarf star because the radius of the giant is much greater than a dwarf of similar mass. Therefore, differences in the spectrum can be interpreted as luminosity effects and a luminosity class can be assigned purely from examination of the spectrum.

A number of different luminosity classes are distinguished[17]

Marginal cases are allowed; for example, a star may be either a supergiant or a bright giant, or may be in between the subgiant and main-sequence classifications. In these cases, two special symbols are used: a slash (/) means that a star is either one class or the other, and a dash (-) means that the star is in between the two classes. For example, a star classified as A3-4III/IV would be in between spectral types A3 and A4, while being either a giant star or a subgiant. Sub-dwarf classes have also been used: VI for sub-dwarfs, stars slightly less luminous than the main sequence; VII and sometimes higher numerals for white dwarf or "hot sub-dwarf" classes.

Spectral peculiarities[edit]

Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum.[27]

Code Spectral peculiarities for stars
 : uncertain spectral value[17]
... Undescribed spectral peculiarities exist
 ! Special peculiarity
comp Composite spectrum[28]
e Emission lines present[28]
[e] "Forbidden" emission lines present
er "Reversed" center of emission lines weaker than edges
eq Emission lines with P Cygni profile
f N III and He II emission[17]
f* NIV λ4058Å is stronger than the NIII λ4634Å, λ4640Å, & λ4642Å lines[29]
f+ SiIV λ4089Å & λ4116Å are emission in addition to the NIII line[29]
(f) N III emission, absence or weak absorption of He II
(f+) [30]
((f)) Displays strong HeII absorption accompanied by weak NIII emissions[31]
((f*)) [30]
h WR stars with emission lines due to hydrogen.[32]
ha WR stars with hydrogen emissions seen on both absorption and emission.[32]
He wk Weak He lines
k Spectra with interstellar absorption features
m Enhanced metal features[28]
n Broad ("nebulous") absorption due to spinning[28]
nn Very broad absorption features[17]
neb A nebula's spectrum mixed in[28]
p Unspecified peculiarity, peculiar star.[nb 3][28]
pq Peculiar spectrum, similar to the spectra of novae
q Red & blue shifts line present
s Narrowly "sharp" absorption lines[28]
ss Very narrow lines
sh Shell star features[28]
var Variable spectral feature[28] (sometimes abbreviated to "v")
wl Weak lines[28] (also "w" & "wk")
Abnormally strong spectral lines of the specified element(s)[28]

For example, 59 Cygni is listed as spectral type B1.5Vnne,[33] indicating a spectrum with the general classification B1.5V, as well as very broad absorption lines and certain emission lines.


The reason for the odd arrangement of letters in the Harvard classification is historical, having evolved from the earlier Secchi classes and been progressively modified as understanding improved.

Secchi classes[edit]

Secchi spectral types (152 Schjellerup is Y Canum Venaticorum)

During the 1860s and 1870s, pioneering stellar spectroscopist Angelo Secchi created the Secchi classes in order to classify observed spectra. By 1866, he had developed three classes of stellar spectra:[34][35][36]

  • 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 Bellatrix. 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:[37]

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

In 1877, he added a fifth class:[38]

In the late 1890s, this classification began to be superseded by the Harvard classification, which is discussed in the remainder of this article.[39][40][41] The roman numerals used for Secchi classes should not be confused with the completely unrelated roman numerals used for Yerkes luminosity classes.

Draper system[edit]

Classifications in the Draper Catalogue of Stellar Spectra[42][43]
Secchi Draper Comment
I A, B, C, D Hydrogen lines dominant.
II E, F, G, H, I, K, L
IV N Did not appear in the catalogue.
  O Wolf–Rayet spectra with bright lines.
  P Planetary nebulae.
  Q Other spectra.

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.[42][43]

Harvard system[edit]

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 twenty-two types numbered from I to XXII.[44][45]

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.[46][47] Finally, by 1912, Cannon had changed the types B, A, B5A, F2G, etc. to B0, A0, B5, F2, etc.[48][49] This is essentially the modern form of the Harvard classification system. A common mnemonic for remembering the spectral type letters is "Oh, Be A Fine Guy/Girl, Kiss Me".

Spectral types[edit]

Class O[edit]

Main article: O-type star
Artist's rendering of Zeta Puppis, an O4 supergiant

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.[nb 4][9] Some of the most massive stars lie within this spectral class. O-type stars frequently have complicated surroundings which make measurement of their spectra difficult.

O-type spectra used to be defined by the ratio of the strength of the He II λ4541 relative to that of He I λ4471, where λ is the wavelength, measured in ångströms. Spectral type O7 was defined to be the point at which the two intensities are equal, with the He I line weakening towards earlier types. Type 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 IV λ4058 to N III λλ4634-40-42.[50]

O-type 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, 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 O used were O5 to O9.5.[51] The MKK scheme was extended to O9.7 in 1971[52] and O4 in 1978,[53] and new classification schemes have subsequently been introduced which add types O2, O3 and O3.5.[54]

Spectral standards:[55]

Class B[edit]

Artist's impression of Aludra, a B5 supergiant

B-type stars are very luminous and blue. Their spectra have neutral helium, which are most prominent at the B2 subclass, and moderate hydrogen lines. As 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 do not, and are unable to, stray far from the area in which they were formed, apart from runaway stars.

The transition from class O to class B was originally defined to be the point at which the He II λ4541 disappears. However, with today's better 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 I violet spectrum, with the maximum intensity corresponding to class B2. For supergiants, lines of silicon are used instead; the Si IV λ4089 and Si III λ4552 lines are indicative of early B. At mid B, the intensity of the latter relative to that of Si II λλ4128-30 is the defining characteristic, while for late B, it is the intensity of Mg II λ4481 relative to that of He I λ4471[50]

These stars tend to be found in their originating OB associations, which are associated with giant molecular clouds. 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 constellation Orion. About 1 in 800 (0.125%) of the main-sequence stars in the solar neighborhood are B-type stars.[nb 4][9]

Spectral standards:[55]

Class A[edit]

Fomalhaut, an A3 main-sequence 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 (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 (0.625%) of the main-sequence stars in the solar neighborhood are A-type stars.[nb 4][9][56]

Spectral standards:[55]

Class F[edit]

Canopus, an F-type supergiant and the second brightest star in the night sky

F-type 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 (3.03%) of the main-sequence stars in the solar neighborhood are F-type stars.[nb 4][9]

Spectral standards:[55]

Class G[edit]

"G star" redirects here. For other uses, see G star (disambiguation).
The Sun, a prototypical G2 main-sequence star

G-type stars, including the Sun[11] have prominent H and K lines of Ca 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 CH molecules. Class G main-sequence stars make up about 7.5%, nearly one in thirteen, of the main-sequence stars in the solar neighborhood.[nb 4][9]

G is host to the "Yellow Evolutionary Void".[57] 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 yellow supergiant G classification as this is an extremely unstable place for a supergiant to be.

Spectral standards:[55]

Class K[edit]

"K-type star" redirects here. For the Korean nuclear fusion project, see KSTAR.
Arcturus, a K1.5 giant

K-type stars are orangish stars that are slightly cooler than the Sun. They make up about 12%, nearly one in eight, of the main-sequence stars in the solar neighborhood.[nb 4][9] There are also giant K-type stars, which range from hypergiants like RW Cephei, to giants and supergiants, such as Arcturus, whereas 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. There is a suggestion that K Spectrum stars may potentially increase the chances of life developing on orbiting planets that are within the habitable zone.[58]

Spectral standards:[55]

Class M[edit]

UY Scuti, an M4 supergiant

Class M stars are by far the most common. About 76% of the main-sequence stars in the Solar neighborhood are class M stars.[nb 4][nb 5][9] However, because main-sequence stars of spectral class M have such low luminosities, none are bright enough to be visible to see with the unaided eye unless under exceptional conditions. The brightest known M-class main-sequence star is M0V Lacaille 8760 at magnitude 6.6 (the limiting magnitude for typical naked-eye visibility under good conditions is typically quoted as 6.5) and it is extremely unlikely that any brighter examples will be found.

Although most class M stars are red dwarfs, the class also hosts most giants and some supergiants such as VY Canis Majoris, Antares and Betelgeuse. Furthermore, 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 a class M star shows lines belonging to oxide molecules, TiO in particular, in the visible and all neutral metals, but absorption lines of hydrogen are usually absent. TiO bands can be strong in class M stars, usually dominating their visible spectrum by about M5. Vanadium monoxide bands become present by late M.

Spectral standards:[55]

Extended spectral types[edit]

A number of new spectral types have been taken into use from newly discovered types of stars.[59]

Hot blue emission star classes[edit]

UGC 5797, an emission-line galaxy where massive bright blue stars are formed[60]

Spectra of some very hot and bluish stars exhibit marked emission lines from carbon or nitrogen, or sometimes oxygen.

Class W: Wolf–Rayet[edit]

Main article: Wolf–Rayet star
Hubble Space Telescope image of the nebula M1-67 and the Wolf–Rayet star WR 124 in the center

Class W or WR represents the Wolf–Rayet stars, notable for spectra lacking hydrogen lines. Instead their spectra are dominated by broad emission lines of highly ionized helium, nitrogen, carbon and sometimes oxygen. They are thought to mostly be dying supergiants with their hydrogen layers blown away by stellar winds, thereby directly exposing their hot helium shells. Class W is further divided into sub classes according to the relative strength of nitrogen and carbon emission lines in their spectra (and outer layers).[32]

WR spectra range is listed below:[61][62]

  • WN,[32] spectrum dominated by NitrogenIII-V and HeliumI-II lines
    • WNE (WN2 to WN5 with some WN6), hotter or "early"
    • WNL (WN7 to WN9 with some WN6), cooler or "late"
    • Extended WN classes WN10 and WN11 sometimes used for the Ofpe/WN9 stars[32]
    • h tag used (e.g. WN9h) for WR with hydrogen emission and ha (e.g. WN6ha) for both hydrogen emission and absorption
  • WN/C, WN stars plus strong CarbonIV lines, intermediate between WN and WC stars[32]
  • WC,[32] spectrum with strong CarbonII-IV lines
    • WCE (WC4 to WC6), hotter or "early"
    • WCL (WC7 to WC9), cooler or "late"
  • WO (WO1 to WO4), strong OxygenVI lines, extremely rare

Although the central stars of most planetary nebulae (CSPNe) show O-type spectra,[63] around 10% are hydrogen-deficient and show WR spectra.[64] These are low-mass stars and to distinguish them from the massive Wolf Rayet stars, their spectra are enclosed in square brackets: e.g. [WC]. Most of these show [WC] spectra, some [WO], and very rarely [WN].

The "Slash" stars[edit]

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 (e.g. Of/WNL[50])

There is a secondary group found with this spectra, a cooler, "intermediate" group with designation of Ofpe/WN9.[50] These stars have also been referred to as WN10 or WN11, but that has become less popular with the realisation of the evolutionary difference to other Wolf–Rayet stars. Recent discoveries of even rarer stars have extended the range of slash stars as far as O2-3.5If*/WN5-7, which are even hotter than the original slash stars.[65]

Cool red and brown dwarf classes[edit]

Main articles: brown dwarf and red dwarf

The new spectral types L, T and Y were created to classify infrared spectra of cool stars. This includes both red dwarfs and brown dwarfs that are very faint in the visual spectrum.[66]

Brown dwarfs, whose energy comes from gravitational attraction alone, cool as they age and so progress to later spectral types. Brown dwarfs start their lives with M-type spectra and will cool through the L, T, and Y spectral classes; faster the less massive they are—the highest-mass brown dwarfs cannot have cooled to Y or even T dwarfs within the age of the universe. Because this leads to a degeneracy between mass and age for a given effective temperature and luminosity, no unique values can be assigned to a given spectral type.[8]

Class L[edit]

Artist's impression 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. Some of these objects have masses large enough to support hydrogen fusion and are therefore stars, but most are of substellar mass and are therefore brown dwarfs. 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.[67][68][69] 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 while in the height of its luminous red nova eruption.

Class T: methane dwarfs[edit]

Artist's impression of a T-dwarf

Class T dwarfs are cool brown dwarfs with surface temperatures between approximately 700 and 1,300 K. Their emission peaks in the infrared. Methane is prominent in their spectra.[67][68]

Class T and L could be more common than all the other classes combined if recent research is accurate. Study of the number of proplyds (protoplanetary discs, clumps of gas in nebulae from which stars and planetary systems are formed) indicates that 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 dwarfs of the L and T classes, which are quite invisible to us. Because brown dwarfs can live so long, these smaller bodies accumulate over time.

Class Y[edit]

Artist's impression of a Y-dwarf

Brown dwarfs of spectral class Y are cooler than those of spectral class T and have qualitatively different spectra from them. A total of 17 objects have been placed in class Y as of August 2013.[70] Although such dwarfs have been modelled[71] and detected within forty light years by the Wide-field Infrared Survey Explorer (WISE)[59][72][73][74][75] there is no well-defined spectral sequence yet with prototypes. Nevertheless, several objects have been assigned spectral classes Y0, Y1, and Y2.[76] The spectra of these objects display absorption around 1.55 micrometers.[77] 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.[77][78] In fact, this ammonia-absorption feature is the main criterion that has been adopted to define this class.[76] However, this feature is difficult to distinguish from absorption by water and methane,[77] and other authors have stated that the assignment of class Y0 is premature.[79]

The brown dwarf with the latest assigned spectral type, WISE 1828+2650, is a >Y2 dwarf with an effective temperature originally estimated around 300 K, the temperature of the human body.[72][73][80] Parallax measurements have however since shown that its luminosity is inconsistent with it being colder than ~400 K; the likely coolest Y dwarf currently known is WD 0806−661B with approximately 350 K.[2]

The mass range for Y dwarfs is 9–25 Jupiter masses, but for young objects might reach below one Jupiter mass, which means that Y class objects straddle the 13 Jupiter mass deuterium-fusion limit that marks the division between brown dwarfs and planets.[76]

Carbon-related late giant star classes[edit]

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 become 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[edit]

Main article: Carbon star
Image of the carbon star R Sculptoris and its striking spiral structure

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.[81] A few main-sequence carbon stars are known, but the overwhelming majority of known carbon stars are giants or supergiants. There are several subclasses:

    • C-R: Formerly a class on its own representing the carbon star equivalent of late G to early K-type stars.
    • C-N: Formerly a class on its own representing the carbon star equivalent of late K to M stars.
    • C-J: A subtype of cool C stars with a high content of 13C.
    • C-H: Population II analogues of the C-R stars.
    • C-Hd: Hydrogen-deficient carbon stars, similar to late G supergiants with CH and C2 bands added.

Class S[edit]

Main article: S-type star

Class S stars have zirconium monoxide lines in addition to (or, rarely, instead of) those of titanium monoxide, and are in between the class M stars and the carbon stars.[82] 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.

Classes MS and SC: intermediary carbon-related classes[edit]

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.

White dwarf classifications[edit]

Sirius A and B (a white dwarf of type DA2) resolved by HST

The class D (for Degenerate) 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.

The white dwarf types are as follows:[83][84]

  • 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/Teff, where Teff 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.[83][85]

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.[83]

Extended white dwarf spectral types:[83]

  • 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.

A different set of spectral peculiarity symbols are used for white dwarfs than for other types of stars:

Code Spectral peculiarities for stars
P Magnetic white dwarf with detectable polarization
E Emission lines present
H Magnetic white dwarf without detectable polarization
V Variable
PEC Spectral peculiarities exist

Degenerate and exotic stars[edit]

Main articles: Neutron star, Black hole and Exotic star

These objects are not stars but are stellar remnants. They are much dimmer and if placed on the HR diagram, would be placed further to the lower left-hand corner.[86]

Color table[edit]

The following is a list of the approximate color of some stellar types, as well as their hex and RGB color models.[87][88]

Class RGB color hexcolor Class RGB color hexcolor Class RGB color hexcolor Class RGB color hexcolor
O5(V) 155 176 255 #9bb0ff K4(V) 255 216 181 #ffd8b5 O9(III) 158 177 255 #9eb1ff M7(III) 255 165 097 #ffa561
O6(V) 162 184 255 #a2b8ff K5(V) 255 210 161 #ffd2a1 B0(III) 158 177 255 #9eb1ff M8(III) 255 167 097 #ffa761
O7(V) 157 177 255 #9db1ff K7(V) 255 199 142 #ffc78e B1(III) 158 177 255 #9eb1ff M9(III) 255 233 154 #ffe99a
O8(V) 157 177 255 #9db1ff K8(V) 255 209 174 #ffd1ae B2(III) 159 180 255 #9fb4ff B2(II) 165 192 255 #a5c0ff
O9(V) 154 178 255 #9ab2ff M0(V) 255 195 139 #ffc38b B3(III) 163 187 255 #a3bbff B5(II) 175 195 255 #afc3ff
B0(V) 156 178 255 #9cb2ff M1(V) 255 204 142 #ffcc8e B5(III) 168 189 255 #a8bdff F0(II) 203 217 255 #cbd9ff
B1(V) 160 182 255 #a0b6ff M2(V) 255 196 131 #ffc483 B7(III) 171 191 255 #abbfff F2(II) 229 233 255 #e5e9ff
B2(V) 160 180 255 #a0b4ff M3(V) 255 206 129 #ffce81 B9(III) 178 195 255 #b2c3ff G5(II) 255 235 203 #ffebcb
B3(V) 165 185 255 #a5b9ff M4(V) 255 201 127 #ffc97f A0(III) 188 205 255 #bccdff M3(II) 255 201 119 #ffc977
B4(V) 164 184 255 #a4b8ff M5(V) 255 204 111 #ffcc6f A3(III) 189 203 255 #bdcbff O9(I) 164 185 255 #a4b9ff
B5(V) 170 191 255 #aabfff M6(V) 255 195 112 #ffc370 A5(III) 202 215 255 #cad7ff B0(I) 161 189 255 #a1bdff
B6(V) 172 189 255 #acbdff M8(V) 255 198 109 #ffc66d A6(III) 209 219 255 #d1dbff B1(I) 168 193 255 #a8c1ff
B7(V) 173 191 255 #adbfff B1(IV) 157 180 255 #9db4ff A7(III) 210 219 255 #d2dbff B2(I) 177 196 255 #b1c4ff
B8(V) 177 195 255 #b1c3ff B2(IV) 159 179 255 #9fb3ff A8(III) 209 219 255 #d1dbff B3(I) 175 194 255 #afc2ff
B9(V) 181 198 255 #b5c6ff B3(IV) 166 188 255 #a6bcff A9(III) 209 219 255 #d1dbff B4(I) 187 203 255 #bbcbff
A0(V) 185 201 255 #b9c9ff B6(IV) 175 194 255 #afc2ff F0(III) 213 222 255 #d5deff B5(I) 179 202 255 #b3caff
A1(V) 181 199 255 #b5c7ff B7(IV) 170 189 255 #aabdff F2(III) 241 241 255 #f1f1ff B6(I) 191 207 255 #bfcfff
A2(V) 187 203 255 #bbcbff B9(IV) 180 197 255 #b4c5ff F4(III) 241 240 255 #f1f0ff B7(I) 195 209 255 #c3d1ff
A3(V) 191 207 255 #bfcfff A0(IV) 179 197 255 #b3c5ff F5(III) 242 240 255 #f2f0ff B8(I) 182 206 255 #b6ceff
A5(V) 202 215 255 #cad7ff A3(IV) 190 205 255 #becdff F6(III) 241 240 255 #f1f0ff B9(I) 204 216 255 #ccd8ff
A6(V) 199 212 255 #c7d4ff A4(IV) 195 210 255 #c3d2ff F7(III) 241 240 255 #f1f0ff A0(I) 187 206 255 #bbceff
A7(V) 200 213 255 #c8d5ff A5(IV) 212 220 255 #d4dcff G0(III) 255 242 233 #fff2e9 A1(I) 214 223 255 #d6dfff
A8(V) 213 222 255 #d5deff A7(IV) 192 207 255 #c0cfff G1(III) 255 243 233 #fff3e9 A2(I) 199 214 255 #c7d6ff
A9(V) 219 224 255 #dbe0ff A9(IV) 224 227 255 #e0e3ff G2(III) 255 243 233 #fff3e9 A5(I) 223 229 255 #dfe5ff
F0(V) 224 229 255 #e0e5ff F0(IV) 218 224 255 #dae0ff G3(III) 255 243 233 #fff3e9 F0(I) 202 215 255 #cad7ff
F2(V) 236 239 255 #ecefff F2(IV) 227 230 255 #e3e6ff G4(III) 255 243 233 #fff3e9 F2(I) 244 243 255 #f4f3ff
F5(V) 248 247 255 #f8f7ff F3(IV) 227 230 255 #e3e6ff G5(III) 255 236 211 #ffecd3 F5(I) 219 225 255 #dbe1ff
F6(V) 244 241 255 #f4f1ff F5(IV) 241 239 255 #f1efff G6(III) 255 236 215 #ffecd7 F8(I) 255 252 247 #fffcf7
F7(V) 246 243 255 #f6f3ff F7(IV) 240 239 255 #f0efff G8(III) 255 231 199 #ffe7c7 G0(I) 255 239 219 #ffefdb
F8(V) 255 247 252 #fff7fc F8(IV) 255 252 253 #fffcfd G9(III) 255 231 196 #ffe7c4 G2(I) 255 236 205 #ffeccd
F9(V) 255 247 252 #fff7fc G0(IV) 255 248 245 #fff8f5 K0(III) 255 227 190 #ffe3be G3(I) 255 231 203 #ffe7cb
G0(V) 255 248 252 #fff8fc G2(IV) 255 244 242 #fff4f2 K1(III) 255 223 181 #ffdfb5 G5(I) 255 230 183 #ffe6b7
G1(V) 255 247 248 #fff7f8 G3(IV) 255 238 226 #ffeee2 K2(III) 255 221 175 #ffddaf G8(I) 255 220 167 #ffdca7
G2(V) 255 245 242 #fff5f2 G4(IV) 255 245 238 #fff5ee K3(III) 255 216 167 #ffd8a7 K0(I) 255 221 181 #ffddb5
G4(V) 255 241 229 #fff1e5 G5(IV) 255 235 213 #ffebd5 K4(III) 255 211 146 #ffd392 K1(I) 255 220 177 #ffdcb1
G5(V) 255 244 234 #fff4ea G6(IV) 255 242 234 #fff2ea K5(III) 255 204 138 #ffcc8a K2(I) 255 211 135 #ffd387
G6(V) 255 244 235 #fff4eb G7(IV) 255 231 205 #ffe7cd K7(III) 255 208 142 #ffd08e K3(I) 255 204 128 #ffcc80
G7(V) 255 244 235 #fff4eb G8(IV) 255 233 211 #ffe9d3 M0(III) 255 203 132 #ffcb84 K4(I) 255 201 118 #ffc976
G8(V) 255 237 222 #ffedde K0(IV) 255 225 189 #ffe1bd M1(III) 255 200 121 #ffc879 K5(I) 255 209 154 #ffd19a
G9(V) 255 239 221 #ffefdd K1(IV) 255 216 171 #ffd8ab M2(III) 255 198 118 #ffc676 M0(I) 255 204 143 #ffcc8f
K0(V) 255 238 221 #ffeedd K2(IV) 255 229 202 #ffe5ca M3(III) 255 200 119 #ffc877 M1(I) 255 202 138 #ffca8a
K1(V) 255 224 188 #ffe0bc K3(IV) 255 219 167 #ffdba7 M4(III) 255 206 127 #ffce7f M2(I) 255 193 104 #ffc168
K2(V) 255 227 196 #ffe3c4 O7(III) 158 177 255 #9eb1ff M5(III) 255 197 124 #ffc57c M3(I) 255 192 118 #ffc076
K3(V) 255 222 195 #ffdec3 O8(III) 157 178 255 #9db2ff M6(III) 255 178 121 #ffb279 M4(I) 255 185 104 #ffb968

Stellar classification, habitability, and the search for life[edit]

Further information: Planetary habitability

The range of stars that are predicted to be able to support life as we know it is limited by a few factors. Of the main-sequence star types, stars more massive than 1.5 times that of the Sun (spectral types O, B, and A) age too quickly for advanced life to develop (using Earth as a guideline). On the other extreme, dwarfs of less than half the mass of the Sun (spectral type M) are likely to tidally lock planets within their habitable zone, along with other problems (see Habitability of red dwarf systems).[89]

Variable star classification[edit]

Main article: Variable star

Stars that exhibit change in luminosity are variable stars. There is a variable star classification scheme that encompasses existing stars that are classified in the spectra classification.

See also[edit]


  1. ^ The conventional color description takes into account only the peak of the stellar spectrum. However, in actuality stars radiate in all parts of the spectrum, and because all spectral colors combined appear white, the actual apparent colors the human eye would observe are lighter than the conventional color descriptions.
  2. ^ Use a different set of spectral types from element-burning stars
  3. ^ When used with A-type stars, this instead refers to abnormally strong metallic spectral lines
  4. ^ a b c d e f g These proportions are fractions of stars brighter than absolute magnitude 16; lowering this limit will render earlier types even rarer, whereas generally adding only to the M class.
  5. ^ This rises to 78.6% if we include all stars. (See the above note.)


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