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Stellar classification

January 13, 2023

For the ranking system, e.g. "four out of five stars", see Star (classification).

In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines. Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary mainly due to the temperature of the photosphere, although in some cases there are true abundance differences. The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature.

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, and 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 classes S and 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+ is used for hypergiants, class I for supergiants, class II for bright giants, class III for regular giants, class IV for subgiants, class V for main-sequence stars, class sd (or VI) for subdwarfs, and class D (or VII) for white dwarfs. The full spectral class for the Sun is then G2V, indicating a main-sequence star with a surface temperature around 5,800 K.

Conventional colour description

Main article: Green star (astronomy)
 
Just-saturated RGB-camera discs

The conventional colour description takes into account only the peak of the stellar spectrum. In actuality, however, stars radiate in all parts of the spectrum. Because all spectral colours combined appear white, the actual apparent colours the human eye would observe are far lighter than the conventional colour descriptions would suggest. This characteristic of 'lightness' indicates that the simplified assignment of colours within the spectrum can be misleading. Excluding colour-contrast effects in dim light, in typical viewing conditions there are no green, cyan, indigo, or violet stars. "Yellow" dwarfs such as the Sun are white, "red" dwarfs are a deep shade of yellow/orange, and "brown" dwarfs do not literally appear brown, but hypothetically would appear dim red or grey/black to a nearby observer.

Modern classification

 
Main-sequence stars arranged from O to M Harvard classes

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, which did not include luminosity[1]) and a luminosity class using Roman numerals as explained below, forming the star's spectral type.

Other modern stellar classification systems, such as the UBV system, are based on color indices—the measured differences in three or more color magnitudes. Those numbers are given labels such as "U−V" or "B−V", which represent the colors passed by two standard filters (e.g. Ultraviolet, Blue and Visual).

Harvard spectral classification

The Harvard system is a one-dimensional classification scheme by astronomer Annie Jump Cannon, who re-ordered and simplified the prior alphabetical system by Draper (see #History). Stars are grouped according to their spectral characteristics by single letters of the alphabet, optionally with numeric subdivisions. 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[2][3] Vega-relative chromaticity[4][5][a] Chromaticity (D65)[6][7][4][b] Main-sequence mass[2][8]
(solar masses) 		Main-sequence radius[2][8]
(solar radii) 		Main-sequence luminosity[2][8]
(bolometric) 		Hydrogen
lines 		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 light 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 orange red light orange red 0.08–0.45 M☉ ≤ 0.7 R☉ ≤ 0.08 L☉ Very weak 76.45%
 
The Hertzsprung–Russell diagram relates stellar classification with absolute magnitude, luminosity, and surface temperature.

A common mnemonic for remembering the order of the spectral type letters, from hottest to coolest, is "Oh, Be A Fine Guy/Girl: Kiss Me!".[10]

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 class A and A9 denotes the coolest ones. Fractional numbers are allowed; for example, the star Mu Normae is classified as O9.7.[11] The Sun is classified as G2.[12]

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[6] 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.[dubious – discuss]

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.[13] 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.[14]

Harvard astronomer Cecilia Payne then demonstrated that the O-B-A-F-G-K-M spectral sequence is actually a sequence in temperature.[15] 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

 
Montage of false color spectra for main-sequence stars[16]

The Yerkes spectral classification, also called the MK, or Morgan-Keenan (alternatively referred to as the MKK, or Morgan-Keenan-Kellman)[17][18] 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.[19] 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 just surface temperature). Later, in 1953, after some revisions to the list of standard stars and classification criteria, the scheme was named the Morgan–Keenan classification, or MK,[20] which remains in 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, as listed in the table below.[21]

Yerkes luminosity classes
Luminosity class Description Examples
0 or Ia+ hypergiants or extremely luminous supergiants Cygnus OB2#12 – B3-4Ia+[22]
Ia luminous supergiants Eta Canis Majoris – B5Ia[23]
Iab intermediate-size luminous supergiants Gamma Cygni – F8Iab[24]
Ib less luminous supergiants Zeta Persei – B1Ib[25]
II bright giants Beta Leporis – G0II[26]
III normal giants Arcturus – K0III[27]
IV subgiants Gamma Cassiopeiae – B0.5IVpe[28]
V main-sequence stars (dwarfs) Achernar – B6Vep[25]
sd (prefix) or VI subdwarfs HD 149382 – sdB5 or B5VI[29]
D (prefix) or VII white dwarfs[c] van Maanen 2 – DZ8[30]

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.
  • 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).

Nominal luminosity class VII (and sometimes higher numerals) is now rarely used for white dwarf or "hot sub-dwarf" classes, since the temperature-letters of the main sequence and giant stars no longer apply to white dwarfs.

Occasionally, letters a and b are applied to luminosity classes other than supergiants; for example, a giant star slightly less luminous than typical may be given a luminosity class of IIIb, while a luminosity class IIIa indicates a star slightly brighter than a typical giant.[31]

A sample of extreme V stars with strong absorption in He II λ4686 spectral lines have been given the Vz designation. An example star is HD 93129 B.[32]

Spectral peculiarities

Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum.[33]

Code Spectral peculiarities for stars
: uncertain spectral value[21]
... Undescribed spectral peculiarities exist
! Special peculiarity
comp Composite spectrum[34]
e Emission lines present[34]
[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[21]
f* N IV 4058Å is stronger than the N III 4634Å, 4640Å, & 4642Å lines[35]
f+ Si IV 4089Å & 4116Å are emitted, in addition to the N III line[35]
f? C III 4647–4650–4652Å emission lines with comparable strength to the N III line[36]
(f) N III emission, absence or weak absorption of He II
(f+) [37]
((f)) Displays strong He II absorption accompanied by weak N III emissions[38]
((f*)) [37]
h WR stars with hydrogen emission lines.[39]
ha WR stars with hydrogen seen in both absorption and emission.[39]
He wk Weak Helium lines
k Spectra with interstellar absorption features
m Enhanced metal features[34]
n Broad ("nebulous") absorption due to spinning[34]
nn Very broad absorption features[21]
neb A nebula's spectrum mixed in[34]
p Unspecified peculiarity, peculiar star.[d][34]
pq Peculiar spectrum, similar to the spectra of novae
q P Cygni profiles
s Narrow ("sharp") absorption lines[34]
ss Very narrow lines
sh Shell star features[34]
var Variable spectral feature[34] (sometimes abbreviated to "v")
wl Weak lines[34] (also "w" & "wk")
Element
symbol 		Abnormally strong spectral lines of the specified element(s)[34]
z indicating abnormally an strong ionised helium line at 468.6 nm[32]

For example, 59 Cygni is listed as spectral type B1.5Vnne,[40] indicating a spectrum with the general classification B1.5V, as well as very broad absorption lines and certain emission lines.

 
Guide for Secchi spectral types ("152 Schjellerup" is Y Canum Venaticorum)

History

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

Secchi classes

During the 1860s and 1870s, pioneering stellar spectroscopist Angelo Secchi created the Secchi classes in order to classify observed spectra. By 1866, he had developed three classes of stellar spectra, shown in the table below.[41][42][43]

In the late 1890s, this classification began to be superseded by the Harvard classification, which is discussed in the remainder of this article.[44][45][46]

Class number Secchi class description
Secchi 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.
Secchi class I
(Orion subtype)		 A subtype of Secchi 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
Secchi 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.
Secchi class III Orange to red stars with complex band spectra, such as Betelgeuse and Antares.
This corresponds to the modern class M.
Secchi class IV In 1868, he discovered carbon stars, which he put into a distinct group:[47]
Red stars with significant carbon bands and lines, corresponding to modern classes C and S.
Secchi class V In 1877, he added a fifth class:[48]
Emission-line stars, such as Gamma Cassiopeiae and Sheliak, which are in modern class Be. In 1891, Edward Charles Pickering proposed that class V should correspond to the modern class O (which then included Wolf–Rayet stars) and stars within planetary nebulae.[49]

The Roman numerals used for Secchi classes should not be confused with the completely unrelated Roman numerals used for Yerkes luminosity classes and the proposed neutron star classes.

Draper system

Classifications in the Draper Catalogue of Stellar Spectra[50][51]
Secchi Draper Comment
I A, B, C, D Hydrogen lines dominant
II E, F, G, H, I, K, L
III M
IV N Did not appear in the catalogue
V O Included Wolf–Rayet spectra with bright lines
V P Planetary nebulae
  Q Other spectra
Classes carried through into the MK system are in bold.

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 and was credited with classifying over 10,000 featured stars and discovering 10 novae and more than 200 variable stars.[52] With the help of the Harvard computers, especially Williamina Fleming, the first iteration of the Henry Draper catalogue was devised to replace the Roman-numeral scheme established by Angelo Secchi.[53]

The catalogue used a scheme in which the previously used Secchi classes (I to V) were subdivided into more specific classes, given letters from A to P. Also, the letter Q was used for stars not fitting into any other class.[50][51] Fleming worked with Pickering to differentiate 17 different classes based on the intensity of hydrogen spectral lines, which causes variation in the wavelengths emanated from stars and results in variation in color appearance. The spectra in class A tended to produce the strongest hydrogen absorption lines while spectra in class O produced virtually no visible lines. The lettering system displayed the gradual decrease in hydrogen absorption in the spectral classes when moving down the alphabet. This classification system was later modified by Annie Jump Cannon and Antonia Maury to produce the Harvard spectral classification scheme.[52][54]

The old Harvard system (1897)

In 1897, another astronomer 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–XXII.[55][56]

Summary of 1897 Harvard system[57]
Groups Summary
I−V included ‘Orion type’ stars that displayed an increasing strength in hydrogen absorption lines from group I to group V
VI acted as an intermediate between the ‘Orion type’ and Secchi type I group
VII−XI were Secchi's type 1 stars, with decreasing strength in hydrogen absorption lines from groups VII−XI
XIII−XVI included Secchi type 2 stars with decreasing hydrogen absorption lines and increasing solar-type metallic lines
XVII−XX included Secchi type 3 stars with increasing spectral lines
XXI included Secchi type 4 stars
XXII included Wolf–Rayet stars

Because the 22 Roman numeral groupings didn't account for additional variations in spectra, three additional divisions were made to further specify differences: Lowercase letters were added to differentiate relative line appearance in spectra; the lines were defined as[57]

Harvard 1897 Subtypes[57]
(a) average width
(b) hazy
(c) sharp

Antonia Maury published her own stellar classification catalogue in 1897 called "Spectra of Bright Stars Photographed with the 11 inch Draper Telescope as Part of the Henry Draper Memorial", which included 4,800 photographs and Maury's analyses of 681 bright northern stars. This was the first instance in which a woman was credited for an observatory publication.[58]

The current Harvard system (1912)

In 1901, Annie Jump Cannon returned to the lettered types, but dropped all letters except O, B, A, F, G, K, M, and N used in that order, as well as P for planetary nebulae and Q for some peculiar spectra. She also used types such as B5A for stars halfway between types B and A, F2G for stars one fifth of the way from F to G, and so on.[59][60]

Finally, by 1912, Cannon had changed the types B, A, B5A, F2G, etc. to B0, A0, B5, F2, etc.[61][62] This is essentially the modern form of the Harvard classification system. This system was developed through the analysis of spectra on photographic plates, which could convert light emanated from stars into a readable spectrum.[63]

Mount Wilson classes

 
Proper motion of stars of early type in ± 200,000 years

A luminosity classification known as the Mount Wilson system was used to distinguish between stars of different luminosities.[64][65][66] This notation system is still sometimes seen on modern spectra.[67]

Class Meaning
sd Subdwarf
d Dwarf
sg Subgiant
g Giant
c Supergiant
 
The movement of stars of late type around the apex (left) and antapex (right) in ± 200,000 years

Spectral types

"Spectral type" redirects here. For spectral types of asteroids, see Asteroid spectral types.

The stellar classification system is taxonomic, based on type specimens, similar to classification of species in biology: The categories are defined by one or more standard stars for each category and sub-category, with an associated description of the distinguishing features.[68]

"Early" and "late" nomenclature

Stars are often referred to as early or late types. "Early" is a synonym for hotter, while "late" is a synonym for cooler.

Depending on the context, "early" and "late" may be absolute or relative terms. "Early" as an absolute term would therefore refer to O or B, and possibly A stars. As a relative reference it relates to stars hotter than others, such as "early K" being perhaps K0, K1, K2 and K3.

"Late" is used in the same way, with an unqualified use of the term indicating stars with spectral types such as K and M, but it can also be used for stars that are cool relative to other stars, as in using "late G" to refer to G7, G8, and G9.

In the relative sense, "early" means a lower Arabic numeral following the class letter, and "late" means a higher number.

This obscure terminology is a hold-over from a late nineteenth century model of stellar evolution, which supposed that stars were powered by gravitational contraction via the Kelvin–Helmholtz mechanism, which is now known to not apply to main-sequence stars. If that were true, then stars would start their lives as very hot "early-type" stars and then gradually cool down into "late-type" stars. This mechanism provided ages of the Sun that were much smaller than what is observed in the geologic record, and was rendered obsolete by the discovery that stars are powered by nuclear fusion.[69] The terms "early" and "late" were carried over, beyond the demise of the model they were based on.

Class O

Main article: O-type star
See also: O-type main-sequence star, Blue giant, and Blue supergiant
 
The spectrum of an O5V star

O-type stars are very hot and extremely luminous, with most of their radiated output in the ultraviolet range. These are the rarest of all main-sequence stars. About 1 in 3,000,000 (0.00003%) of the main-sequence stars in the solar neighborhood are O-type stars.[e][9] Some of the most massive stars lie within this spectral class. O-type stars frequently have complicated surroundings that make measurement of their spectra difficult.

O-type spectra formerly were defined by the ratio of the strength of the He II λ4541 relative to that of He I λ4471, where λ is the radiation wavelength. Spectral type O7 was defined to be the point at which the two intensities are equal, with the He 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.[70]

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. Higher-mass O-type stars do not retain extensive atmospheres due to the extreme velocity of their stellar wind, which may reach 2,000 km/s. Because they are so massive, O-type stars have very hot cores and burn through their hydrogen fuel very quickly, so they are the first stars to leave the main sequence.

When the MKK classification scheme was first described in 1943, the only subtypes of class O used were O5 to O9.5.[71] The MKK scheme was extended to O9.7 in 1971[72] and O4 in 1978,[73] and new classification schemes that add types O2, O3, and O3.5 have subsequently been introduced.[74]

Spectral standards:[68]

Class B

See also: B-type main-sequence star, Blue giant, and Blue supergiant
 
B-class stars in the Jewel Box cluster (Credit: ESO VLT)

B-type stars are very luminous and blue. Their spectra have neutral helium lines, which are most prominent at the B2 subclass, and moderate hydrogen lines. As O- and B-type stars are so energetic, they only live for a relatively short time. Thus, due to the low probability of kinematic interaction during their lifetime, they are unable to stray far from the area in which they formed, apart from runaway 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 modern equipment, the line is still apparent in the early B-type stars. Today for main-sequence stars, the B class is instead defined by the intensity of the He 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.[70]

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 main-sequence stars.[e][9] Due to them being rare, the closest B type star is Regulus, at around 80 light years.[75]

Massive yet non-supergiant entities known as "Be stars" are main-sequence stars that notably have, or had at some time, one or more Balmer lines in emission, with the hydrogen-related electromagnetic radiation series projected out by the stars being of particular interest. Be stars are generally thought to feature unusually strong stellar winds, high surface temperatures, and significant attrition of stellar mass as the objects rotate at a curiously rapid rate.[76]

Objects known as "B(e)" or "B[e]" stars possess distinctive neutral or low ionisation emission lines that are considered to have 'forbidden mechanisms', undergoing processes not normally allowed under current understandings of quantum mechanics.

Spectral standards:[68]

Class A

See also: A-type main-sequence star
 
Class A Vega (left) compared to the Sun (right)

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,[e][9] which includes 9 stars within 15 parsecs.[77]

Spectral standards:[68]

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

Class F

See also: F-type main-sequence star

F-type stars have strengthening spectral lines H and K 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,[e][9] including 12 stars within 10 pc.[citation needed]

Spectral standards:[68]

Class G

"G star" redirects here. For other uses, see G star (disambiguation).
See also: G-type main-sequence star, Yellow supergiant, and Yellow hypergiant
 
The Sun, a G2 main-sequence star, with dark sunspots

G-type stars, including the Sun,[12] have prominent spectral lines H and K 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 CN molecules. Class G main-sequence stars make up about 7.5%, nearly one in thirteen, of the main-sequence stars in the solar neighborhood. There are 21 G-type stars within 10pc.[e][9]

Class G contains the "Yellow Evolutionary Void".[78] Supergiant stars often swing between O or B (blue) and K or M (red). While they do this, they do not stay for long in the unstable yellow supergiant class.

Spectral standards:[68]

Class K

See also: K-type main-sequence star
"K-type star" redirects here. For the Korean nuclear fusion project, see KSTAR.
 
Arcturus, a K1.5 giant compared to the Sun and Antares

K-type stars are orangish stars that are slightly cooler than the Sun. They make up about 12% of the main-sequence stars in the solar neighborhood.[e][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 those are present at all, and mostly neutral metals (Mn I, Fe I, Si I). By late K, molecular bands of titanium oxide become present. Mainstream theories (those rooted in lower harmful radioactivity and star longevity) would thus suggest such stars have the optimal chances of heavily evolved life developing on orbiting planets (if such life is directly analogous to earth's) due to a broad habitable zone yet much lower harmful periods of emission compared to those with the broadest such zones.[79][80]

Spectral standards:[68]

Class M

See also: Red dwarf, Red giant, and Red 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.[e][f][9] However, class M main-sequence stars (red dwarfs) have such low luminosities that none are bright enough to be seen with the unaided eye, unless under exceptional conditions. The brightest-known M class main-sequence star is Lacaille 8760, class M0V, with magnitude 6.7 (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, most of the largest-known supergiant stars in the Milky Way are class M stars, such as VV Cephei, Antares, and Betelgeuse. Furthermore, some larger, hotter brown dwarfs are late class M, usually in the range of M6.5 to M9.5.

The spectrum of a class M star contains lines from oxide molecules (in the visible spectrum, especially TiO) 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(II) oxide bands become present by late M.

Spectral standards:[68]

Extended spectral types

A number of new spectral types have been taken into use from newly discovered types of stars.[81]

Hot blue emission star classes

 
UGC 5797, an emission-line galaxy where massive bright blue stars are formed[82]

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

Class W: Wolf–Rayet

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

Once included as type O stars, the Wolf–Rayet stars of class W or WR are 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 subclasses according to the relative strength of nitrogen and carbon emission lines in their spectra (and outer layers).[39]

WR spectra range is listed below:[83][84]

  • WN[39] – spectrum dominated by N III-V and He I-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[39]
    • 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 C IV lines, intermediate between WN and WC stars[39]
  • WC[39] – spectrum with strong C II-IV lines
    • WCE (WC4 to WC6) – hotter or "early"
    • WCL (WC7 to WC9) – cooler or "late"
  • WO (WO1 to WO4) – strong O VI lines, extremely rare, extension of the WCE class into incredibly hot temperatures (up to 200 kK or more)

Although the central stars of most planetary nebulae (CSPNe) show O-type spectra,[85] around 10% are hydrogen-deficient and show WR spectra.[86] 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].

Slash stars

Main article: Slash star

The slash stars are O-type stars with WN-like lines in their spectra. The name "slash" comes from their printed spectral type having a slash in it (e.g. "Of/WNL"[70]).

There is a secondary group found with these spectra, a cooler, "intermediate" group designated "Ofpe/WN9".[70] These stars have also been referred to as WN10 or WN11, but that has become less popular with the realisation of the evolutionary difference from 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.[87]

Magnetic O stars

They are O stars with strong magnetic fields. Designation is Of?p.[70]

Cool red and brown dwarf classes

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 visible spectrum.[88]

Brown dwarfs, stars that do not undergo hydrogen fusion, 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 an unresolvable overlap between spectral types' effective temperature and luminosity for some masses and ages of different L-T-Y types, no distinct temperature or luminosity values can be given.[8]

Class L

 
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.[89][90][91]

Due to low surface gravity in giant stars, TiO- and VO-bearing condensates never form. Thus, L-type stars larger than dwarfs can never form in an isolated environment. However, it may be possible for these L-type supergiants to form through stellar collisions, an example of which is V838 Monocerotis while in the height of its luminous red nova eruption.

Class T: methane dwarfs

 
Artist's impression of a T-dwarf

Class T dwarfs are cool brown dwarfs with surface temperatures between approximately 550 and 1,300 K (277 and 1,027 °C; 530 and 1,880 °F). Their emission peaks in the infrared. Methane is prominent in their spectra.[89][90]

Study of the number of proplyds (protoplanetary disks, 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 was previously conjectured. It is theorized that these proplyds are in a race with each other. The first one to form will become a protostar, 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.[92]

Class Y

See also: Sub-brown dwarf and Substellar object
 
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.[93] Although such dwarfs have been modelled[94] and detected within forty light-years by the Wide-field Infrared Survey Explorer (WISE)[81][95][96][97][98] there is no well-defined spectral sequence yet and no prototypes. Nevertheless, several objects have been proposed as spectral classes Y0, Y1, and Y2.[99]

The spectra of these prospective Y objects display absorption around 1.55 micrometers.[100] Delorme et al. have suggested that this feature is due to absorption from ammonia, and that this should be taken as the indicative feature for the T-Y transition.[100][101] In fact, this ammonia-absorption feature is the main criterion that has been adopted to define this class.[99] However, this feature is difficult to distinguish from absorption by water and methane,[100] and other authors have stated that the assignment of class Y0 is premature.[102]

The latest brown dwarf proposed for the Y spectral type, WISE 1828+2650, is a > Y2 dwarf with an effective temperature originally estimated around 300 K, the temperature of the human body.[95][96][103] Parallax measurements have, however, since shown that its luminosity is inconsistent with it being colder than ~400 K. The coolest Y dwarf currently known is WISE 0855−0714 with an approximate temperature of 250 K, and a mass just seven times that of Jupiter.[104]

The mass range for Y dwarfs is 9–25 Jupiter masses, but young objects might reach below one Jupiter mass (although they cool to become planets), which means that Y class objects straddle the 13 Jupiter mass deuterium-fusion limit that marks the current IAU division between brown dwarfs and planets.[99]

Peculiar brown dwarfs

Symbols used for peculiar brown dwarfs
pec This suffix stands for "peculiar" (e.g. L2pec).[105]
sd This prefix (e.g. sdL0) stands for subdwarf and indicates a low metallicity and blue color[106]
β Objects with the beta (β) suffix (e.g. L4β) have an intermediate surface gravity.[107]
γ Objects with the gamma (γ) suffix (e.g. L5γ) have a low surface gravity.[107]
red The red suffix (e.g. L0red) indicates objects without signs of youth, but high dust content.[108]
blue The blue suffix (e.g. L3blue) indicates unusual blue near-infrared colors for L-dwarfs without obvious low metallicity.[109]

Young brown dwarfs have low surface gravities because they have larger radii and lower masses compared to the field stars of similar spectral type. These sources are marked by a letter beta (β) for intermediate surface gravity and gamma (γ) for low surface gravity. Indication for low surface gravity are weak CaH, KI and NaI lines, as well as strong VO line.[107] Alpha (α) stands for normal surface gravity and is usually dropped. Sometimes an extremely low surface gravity is denoted by a delta (δ).[109] The suffix "pec" stands for peculiar. The peculiar suffix is still used for other features that are unusual and summarizes different properties, indicative of low surface gravity, subdwarfs and unresolved binaries.[110] The prefix sd stands for subdwarf and only includes cool subdwarfs. This prefix indicates a low metallicity and kinematic properties that are more similar to halo stars than to disk stars.[106] Subdwarfs appear bluer than disk objects.[111] The red suffix describes objects with red color, but an older age. This is not interpreted as low surface gravity, but as a high dust content.[108][109] The blue suffix describes objects with blue near-infrared colors that cannot be explained with low metallicity. Some are explained as L+T binaries, others are not binaries, such as 2MASS J11263991−5003550 and are explained with thin and/or large-grained clouds.[109]

Late giant carbon-star classes

Carbon-stars are stars whose spectra indicate production of carbon – a byproduct of triple-alpha helium 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. Equivalent classes for carbon-rich stars are S and C.

The giants among those stars are presumed to produce this carbon themselves, but some stars in this class are double stars, whose odd atmosphere is suspected of having been transferred from a companion that is now a white dwarf, when the companion was a carbon-star.

Class C: carbon stars

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 C–J-type stars, which are characterized by the strong presence of molecules of 13CN in addition to those of 12CN.[112] 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 its own class (R) representing the carbon star equivalent of late G- to early K-type stars.
  • C-N – Formerly its own class representing the carbon star equivalent of late K- to M-type 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

Main article: S-type star

Class S stars form a continuum between class M stars and carbon stars. Those most similar to class M stars have strong ZrO absorption bands analogous to the TiO bands of class M stars, whereas those most similar to carbon stars have strong sodium D lines and weak C2 bands.[113] Class S stars have excess amounts of zirconium and other elements produced by the s-process, and have more similar carbon and oxygen abundances than class M or carbon stars. Like carbon stars, nearly all known class S stars are asymptotic-giant-branch stars.

The spectral type is formed by the letter S and a number between zero and ten. This number corresponds to the temperature of the star and approximately follows the temperature scale used for class M giants. The most common types are S3 to S5. The non-standard designation S10 has only been used for the star Chi Cygni when at an extreme minimum.

The basic classification is usually followed by an abundance indication, following one of several schemes: S2,5; S2/5; S2 Zr4 Ti2; or S2*5. A number following a comma is a scale between 1 and 9 based on the ratio of ZrO and TiO. A number following a slash is a more-recent but less-common scheme designed to represent the ratio of carbon to oxygen on a scale of 1 to 10, where a 0 would be an MS star. Intensities of zirconium and titanium may be indicated explicitly. Also occasionally seen is a number following an asterisk, which represents the strength of the ZrO bands on a scale from 1 to 5.

Classes MS and SC: Intermediate carbon-related classes

In between the M and S classes, border cases are named MS stars. In a similar way, border cases between the S and C-N classes are named SC or CS. The sequence M → MS → S → SC → C-N is hypothesized to be a sequence of increased carbon abundance with age for carbon stars in the asymptotic giant branch.

White dwarf classifications

Main article: White dwarf spectroscopy

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:[114][115]

  • 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.(For example DA1.5 for IK Pegasi B)[114][116]

Two or more of the type letters may be used to indicate a white dwarf that displays more than one of the spectral features above.[114]

Extended white dwarf spectral types

 
Sirius A and B (a white dwarf of type DA2) resolved by Hubble
  • 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:[114]

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

Non-stellar spectral types: Classes P and Q

Finally, the classes P and Q are left over from the system developed by Cannon for the Henry Draper Catalogue. They are occasionally used for certain non-stellar objects: Type P objects are stars within planetary nebulae (typically young white dwarfs or hydrogen-poor M giants); type Q objects are novae.[citation needed]

Stellar remnants

Main articles: Neutron star, Black hole, and Exotic star

Stellar remnants are objects associated with the death of stars. Included in the category are white dwarfs, and as can be seen from the radically different classification scheme for class D, non-stellar objects are difficult to fit into the MK system.

The Hertzsprung–Russell diagram, which the MK system is based on, is observational in nature so these remnants cannot easily be plotted on the diagram, or cannot be placed at all. Old neutron stars are relatively small and cold, and would fall on the far right side of the diagram. Planetary nebulae are dynamic and tend to quickly fade in brightness as the progenitor star transitions to the white dwarf branch. If shown, a planetary nebula would be plotted to the right of the diagram's upper right quadrant. A black hole emits no visible light of its own, and therefore would not appear on the diagram.[117]

A classification system for neutron stars using Roman numerals has been proposed: type I for less massive neutron stars with low cooling rates, type II for more massive neutron stars with higher cooling rates, and a proposed type III for more massive neutron stars (possible exotic star candidates) with higher cooling rates.[118] The more massive a neutron star is, the higher neutrino flux it carries. These neutrinos carry away so much heat energy that after only a few years the temperature of an isolated neutron star falls from the order of billions to only around a million Kelvin. This proposed neutron star classification system is not to be confused with the earlier Secchi spectral classes and the Yerkes luminosity classes.

Replaced spectral classes

Several spectral types, all previously used for non-standard stars in the mid-20th century, have been replaced during revisions of the stellar classification system. They may still be found in old editions of star catalogs: R and N have been subsumed into the new C class as C-R and C-N.

Stellar classification, habitability, and the search for life

See also: Planetary habitability

While humans may eventually be able to colonize any kind of stellar habitat, this section will address the probability of life arising around other stars.

Stability, luminosity, and lifespan are all factors in stellar habitability. We only know of one star that hosts life, and that is our own—a G-class star with an abundance of heavy elements and low variability in brightness. It is also unlike many stellar systems in that it only has one star in it (see Habitability of binary star systems).

Working from these constraints and the problems of having an empirical sample set of only one, 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 our Sun (spectral type M) are likely to tidally lock planets within their habitable zone, along with other problems (see Habitability of red dwarf systems).[119] While there are many problems facing life on red dwarfs, many astronomers continue to model these systems due to their sheer numbers and longevity.

For these reasons NASA's Kepler Mission is searching for habitable planets at nearby main-sequence stars that are less massive than spectral type A but more massive than type M—making the most probable stars to host life dwarf stars of types F, G, and K.[119]

See also

Explanatory notes

  1. ^ This is the relative color of the star if Vega, generally considered a bluish star, is used as a standard for "white".
  2. ^ Chromaticity can vary significantly within a class; for example, the Sun (a G2 star) is white, while a G9 star is yellow.
  3. ^ Technically, white dwarfs are no longer "live" stars but, rather, the "dead" remains of extinguished stars. Their classification uses a different set of spectral types from element-burning "live" stars.
  4. ^ When used with A-type stars, this instead refers to abnormally strong metallic spectral lines
  5. ^ 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.
  6. ^ This rises to 78.6% if we include all stars. (See the above note.)

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External links

 
Look up late-type star or early-type star in Wiktionary, the free dictionary.
  • Libraries of stellar spectra by D. Montes, UCM
  • Spectral Types for Hipparcos Catalogue Entries
  • Stellar Spectral Classification by Richard O. Gray and Christopher J. Corbally
  • Spectral models of stars by P. Coelho
  • Merrifield, Michael; Bauer, Amanda; Häußler, Boris (2010). "Star Classification". Sixty Symbols. Brady Haran for the University of Nottingham.
  • Stellar classification table

January 13, 2023
stellar, classification, ranking, system, four, five, stars, star, classification, hertzsprung, russell, diagram, spectral, type, brown, dwarfs, white, dwarfs, dwarfs, subdwarfs, main, sequence, dwarfs, subgiants, giants, giants, blue, giants, bright, giants, . For the ranking system e g four out of five stars see Star classification Hertzsprung Russell diagram Spectral type O B A F G K M L T Brown dwarfs White dwarfs Red dwarfs Subdwarfs Main sequence dwarfs Subgiants Giants Red giants Blue giants Bright giants Supergiants Red supergiant Hypergiants absolutemagni tude MV In astronomy stellar classification is the classification of stars based on their spectral characteristics Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines Each line indicates a particular chemical element or molecule with the line strength indicating the abundance of that element The strengths of the different spectral lines vary mainly due to the temperature of the photosphere although in some cases there are true abundance differences The spectral class of a star is a short code primarily summarizing the ionization state giving an objective measure of the photosphere s temperature 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 and 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 classes S and 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 is used for hypergiants class I for supergiants class II for bright giants class III for regular giants class IV for subgiants class V for main sequence stars class sd or VI for subdwarfs and class D or VII for white dwarfs The full spectral class for the Sun is then G2V indicating a main sequence star with a surface temperature around 5 800 K Contents 1 Conventional colour description 2 Modern classification 2 1 Harvard spectral classification 2 2 Yerkes spectral classification 2 3 Spectral peculiarities 3 History 3 1 Secchi classes 3 2 Draper system 3 3 The old Harvard system 1897 3 4 The current Harvard system 1912 3 5 Mount Wilson classes 4 Spectral types 4 1 Early and late nomenclature 4 2 Class O 4 3 Class B 4 4 Class A 4 5 Class F 4 6 Class G 4 7 Class K 4 8 Class M 5 Extended spectral types 5 1 Hot blue emission star classes 5 1 1 Class W Wolf Rayet 5 1 2 Slash stars 5 1 3 Magnetic O stars 5 2 Cool red and brown dwarf classes 5 2 1 Class L 5 2 2 Class T methane dwarfs 5 2 3 Class Y 5 2 4 Peculiar brown dwarfs 5 3 Late giant carbon star classes 5 3 1 Class C carbon stars 5 3 2 Class S 5 3 3 Classes MS and SC Intermediate carbon related classes 5 4 White dwarf classifications 5 4 1 Extended white dwarf spectral types 5 5 Non stellar spectral types Classes P and Q 6 Stellar remnants 7 Replaced spectral classes 8 Stellar classification habitability and the search for life 9 See also 10 Explanatory notes 11 References 12 External linksConventional colour description EditThis section does not cite any sources Please help improve this section by adding citations to reliable sources Unsourced material may be challenged and removed August 2022 Learn how and when to remove this template message Main article Green star astronomy Just saturated RGB camera discs The conventional colour description takes into account only the peak of the stellar spectrum In actuality however stars radiate in all parts of the spectrum Because all spectral colours combined appear white the actual apparent colours the human eye would observe are far lighter than the conventional colour descriptions would suggest This characteristic of lightness indicates that the simplified assignment of colours within the spectrum can be misleading Excluding colour contrast effects in dim light in typical viewing conditions there are no green cyan indigo or violet stars Yellow dwarfs such as the Sun are white red dwarfs are a deep shade of yellow orange and brown dwarfs do not literally appear brown but hypothetically would appear dim red or grey black to a nearby observer Modern classification EditThis section needs additional citations for verification Please help improve this article by adding citations to reliable sources Unsourced material may be challenged and removed September 2021 Learn how and when to remove this template message Main sequence stars arranged from O to M Harvard classes 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 which did not include luminosity 1 and a luminosity class using Roman numerals as explained below forming the star s spectral type Other modern stellar classification systems such as the UBV system are based on color indices the measured differences in three or more color magnitudes Those numbers are given labels such as U V or B V which represent the colors passed by two standard filters e g Ultraviolet Blue and Visual Harvard spectral classification Edit The Harvard system is a one dimensional classification scheme by astronomer Annie Jump Cannon who re ordered and simplified the prior alphabetical system by Draper see History Stars are grouped according to their spectral characteristics by single letters of the alphabet optionally with numeric subdivisions 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 2 3 Vega relative chromaticity 4 5 a Chromaticity D65 6 7 4 b Main sequence mass 2 8 solar masses Main sequence radius 2 8 solar radii Main sequence luminosity 2 8 bolometric Hydrogenlines Fraction of allmain 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 light 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 orange red light orange red 0 08 0 45 M 0 7 R 0 08 L Very weak 76 45 The Hertzsprung Russell diagram relates stellar classification with absolute magnitude luminosity and surface temperature A common mnemonic for remembering the order of the spectral type letters from hottest to coolest is Oh Be A Fine Guy Girl Kiss Me 10 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 class A and A9 denotes the coolest ones Fractional numbers are allowed for example the star Mu Normae is classified as O9 7 11 The Sun is classified as G2 12 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 6 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 dubious discuss 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 13 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 14 Harvard astronomer Cecilia Payne then demonstrated that the O B A F G K M spectral sequence is actually a sequence in temperature 15 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 Montage of false color spectra for main sequence stars 16 The Yerkes spectral classification also called the MK or Morgan Keenan alternatively referred to as the MKK or Morgan Keenan Kellman 17 18 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 19 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 just surface temperature Later in 1953 after some revisions to the list of standard stars and classification criteria the scheme was named the Morgan Keenan classification or MK 20 which remains in 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 as listed in the table below 21 Yerkes luminosity classes Luminosity class Description Examples0 or Ia hypergiants or extremely luminous supergiants Cygnus OB2 12 B3 4Ia 22 Ia luminous supergiants Eta Canis Majoris B5Ia 23 Iab intermediate size luminous supergiants Gamma Cygni F8Iab 24 Ib less luminous supergiants Zeta Persei B1Ib 25 II bright giants Beta Leporis G0II 26 III normal giants Arcturus K0III 27 IV subgiants Gamma Cassiopeiae B0 5IVpe 28 V main sequence stars dwarfs Achernar B6Vep 25 sd prefix or VI subdwarfs HD 149382 sdB5 or B5VI 29 D prefix or VII white dwarfs c van Maanen 2 DZ8 30 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 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 Nominal luminosity class VII and sometimes higher numerals is now rarely used for white dwarf or hot sub dwarf classes since the temperature letters of the main sequence and giant stars no longer apply to white dwarfs Occasionally letters a and b are applied to luminosity classes other than supergiants for example a giant star slightly less luminous than typical may be given a luminosity class of IIIb while a luminosity class IIIa indicates a star slightly brighter than a typical giant 31 A sample of extreme V stars with strong absorption in He II l4686 spectral lines have been given the Vz designation An example star is HD 93129 B 32 Spectral peculiarities Edit Additional nomenclature in the form of lower case letters can follow the spectral type to indicate peculiar features of the spectrum 33 Code Spectral peculiarities for stars uncertain spectral value 21 Undescribed spectral peculiarities exist Special peculiaritycomp Composite spectrum 34 e Emission lines present 34 e Forbidden emission lines presenter Reversed center of emission lines weaker than edgeseq Emission lines with P Cygni profilef N III and He II emission 21 f N IV 4058A is stronger than the N III 4634A 4640A amp 4642A lines 35 f Si IV 4089A amp 4116A are emitted in addition to the N III line 35 f C III 4647 4650 4652A emission lines with comparable strength to the N III line 36 f N III emission absence or weak absorption of He II f 37 f Displays strong He II absorption accompanied by weak N III emissions 38 f 37 h WR stars with hydrogen emission lines 39 ha WR stars with hydrogen seen in both absorption and emission 39 He wk Weak Helium linesk Spectra with interstellar absorption featuresm Enhanced metal features 34 n Broad nebulous absorption due to spinning 34 nn Very broad absorption features 21 neb A nebula s spectrum mixed in 34 p Unspecified peculiarity peculiar star d 34 pq Peculiar spectrum similar to the spectra of novaeq P Cygni profiless Narrow sharp absorption lines 34 ss Very narrow linessh Shell star features 34 var Variable spectral feature 34 sometimes abbreviated to v wl Weak lines 34 also w amp wk Elementsymbol Abnormally strong spectral lines of the specified element s 34 z indicating abnormally an strong ionised helium line at 468 6 nm 32 For example 59 Cygni is listed as spectral type B1 5Vnne 40 indicating a spectrum with the general classification B1 5V as well as very broad absorption lines and certain emission lines Guide for Secchi spectral types 152 Schjellerup is Y Canum Venaticorum History EditThe 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 During the 1860s and 1870s pioneering stellar spectroscopist Angelo Secchi created the Secchi classes in order to classify observed spectra By 1866 he had developed three classes of stellar spectra shown in the table below 41 42 43 In the late 1890s this classification began to be superseded by the Harvard classification which is discussed in the remainder of this article 44 45 46 Class number Secchi class descriptionSecchi 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 Secchi class I Orion subtype A subtype of Secchi class I with narrow lines in place of wide bands such as Rigel and Bellatrix In modern terms this corresponds to early B type starsSecchi 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 Secchi class III Orange to red stars with complex band spectra such as Betelgeuse and Antares This corresponds to the modern class M Secchi class IV In 1868 he discovered carbon stars which he put into a distinct group 47 Red stars with significant carbon bands and lines corresponding to modern classes C and S Secchi class V In 1877 he added a fifth class 48 Emission line stars such as Gamma Cassiopeiae and Sheliak which are in modern class Be In 1891 Edward Charles Pickering proposed that class V should correspond to the modern class O which then included Wolf Rayet stars and stars within planetary nebulae 49 The Roman numerals used for Secchi classes should not be confused with the completely unrelated Roman numerals used for Yerkes luminosity classes and the proposed neutron star classes Draper system Edit Classifications in the Draper Catalogue of Stellar Spectra 50 51 Secchi Draper CommentI A B C D Hydrogen lines dominantII E F G H I K LIII MIV N Did not appear in the catalogueV O Included Wolf Rayet spectra with bright linesV P Planetary nebulae Q Other spectraClasses carried through into the MK system are in bold 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 and was credited with classifying over 10 000 featured stars and discovering 10 novae and more than 200 variable stars 52 With the help of the Harvard computers especially Williamina Fleming the first iteration of the Henry Draper catalogue was devised to replace the Roman numeral scheme established by Angelo Secchi 53 The catalogue used a scheme in which the previously used Secchi classes I to V were subdivided into more specific classes given letters from A to P Also the letter Q was used for stars not fitting into any other class 50 51 Fleming worked with Pickering to differentiate 17 different classes based on the intensity of hydrogen spectral lines which causes variation in the wavelengths emanated from stars and results in variation in color appearance The spectra in class A tended to produce the strongest hydrogen absorption lines while spectra in class O produced virtually no visible lines The lettering system displayed the gradual decrease in hydrogen absorption in the spectral classes when moving down the alphabet This classification system was later modified by Annie Jump Cannon and Antonia Maury to produce the Harvard spectral classification scheme 52 54 The old Harvard system 1897 Edit In 1897 another astronomer 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 XXII 55 56 Summary of 1897 Harvard system 57 Groups SummaryI V included Orion type stars that displayed an increasing strength in hydrogen absorption lines from group I to group VVI acted as an intermediate between the Orion type and Secchi type I groupVII XI were Secchi s type 1 stars with decreasing strength in hydrogen absorption lines from groups VII XIXIII XVI included Secchi type 2 stars with decreasing hydrogen absorption lines and increasing solar type metallic linesXVII XX included Secchi type 3 stars with increasing spectral linesXXI included Secchi type 4 starsXXII included Wolf Rayet starsBecause the 22 Roman numeral groupings didn t account for additional variations in spectra three additional divisions were made to further specify differences Lowercase letters were added to differentiate relative line appearance in spectra the lines were defined as 57 Harvard 1897 Subtypes 57 a average width b hazy c sharpAntonia Maury published her own stellar classification catalogue in 1897 called Spectra of Bright Stars Photographed with the 11 inch Draper Telescope as Part of the Henry Draper Memorial which included 4 800 photographs and Maury s analyses of 681 bright northern stars This was the first instance in which a woman was credited for an observatory publication 58 The current Harvard system 1912 Edit In 1901 Annie Jump Cannon returned to the lettered types but dropped all letters except O B A F G K M and N used in that order as well as P for planetary nebulae and Q for some peculiar spectra She also used types such as B5A for stars halfway between types B and A F2G for stars one fifth of the way from F to G and so on 59 60 Finally by 1912 Cannon had changed the types B A B5A F2G etc to B0 A0 B5 F2 etc 61 62 This is essentially the modern form of the Harvard classification system This system was developed through the analysis of spectra on photographic plates which could convert light emanated from stars into a readable spectrum 63 Mount Wilson classes Edit Proper motion of stars of early type in 200 000 years A luminosity classification known as the Mount Wilson system was used to distinguish between stars of different luminosities 64 65 66 This notation system is still sometimes seen on modern spectra 67 Class Meaningsd Subdwarfd Dwarfsg Subgiantg Giantc Supergiant The movement of stars of late type around the apex left and antapex right in 200 000 yearsSpectral types Edit Spectral type redirects here For spectral types of asteroids see Asteroid spectral types The stellar classification system is taxonomic based on type specimens similar to classification of species in biology The categories are defined by one or more standard stars for each category and sub category with an associated description of the distinguishing features 68 Early and late nomenclature Edit Stars are often referred to as early or late types Early is a synonym for hotter while late is a synonym for cooler Depending on the context early and late may be absolute or relative terms Early as an absolute term would therefore refer to O or B and possibly A stars As a relative reference it relates to stars hotter than others such as early K being perhaps K0 K1 K2 and K3 Late is used in the same way with an unqualified use of the term indicating stars with spectral types such as K and M but it can also be used for stars that are cool relative to other stars as in using late G to refer to G7 G8 and G9 In the relative sense early means a lower Arabic numeral following the class letter and late means a higher number This obscure terminology is a hold over from a late nineteenth century model of stellar evolution which supposed that stars were powered by gravitational contraction via the Kelvin Helmholtz mechanism which is now known to not apply to main sequence stars If that were true then stars would start their lives as very hot early type stars and then gradually cool down into late type stars This mechanism provided ages of the Sun that were much smaller than what is observed in the geologic record and was rendered obsolete by the discovery that stars are powered by nuclear fusion 69 The terms early and late were carried over beyond the demise of the model they were based on Class O Edit Main article O type star See also O type main sequence star Blue giant and Blue supergiant The spectrum of an O5V star O type stars are very hot and extremely luminous with most of their radiated output in the ultraviolet range These are the rarest of all main sequence stars About 1 in 3 000 000 0 00003 of the main sequence stars in the solar neighborhood are O type stars e 9 Some of the most massive stars lie within this spectral class O type stars frequently have complicated surroundings that make measurement of their spectra difficult O type spectra formerly were defined by the ratio of the strength of the He II l4541 relative to that of He I l4471 where l is the radiation wavelength 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 l4058 to N III ll4634 40 42 70 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 Higher mass O type stars do not retain extensive atmospheres due to the extreme velocity of their stellar wind which may reach 2 000 km s Because they are so massive O type stars have very hot cores and burn through their hydrogen fuel very quickly so they are the first stars to leave the main sequence When the MKK classification scheme was first described in 1943 the only subtypes of class O used were O5 to O9 5 71 The MKK scheme was extended to O9 7 in 1971 72 and O4 in 1978 73 and new classification schemes that add types O2 O3 and O3 5 have subsequently been introduced 74 Spectral standards 68 O7V S Monocerotis O9V 10 LacertaeClass B Edit See also B type main sequence star Blue giant and Blue supergiant B class stars in the Jewel Box cluster Credit ESO VLT B type stars are very luminous and blue Their spectra have neutral helium lines which are most prominent at the B2 subclass and moderate hydrogen lines As O and B type stars are so energetic they only live for a relatively short time Thus due to the low probability of kinematic interaction during their lifetime they are unable to stray far from the area in which they formed apart from runaway stars The transition from class O to class B was originally defined to be the point at which the He II l4541 disappears However with modern equipment the line is still apparent in the early B type stars Today for main sequence stars the B class is instead defined by the intensity of the He I violet spectrum with the maximum intensity corresponding to class B2 For supergiants lines of silicon are used instead the Si IV l4089 and Si III l4552 lines are indicative of early B At mid B the intensity of the latter relative to that of Si II ll4128 30 is the defining characteristic while for late B it is the intensity of Mg II l4481 relative to that of He I l4471 70 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 main sequence stars e 9 Due to them being rare the closest B type star is Regulus at around 80 light years 75 Massive yet non supergiant entities known as Be stars are main sequence stars that notably have or had at some time one or more Balmer lines in emission with the hydrogen related electromagnetic radiation series projected out by the stars being of particular interest Be stars are generally thought to feature unusually strong stellar winds high surface temperatures and significant attrition of stellar mass as the objects rotate at a curiously rapid rate 76 Objects known as B e or B e stars possess distinctive neutral or low ionisation emission lines that are considered to have forbidden mechanisms undergoing processes not normally allowed under current understandings of quantum mechanics Spectral standards 68 B0V Upsilon Orionis B0Ia Alnilam B2Ia Chi2 Orionis B2Ib 9 Cephei B3V Eta Ursae Majoris B3V Eta Aurigae B3Ia Omicron2 Canis Majoris B5Ia Eta Canis Majoris B8Ia RigelClass A Edit See also A type main sequence star Class A Vega left compared to the Sun right 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 e 9 which includes 9 stars within 15 parsecs 77 Spectral standards 68 A0Van Gamma Ursae Majoris A0Va Vega A0Ib Eta Leonis A0Ia HD 21389 A1V Sirius A A2Ia Deneb A3Va Fomalhaut Canopus an A type supergiant and the second brightest star in the night sky Class F Edit See also F type main sequence star F type stars have strengthening spectral lines H and K 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 e 9 including 12 stars within 10 pc citation needed Spectral standards 68 F0IIIa Zeta Leonis F0Ib Alpha Leporis F2V 78 Ursae MajorisClass G Edit G star redirects here For other uses see G star disambiguation See also G type main sequence star Yellow supergiant and Yellow hypergiant The Sun a G2 main sequence star with dark sunspots G type stars including the Sun 12 have prominent spectral lines H and K 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 CN molecules Class G main sequence stars make up about 7 5 nearly one in thirteen of the main sequence stars in the solar neighborhood There are 21 G type stars within 10pc e 9 Class G contains the Yellow Evolutionary Void 78 Supergiant stars often swing between O or B blue and K or M red While they do this they do not stay for long in the unstable yellow supergiant class Spectral standards 68 G0V Beta Canum Venaticorum G0IV Eta Bootis G0Ib Beta Aquarii G2V Sun G5V Kappa1 Ceti G5IV Mu Herculis G5Ib 9 Pegasi G8V 61 Ursae Majoris G8IV Beta Aquilae G8IIIa Kappa Geminorum G8IIIab Epsilon Virginis G8Ib Epsilon GeminorumClass K Edit See also K type main sequence star K type star redirects here For the Korean nuclear fusion project see KSTAR Arcturus a K1 5 giant compared to the Sun and Antares K type stars are orangish stars that are slightly cooler than the Sun They make up about 12 of the main sequence stars in the solar neighborhood e 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 those are present at all and mostly neutral metals Mn I Fe I Si I By late K molecular bands of titanium oxide become present Mainstream theories those rooted in lower harmful radioactivity and star longevity would thus suggest such stars have the optimal chances of heavily evolved life developing on orbiting planets if such life is directly analogous to earth s due to a broad habitable zone yet much lower harmful periods of emission compared to those with the broadest such zones 79 80 Spectral standards 68 K0V Sigma Draconis K0III Pollux K0III Epsilon Cygni K2V Epsilon Eridani K2III Kappa Ophiuchi K3III Rho Bootis K5V 61 Cygni A K5III Gamma DraconisClass M Edit See also Red dwarf Red giant and Red 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 e f 9 However class M main sequence stars red dwarfs have such low luminosities that none are bright enough to be seen with the unaided eye unless under exceptional conditions The brightest known M class main sequence star is Lacaille 8760 class M0V with magnitude 6 7 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 most of the largest known supergiant stars in the Milky Way are class M stars such as VV Cephei Antares and Betelgeuse Furthermore some larger hotter brown dwarfs are late class M usually in the range of M6 5 to M9 5 The spectrum of a class M star contains lines from oxide molecules in the visible spectrum especially TiO 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 II oxide bands become present by late M Spectral standards 68 M0IIIa Beta Andromedae M2III Chi Pegasi M1 M2Ia Iab Betelgeuse M2Ia Mu Cephei Herschel s garnet Extended spectral types EditA number of new spectral types have been taken into use from newly discovered types of stars 81 Hot blue emission star classes Edit UGC 5797 an emission line galaxy where massive bright blue stars are formed 82 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 Once included as type O stars the Wolf Rayet stars of class W or WR are 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 subclasses according to the relative strength of nitrogen and carbon emission lines in their spectra and outer layers 39 WR spectra range is listed below 83 84 WN 39 spectrum dominated by N III V and He I 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 39 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 C IV lines intermediate between WN and WC stars 39 WC 39 spectrum with strong C II IV lines WCE WC4 to WC6 hotter or early WCL WC7 to WC9 cooler or late WO WO1 to WO4 strong O VI lines extremely rare extension of the WCE class into incredibly hot temperatures up to 200 kK or more Although the central stars of most planetary nebulae CSPNe show O type spectra 85 around 10 are hydrogen deficient and show WR spectra 86 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 Slash stars Edit Main article Slash star The slash stars are O type stars with WN like lines in their spectra The name slash comes from their printed spectral type having a slash in it e g Of WNL 70 There is a secondary group found with these spectra a cooler intermediate group designated Ofpe WN9 70 These stars have also been referred to as WN10 or WN11 but that has become less popular with the realisation of the evolutionary difference from 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 87 Magnetic O stars Edit They are O stars with strong magnetic fields Designation is Of p 70 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 visible spectrum 88 Brown dwarfs stars that do not undergo hydrogen fusion 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 an unresolvable overlap between spectral types effective temperature and luminosity for some masses and ages of different L T Y types no distinct temperature or luminosity values can be given 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 89 90 91 Due to low surface gravity in giant stars TiO and VO bearing condensates never form Thus L type stars larger than dwarfs can never form in an isolated environment However it may be possible for these L type supergiants to form through stellar collisions 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 550 and 1 300 K 277 and 1 027 C 530 and 1 880 F Their emission peaks in the infrared Methane is prominent in their spectra 89 90 Study of the number of proplyds protoplanetary disks 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 was previously conjectured It is theorized that these proplyds are in a race with each other The first one to form will become a protostar 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 92 Class Y Edit See also Sub brown dwarf and Substellar object 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 93 Although such dwarfs have been modelled 94 and detected within forty light years by the Wide field Infrared Survey Explorer WISE 81 95 96 97 98 there is no well defined spectral sequence yet and no prototypes Nevertheless several objects have been proposed as spectral classes Y0 Y1 and Y2 99 The spectra of these prospective Y objects display absorption around 1 55 micrometers 100 Delorme et al have suggested that this feature is due to absorption from ammonia and that this should be taken as the indicative feature for the T Y transition 100 101 In fact this ammonia absorption feature is the main criterion that has been adopted to define this class 99 However this feature is difficult to distinguish from absorption by water and methane 100 and other authors have stated that the assignment of class Y0 is premature 102 The latest brown dwarf proposed for the Y spectral type WISE 1828 2650 is a gt Y2 dwarf with an effective temperature originally estimated around 300 K the temperature of the human body 95 96 103 Parallax measurements have however since shown that its luminosity is inconsistent with it being colder than 400 K The coolest Y dwarf currently known is WISE 0855 0714 with an approximate temperature of 250 K and a mass just seven times that of Jupiter 104 The mass range for Y dwarfs is 9 25 Jupiter masses but young objects might reach below one Jupiter mass although they cool to become planets which means that Y class objects straddle the 13 Jupiter mass deuterium fusion limit that marks the current IAU division between brown dwarfs and planets 99 Peculiar brown dwarfs Edit Symbols used for peculiar brown dwarfspec This suffix stands for peculiar e g L2pec 105 sd This prefix e g sdL0 stands for subdwarf and indicates a low metallicity and blue color 106 b Objects with the beta b suffix e g L4b have an intermediate surface gravity 107 g Objects with the gamma g suffix e g L5g have a low surface gravity 107 red The red suffix e g L0red indicates objects without signs of youth but high dust content 108 blue The blue suffix e g L3blue indicates unusual blue near infrared colors for L dwarfs without obvious low metallicity 109 Young brown dwarfs have low surface gravities because they have larger radii and lower masses compared to the field stars of similar spectral type These sources are marked by a letter beta b for intermediate surface gravity and gamma g for low surface gravity Indication for low surface gravity are weak CaH KI and NaI lines as well as strong VO line 107 Alpha a stands for normal surface gravity and is usually dropped Sometimes an extremely low surface gravity is denoted by a delta d 109 The suffix pec stands for peculiar The peculiar suffix is still used for other features that are unusual and summarizes different properties indicative of low surface gravity subdwarfs and unresolved binaries 110 The prefix sd stands for subdwarf and only includes cool subdwarfs This prefix indicates a low metallicity and kinematic properties that are more similar to halo stars than to disk stars 106 Subdwarfs appear bluer than disk objects 111 The red suffix describes objects with red color but an older age This is not interpreted as low surface gravity but as a high dust content 108 109 The blue suffix describes objects with blue near infrared colors that cannot be explained with low metallicity Some are explained as L T binaries others are not binaries such as 2MASS J11263991 5003550 and are explained with thin and or large grained clouds 109 Late giant carbon star classes Edit Carbon stars are stars whose spectra indicate production of carbon a byproduct of triple alpha helium 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 Equivalent classes for carbon rich stars are S and C The giants among those stars are presumed to produce this carbon themselves but some stars in this class are double stars whose odd atmosphere is suspected of having been transferred from a companion that is now a white dwarf when the companion was a carbon star 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 C J type stars which are characterized by the strong presence of molecules of 13CN in addition to those of 12CN 112 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 its own class R representing the carbon star equivalent of late G to early K type stars C N Formerly its own class representing the carbon star equivalent of late K to M type 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 form a continuum between class M stars and carbon stars Those most similar to class M stars have strong ZrO absorption bands analogous to the TiO bands of class M stars whereas those most similar to carbon stars have strong sodium D lines and weak C2 bands 113 Class S stars have excess amounts of zirconium and other elements produced by the s process and have more similar carbon and oxygen abundances than class M or carbon stars Like carbon stars nearly all known class S stars are asymptotic giant branch stars The spectral type is formed by the letter S and a number between zero and ten This number corresponds to the temperature of the star and approximately follows the temperature scale used for class M giants The most common types are S3 to S5 The non standard designation S10 has only been used for the star Chi Cygni when at an extreme minimum The basic classification is usually followed by an abundance indication following one of several schemes S2 5 S2 5 S2 Zr4 Ti2 or S2 5 A number following a comma is a scale between 1 and 9 based on the ratio of ZrO and TiO A number following a slash is a more recent but less common scheme designed to represent the ratio of carbon to oxygen on a scale of 1 to 10 where a 0 would be an MS star Intensities of zirconium and titanium may be indicated explicitly Also occasionally seen is a number following an asterisk which represents the strength of the ZrO bands on a scale from 1 to 5 Classes MS and SC Intermediate carbon related classes Edit In between the M and S classes border cases are named MS stars In a similar way border cases between the S and C N classes are named SC or CS The sequence M MS S SC C N is hypothesized to be a sequence of increased carbon abundance with age for carbon stars in the asymptotic giant branch White dwarf classifications Edit Main article White dwarf spectroscopy 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 114 115 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 For example DA1 5 for IK Pegasi B 114 116 Two or more of the type letters may be used to indicate a white dwarf that displays more than one of the spectral features above 114 Extended white dwarf spectral types Edit Sirius A and B a white dwarf of type DA2 resolved by Hubble 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 dwarfA different set of spectral peculiarity symbols are used for white dwarfs than for other types of stars 114 Code Spectral peculiarities for starsP Magnetic white dwarf with detectable polarizationE Emission lines presentH Magnetic white dwarf without detectable polarizationV VariablePEC Spectral peculiarities existNon stellar spectral types Classes P and Q Edit Finally the classes P and Q are left over from the system developed by Cannon for the Henry Draper Catalogue They are occasionally used for certain non stellar objects Type P objects are stars within planetary nebulae typically young white dwarfs or hydrogen poor M giants type Q objects are novae citation needed Stellar remnants EditMain articles Neutron star Black hole and Exotic star Stellar remnants are objects associated with the death of stars Included in the category are white dwarfs and as can be seen from the radically different classification scheme for class D non stellar objects are difficult to fit into the MK system The Hertzsprung Russell diagram which the MK system is based on is observational in nature so these remnants cannot easily be plotted on the diagram or cannot be placed at all Old neutron stars are relatively small and cold and would fall on the far right side of the diagram Planetary nebulae are dynamic and tend to quickly fade in brightness as the progenitor star transitions to the white dwarf branch If shown a planetary nebula would be plotted to the right of the diagram s upper right quadrant A black hole emits no visible light of its own and therefore would not appear on the diagram 117 A classification system for neutron stars using Roman numerals has been proposed type I for less massive neutron stars with low cooling rates type II for more massive neutron stars with higher cooling rates and a proposed type III for more massive neutron stars possible exotic star candidates with higher cooling rates 118 The more massive a neutron star is the higher neutrino flux it carries These neutrinos carry away so much heat energy that after only a few years the temperature of an isolated neutron star falls from the order of billions to only around a million Kelvin This proposed neutron star classification system is not to be confused with the earlier Secchi spectral classes and the Yerkes luminosity classes Replaced spectral classes EditSeveral spectral types all previously used for non standard stars in the mid 20th century have been replaced during revisions of the stellar classification system They may still be found in old editions of star catalogs R and N have been subsumed into the new C class as C R and C N Stellar classification habitability and the search for life EditSee also Planetary habitability While humans may eventually be able to colonize any kind of stellar habitat this section will address the probability of life arising around other stars Stability luminosity and lifespan are all factors in stellar habitability We only know of one star that hosts life and that is our own a G class star with an abundance of heavy elements and low variability in brightness It is also unlike many stellar systems in that it only has one star in it see Habitability of binary star systems Working from these constraints and the problems of having an empirical sample set of only one 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 our Sun spectral type M are likely to tidally lock planets within their habitable zone along with other problems see Habitability of red dwarf systems 119 While there are many problems facing life on red dwarfs many astronomers continue to model these systems due to their sheer numbers and longevity For these reasons NASA s Kepler Mission is searching for habitable planets at nearby main sequence stars that are less massive than spectral type A but more massive than type M making the most probable stars to host life dwarf stars of types F G and K 119 See also EditAstrograph Type of telescope Guest star Ancient Chinese name for cataclysmic variable stars Spectral signature Variation of reflectance or emittance of a material with respect to wavelengths Star count survey of stars Stellar dynamicsExplanatory notes Edit This is the relative color of the star if Vega generally considered a bluish star is used as a standard for white Chromaticity can vary significantly within a class for example the Sun a G2 star is white while a G9 star is yellow Technically white dwarfs are no longer live stars but rather the dead remains of extinguished stars Their classification uses a different set of spectral types from element burning live stars When used with A type stars this instead refers to abnormally strong metallic spectral lines a b c d e f g These proportions are fractions of stars 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2002 The cooling neutron star in 3C 58 Astronomy amp Astrophysics 389 L24 L27 arXiv astro ph 0204233 Bibcode 2002A amp A 389L 24Y doi 10 1051 0004 6361 20020699 S2CID 6247160 a b Stars and Habitable Planets www solstation com External links Edit Look up late type star or early type star in Wiktionary the free dictionary Libraries of stellar spectra by D Montes UCM Spectral Types for Hipparcos Catalogue Entries Stellar Spectral Classification by Richard O Gray and Christopher J Corbally Spectral models of stars by P Coelho Merrifield Michael Bauer Amanda Haussler Boris 2010 Star Classification Sixty Symbols Brady Haran for the University of Nottingham Stellar classification table Portals Physics Astronomy Stars Outer space Retrieved from https en wikipedia org w index php title Stellar classification amp oldid 1133143963 Class K, wikipedia, wiki, book, books, library,

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