Stellar classification

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Hertzsprung–Russell diagram

Spectral type

Brown dwarfs

White dwarfs

Red dwarfs

Subdwarfs

Main sequence
("dwarfs")

Subgiants

Giants

Bright giants

Supergiants

Hypergiants

absolute

magni-

tude

(M_V)

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

Most stars are currently classified under the Morgan–Keenan (MKK) system using the letters O, B, A, F, G, K, M, L, T and Y, a sequence from the hottest (O type) to the coolest (Y type). The types R and N are carbon-based stars, and the type S is zirconium-monoxide-based stars. Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. A8, A9, F0, F1 form a sequence from hotter to cooler).

In the MKK 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 stars for hypergiants, class I stars for supergiants, class II for bright giants, class III for regular giants, class IV for sub-giants, class V for main-sequence stars, class VI for sub-dwarfs, class VII for white dwarfs, and class VIII for brown dwarfs. The full spectral class for the Sun is then G2V, indicating a main-sequence star with a temperature around 5,800K.

Harvard spectral classification

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

Class	Effective temperature[1][2][3]	Conventional color description	Actual apparent color[4][5][6]	Mass[1][7] (solar masses)	Radius[1][7] (solar radii)	Luminosity[1][7] (bolometric)	Hydrogen lines	Fraction of all main-sequence stars[8]
O	≥ 30,000 K	blue	blue	≥ 16 M☉	≥ 6.6 R☉	≥ 30,000 L☉	Weak	~0.00003%
B	10,000–30,000 K	blue white	deep blue white	2.1–16 M☉	1.8–6.6 R☉	25–30,000 L☉	Medium	0.13%
A	7,500–10,000 K	white	blue white	1.4–2.1 M☉	1.4–1.8 R☉	5–25 L☉	Strong	0.6%
F	6,000–7,500 K	yellow white	white	1.04–1.4 M☉	1.15–1.4 R☉	1.5–5 L☉	Medium	3%
G	5,200–6,000 K	yellow	yellowish white	0.8–1.04 M☉	0.96–1.15 R☉	0.6–1.5 L☉	Weak	7.6%
K	3,700–5,200 K	orange	pale yellow orange	0.45–0.8 M☉	0.7–0.96 R☉	0.08–0.6 L☉	Very weak	12.1%
M	2,400–3,700 K	red	light orange red	0.08–0.45 M☉	≤ 0.7 R☉	≤ 0.08 L☉	Very weak	76.45%
L	1,300–2,400 K	red brown[citation needed]	scarlet[citation needed]	0.005–0.08 M☉	0.08–0.15 R☉	0.000,05–0.001 L☉	Extremely weak
T	500–1,300 K	brown[citation needed]	magenta[9][10]	0.001–0.07 M☉	0.08–0.14 R☉	0.000,001–0.000,05 L☉	Extremely weak
Y	≤ 500 K	dark brown[citation needed]	black[citation needed]	0.0005–0.02 M☉	0.08–0.14 R☉	0.000,000,1–0.000,001 L☉	Extremely weak

Notes:

• The conventional color description takes into account only the peak of the stellar spectrum. However, in actuality stars radiate in all parts of the spectrum, and because all spectral colors combined appear white, the actual apparent colors the human eye would observe are lighter than the conventional color descriptions. See Conventional and apparent colors.

• The mass, radius, and luminosity shown in the table for each class are appropriate only for stars on the main-sequence portion of their lives. The temperatures of giant and supergiant stars of the same spectral type are somewhat different, whereas the radius and luminosity can be dramatically different.

The spectral classes O through Y are subdivided by Arabic numerals (0–9), which can be further divided into half-subtypes. For example, A0 denotes the hottest stars in the A class and A9 denotes the coolest ones. The Sun is classified as G2. So far however, only Y dwarfs of subtypes 0–≈2 have been discovered.^[11]

O, B, and A stars are sometimes called "early type″, whereas K and M stars are said to be "late type″. This stems from an early 20th-century model of stellar evolution in which stars were powered by gravitational contraction via the Kelvin–Helmholtz mechanism whereby stars start their lives as very hot "early-type" stars, and then gradually cool down, evolving into "late-type″ stars. This mechanism provided ages of the Sun that were much smaller than what is observed, and was rendered obsolete by the discovery that stars are powered by nuclear fusion — these span spectral classes down to late L. However, brown dwarfs, whose energy comes from gravitational attraction alone, cool as they age and so progress to later spectral types. Brown dwarfs start their lives with M-type spectra and will cool through the L, T, and Y spectral classes; the faster the less massive they are — the highest-mass brown dwarfs can not have cooled to Y or even T dwarfs within the age of the universe . As this leads to a degeneracy between mass and age for given effective temperature and luminosity, no unique values can be assigned to a given spectral type.^[7]

Spectrum images by Harvard spectral class

Yerkes spectral classification

The Morgan–Keenan spectral classification

The Yerkes spectral classification, also called the MKK system from the authors' initials, is a system of stellar spectral classification introduced in 1943 by William Wilson Morgan, Philip C. Keenan, and Edith Kellman from Yerkes Observatory.^[12] This two-dimensional (temperature and luminosity) classification scheme is based on spectral lines sensitive to stellar temperature and surface gravity which is related to luminosity (whilst the Harvard classification is based on surface temperature only). Later, in 1953, after some revisions of list of standard stars and classification criteria, the scheme was named MK (by William Wilson Morgan and Philip C. Keenan's initials).^[13]

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:

• 0, Ia-0, Ia⁺ hypergiants or extremely luminous supergiants (later addition)), Example: Eta Carinae (spectrum-peculiar)
• Ia (luminous supergiants), Example: Deneb (spectrum is A2 Ia)
• Iab (intermediate luminous supergiants), Example: Betelgeuse (spectrum is M2 Iab)
• Ib (less luminous supergiants), Example:
• II bright giants, Example: β Scuti (HD 173764) (spectrum is G5 II)
• III normal giants, Example: ρ Persei (spectrum is O7.5 III(n)((f)))
• IV subgiants, Example: ε Reticuli (spectrum is K1–2 IV)
• V main-sequence stars (dwarfs), Example: AD Leonis (spectrum M3.5e V)
• VI subdwarfs, Example: SSSPM J1930-4311 (spectrum sdM7)
• VII (uncommon) white dwarfs. White dwarfs are represented with a prescript wD or WD.

Marginal cases are allowed; for instance a star classified as Ia-0 would be a very luminous supergiant, verging on hypergiant. Examples are below. The spectral type of the star is not a factor.

Marginal symbols	Example	Explanation
-	G2 I-II	A star is between supergiant and bright giant.
+	O9.5 Ia+	A star is a hypergiant star.
/	F2 IV/V	A star is either a subgiant or a dwarf star.

Spectral peculiarities

Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum.^[14]

Code	Spectral peculiarities for stars
:	Blending and/or uncertain spectral value
...	Undescribed spectral peculiarities exist
!	Special peculiarity
comp	Composite spectrum
e	Emission lines present
[e]	"Forbidden" emission lines present
er	"Reversed" center of emission lines weaker than edges
ep	Emission lines with peculiarity
eq	Emission lines with P Cygni profile
ev	Spectral emission that exhibits variability
f	N III and He II emission (for element name followed by Roman numeral see spectral line)
f*	NIV λ4058Å is stronger than the NIII λ4634Å, λ4640Å, & λ4642Å lines[15]
f+	SiIV λ4089Å & λ4116Å are emission in addition to the NIII line[15]
(f)	N III emission, absence or weak absorption of He II
(f+)	[16]
((f))	Displays strong HeII absorption accompanied by weak NIII emissions[17]
((f*))	[16]
h	WR stars with emission lines due to hydrogen.[18]
ha	WR stars with hydrogen emissions seen on both absorption and emission.[18]
He wk	Weak He lines
k	Spectra with interstellar absorption features
m	Enhanced metal features
n	Broad ("nebulous") absorption due to spinning
nn	Very broad absorption features due to spinning very fast[19]
neb	A nebula's spectrum mixed in
p	Unspecified peculiarity, peculiar star.
pq	Peculiar spectrum, similar to the spectra of novae
q	Red & blue shifts line present
s	Narrowly "sharp" absorption lines
ss	Very narrow lines
sh	Shell star features
v	Variable spectral feature (also "var")
w	Weak lines (also "wl" & "wk")
Code	Abnormal CNO features[20]
C	C absorption enhanced, N deficient
N	N absorption enhanced, C and O deficient
Code	If spectrum shows enhanced metal features
Ba	Abnormally strong Barium
Ca	Abnormally strong Calcium
Cr	Abnormally strong Chromium
Eu	Abnormally strong Europium
He	Abnormally strong Helium
Hg	Abnormally strong Mercury
Mn	Abnormally strong Manganese
Si	Abnormally strong Silicon
Sr	Abnormally strong Strontium
Tc	Technetium is present
Code	Spectral peculiarities for white dwarfs
P	Magnetic white dwarf with detectable polarization
E	Emission lines present
H	Magnetic white dwarf without detectable polarization
V	Variable
PEC	Spectral peculiarities exist

For example, Epsilon Ursae Majoris is listed as spectral type A0pCr, indicating general classification A0 with strong emission lines of the element chromium. There are several common classes of chemically peculiar stars, where the spectral lines of a number of elements appear abnormally strong.

Conventional and apparent colors

The conventional color descriptions are traditional in astronomy, and represent colors relative to the mean color of an A-class star which is considered to be white. The apparent color^[4] 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. The table colors used are D65 standard colors calculated for the midpoint of each spectral class (e.g. G5 or A5) and for main-sequence stars.^[6] These are accurate representations of the actual color of the disk of a star. 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. Even the brightest stars do not have strong colors, and within color systems such CIE all blackbody colors fall within a central "white" area of unsaturated colors. Of course most people have never seen an M-class main-sequence star, certainly not bright enough to notice color. 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, although they cannot be accurately reproduced on an RGB monitor. For example,

Cool red dwarf M5V

Betelgeuse M2Ia

Y CVn C5

The Sun itself is white and approximates a black body of 5780 K (see color temperature). It is sometimes called a yellow star, spectroscopically relative to Vega, may appear yellow or red when viewed through the atmosphere, or appear white if viewed when too bright for the eye to see any color. This is a natural consequence of the evolution of human optical senses: the response curve that maximizes the overall efficiency against solar illumination will by definition perceive the Sun as white although there is some subjective variation between observers. The D65 standard closely mimics the spectrum of the Sun although it is intended to represent a 6500K blackbody light source. The color of the Sun is subtly different from D65 and is a faint peachy shade relative to it.^[21]
There are a few apparently green stars, but this is an optical illusion caused by the star being close to a bright red star.

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 Father Angelo Secchi created the Secchi classes in order to classify observed spectra. By 1866, he had developed three classes of stellar spectra:^[22][23][24]

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

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

• Class II: yellow stars—hydrogen less strong, but evident metallic lines, such as the Sun, Arcturus, and Capella. This includes the modern classes G and K as well as late class F.
• Class III: orange to red stars with complex band spectra, such as Betelgeuse and Antares. This corresponds to the modern class M.

In 1868, he discovered carbon stars, which he put into a distinct group:^[25]

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

In 1877, he added a fifth class:^[26]

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

In the late 1890s, this classification began to be superseded by the Harvard classification, which is discussed in the remainder of this article.^[27][28][29]

Draper system

Classifications in the Draper Catalogue of Stellar Spectra^[30][31]

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.
	O	Wolf–Rayet spectra with bright lines.
	P	Planetary nebulae.
	Q	Other spectra.

In the 1880s, the astronomer Edward C. Pickering began to make a survey of stellar spectra at the Harvard College Observatory, using the objective-prism method. A first result of this work was the Draper Catalogue of Stellar Spectra, published in 1890. Williamina Fleming classified most of the spectra in this catalogue. It used a scheme in which the previously used Secchi classes (I to IV) were divided into more specific classes, given letters from A to N. Also, the letters O, P and Q were used, O for stars whose spectra consisted mainly of bright lines, P for planetary nebulae, and Q for stars not fitting into any other class.^[30][31]

Harvard system

In 1897, another worker at Harvard, Antonia Maury, placed the Orion subtype of Secchi class I ahead of the remainder of Secchi class I, thus placing the modern type B ahead of the modern type A. She was the first to do so, although she did not use lettered spectral types, but rather a series of twenty-two types numbered from I to XXII.^[32][33]

In 1901, Annie Jump Cannon returned to the lettered types, but dropped all letters except O, B, A, F, G, K, and M, used in that order, as well as P for planetary nebulae and Q for some peculiar spectra. She also used types such as B5A for stars halfway between types B and A, F2G for stars one-fifth of the way from F to G, and so forth.^[34][35] Finally, by 1912, Cannon had changed the types B, A, B5A, F2G, etc. to B0, A0, B5, F2, etc.^[36][37] This is essentially the modern form of the Harvard classification system.

Useful mnemonics for remembering the spectral type letters are "Oh, Be A Fine Guy/Girl, Kiss Me" or "Oh Boy, An F Grade Kills Me". To also include the colder spectral classes L, T and Y, the first mnemonic can be extended by adding "Love To You."

Modern interpretation

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

The fact that the Harvard classification of a star indicated its surface or photospheric temperature (or more precisely, its effective temperature) was not fully understood until after its development, though by the time the first Hertzsprung–Russell diagram was formulated (by 1914), this was generally suspected to be true.^[38] 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.^[39] The Harvard astronomer Cecilia Helena Payne (later to become Cecilia Payne-Gaposchkin) then demonstrated that the OBAFGKM spectral sequence is actually a sequence in temperature.^[40] 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.

Spectral types

The following illustration represents star classes with the colors very close to those actually perceived by the human eye. The relative sizes are for main-sequence (or "dwarf") stars.

Class O

Class O 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 class O stars.^{[nb 1][8]} Some of the most massive stars lie within this spectral class. Class O stars frequently have complicated surroundings which make measurement of their spectra difficult.

Spectrum of an O5 V star

O stars have dominant lines of absorption and sometimes emission for He II lines, prominent ionized (Si IV, O III, N III, and C III) and neutral helium lines, strengthening from O5 to O9, and prominent hydrogen Balmer lines, although not as strong as in later types. Because they are so massive, class O stars have very hot cores 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.^[41] The MKK scheme was extended to O9.7 in 1971^[42] and O4 in 1978,^[43] and new classification schemes have subsequently been introduced which add types O2, O3 and O3.5.^[44]

Examples:

• Alpha Camelopardalis
• Tau Canis Majoris
• Lambda Cephei
• Delta Circini A
• Plaskett's star
• Zeta Ophiuchi
• Delta Orionis
• Theta¹ Orionis C
• Sigma Orionis A
• Xi Persei
• Zeta Puppis
• Pismis 24-1
• Pistol Star

Class B

Proper motion of stars spectral classes B and A in -/+ 200 000 years

3D red cyan glasses are recommended to view this image correctly.

Class B stars are very luminous and blue. Their spectra have neutral helium, which are most prominent at the B2 subclass, and moderate hydrogen lines. Ionized metal lines include Mg II and Si II. As O and B stars are so powerful, they only live for a relatively short time, and thus they do not stray far from the area in which they were formed.

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 our galaxy 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 class B stars.^{[nb 1][8]}

Examples:

• Achernar
• Regulus
• Rigel
• Bellatrix
• Alnilam
• Saiph
• Algol A
• Spica

Class A

Class A Vega (left) compared to the Sun (right).

Class A 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 class A stars.^{[nb 1][8]}

Two class F stars: Supergiant Polaris A and its distant companion Polaris B^[45]

Examples: Sirius, Deneb, Altair, Vega, Fomalhaut           

Class F

Class F stars have strengthening H and K lines of Ca II. Neutral metals (Fe I, Cr I) beginning to gain on ionized metal lines by late F. Their spectra are characterized by the weaker hydrogen lines and ionized metals. Their color is white. About 1 in 33 (3.03%) of the main-sequence stars in the solar neighborhood are class F stars.^{[nb 1][8]}

Examples: Alrakis, Canopus, Dubhe B, Polaris, Procyon, Wezen

Class G

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

The movement of stars of spectral class G around the apex (left) and antapex (right) in -/+ 200 000 years

3D red cyan glasses are recommended to view this image correctly.

Class G stars are probably the best known, if only for the reason that the Sun is of this class. They make up about 7.5%, nearly one in thirteen, of the main-sequence stars in the solar neighborhood.^{[nb 1][8]}

Most notable are the H and K lines of Ca II, which are most prominent at G2. They have even weaker hydrogen lines than F, but along with the ionized metals, they have neutral metals. There is a prominent spike in the G band of CH molecules. G is host to the "Yellow Evolutionary Void".^[46] Supergiant stars often swing between O or B (blue) and K or M (red). While they do this, they do not stay for long in the yellow supergiant G classification as this is an extremely unstable place for a supergiant to be.

Examples: The Sun, Alpha Centauri A, Capella, Tau Ceti, Kepler-22

Class K

"K STAR" redirects here. For the Korean nuclear fusion project, see KSTAR.

Comparison between the class K star Arcturus, the class M star Antares, and Sol.

Class K stars are orangish stars that are slightly cooler than the Sun. They make up about 12%, nearly one in eight, of the main-sequence stars in the solar neighborhood.^{[nb 1][8]} Some K stars are giants and supergiants, such as Arcturus, whereas orange dwarfs, like Alpha Centauri B, are main-sequence stars.

They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals (Mn I, Fe I, Si I). By late K, molecular bands of titanium oxide become present. There is a suggestion that K Spectrum stars may potentially increase the chances of life developing on orbiting planets that are within the habitable zone.^[47]

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

Class M

Class M stars are by far the most common. About 76% of the main-sequence stars in the Solar neighborhood are class M stars.^{[nb 1][nb 2][8]} However, because main-sequence stars of spectral class M have such low luminosities, none are bright enough to be visible to see with the unaided eye. The brightest known M-class main-sequence star is M0V Lacaille 8760 at magnitude 6.6 (the fractionally brighter Groombridge 1618 was once considered to be class M0 but is now considered to be as K5) and it is extremely unlikely that any brighter examples will be found.
Although most class M stars are red dwarfs, the class also hosts most giants and some supergiants such as VY Canis Majoris, Antares and Betelgeuse, as well as Mira variables. Furthermore, the late-M group holds hotter brown dwarfs that are above the L spectrum. This is usually in the range of M6.5 to M9.5. The spectrum of a class M star shows lines belonging to oxide molecules, TiO in particular, in the visible and all neutral metals, but absorption lines of hydrogen are usually absent. TiO bands can be strong in class M stars, usually dominating their visible spectrum by about M5. Vanadium monoxide bands become present by late M.

VY Canis Majoris is a class M hypergiant. This has at times been reported as the largest known star, but its precise size is debated due to uncertainties over its distance, luminosity, and temperature. Artist's impression.

Examples: NML Cygni, WOH G64, VY Canis Majoris (hypergiants)

Examples: Betelgeuse, Antares (supergiants)

Examples: Rasalgethi, Beta Pegasi (giants)

Examples: Proxima Centauri, Barnard's star, Gliese 581, AD Leonis (red dwarfs)

Examples: LEHPM 2-59,^[48] SSSPM J1930-4311 (subdwarf)

Example: APMPM J0559-2903 (extreme subdwarf)

Examples: Teide 1 (field brown dwarf), GSC 08047-00232 B^[49] (companion brown dwarf)

Extended spectral types

A number of new spectral types have been taken into use from newly discovered types of stars.^[50]

Hot blue emission star classes

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

Class W: Wolf–Rayet

Class W or WR represents the Wolf–Rayet stars, notable for spectra lacking hydrogen lines. Instead their spectra are dominated by broad emission lines of highly ionised 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).^[18]
WR spectra range is listed below:^[51][52]

• WN,^[18] spectrum dominated by Nitrogen_III-V and Helium_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^[18]
  • 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 Carbon_IV lines, intermediate between WN and WC stars^[18]
• WC,^[18] spectrum with strong Carbon_II-IV lines
  • WCE (WC4 to WC6), hotter or "early"
  • WCL (WC7 to WC9), cooler or "late"
• WO (WO1 to WO4), strong Oxygen_VI lines, extremely rare

Although the central stars of most planetary nebulae (CSPNe) show O-type spectra,^[53] around 10% are hydrogen-deficient and show WR spectra.^[54] 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].

Wolf–Rayet examples:

Example: R136a1 (WN5h)

Example: Gamma² Velorum A (WC8)

Example: WR93B (WO3)

The "Slash" stars

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

Example spectra: Of/WNL^[55]

There is a secondary group found with this spectra, a cooler, "intermediate" group with designation of Ofpe/WN9.^[55] These stars have also been referred to as WN10 or WN11, but that has become less popular with the realisation of the evolutionary difference to other Wolf–Rayet stars. Recent discoveries of even rarer stars have extended the range of slash stars as far as O2-3.5If^*/WN5-7, which are even hotter than the original slash stars.^[56]

The magnetic O stars

They are O stars with strong magnetic fields. Designation is Of?p^[55]

The "class" OB

In lists of spectra, the "spectrum OB" may occur. This is in fact not a spectrum, but a marker which means that "the spectrum of this star is unknown, but it belongs to an OB association, so probably either a class O or class B star, or perhaps a fairly hot class A star."

Cool red and brown dwarf classes

The new spectral types L, T and Y were created to classify infrared spectra of cool stars. This includes both red dwarfs and brown dwarfs that are very faint in the visual spectrum.^[57]

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. L does not mean lithium dwarf; a large fraction of these stars do not have lithium in their spectra. Some of these objects have masses large enough to support hydrogen fusion, but some are of substellar mass and do not, so collectively these objects should be referred to as L dwarfs, not L stars. They are a very dark red in color and brightest in infrared. Their atmosphere is cool enough to allow metal hydrides and alkali metals to be prominent in their spectra.^[58][59][60] Due to low gravities in giant stars, TiO- and VO-bearing condensates never form. Thus, larger L-type stars can never form in an isolated environment. It may be possible for these L-type supergiants to form through stellar collisions, however, an example of which is V838 Monocerotis.

• L: 1,300–2,400 K, dwarfs (some stellar, some substellar) with metal hydrides and alkali metals prominent in their spectra.

Example: VW Hyi

Example: 2MASSW J0746425+2000321 binary^[61]

Component A is an L dwarf star

Component B is an L brown dwarf

Example: LSR 1610-0040 (subdwarf)^[62]

Example: V838 Monocerotis (supergiants)

Class T: methane dwarfs

Artist's impression of a T-dwarf

Class T dwarfs are cool brown dwarfs with surface temperatures between approximately 700 and 1,300 K. Their emission peaks in the infrared. Methane is prominent in their spectra.^[58][59]

• T: ~500–1,300 K, cooler brown dwarfs with methane in the spectrum

Examples: SIMP 0136 (the brightest T dwarf discovered in northern hemisphere)^[63]

Examples: Epsilon Indi Ba & Epsilon Indi Bb

Examples: CFBDS 1448 a T dwarf with an exoplanet orbiting it

Class T and L could be more common than all the other classes combined if recent research is accurate. Study of the number of proplyds (protoplanetary discs, clumps of gas in nebulae from which stars and planetary systems are formed) indicates that the number of stars in the galaxy should be several orders of magnitude higher than what we know about. It is theorized that these proplyds are in a race with each other. The first one to form will become a proto-star, which are very violent objects and will disrupt other proplyds in the vicinity, stripping them of their gas. The victim proplyds will then probably go on to become main-sequence stars or brown dwarfs of the L and T classes, which are quite invisible to us. Because brown dwarfs can live so long, these smaller bodies accumulate over time.

Class Y

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.^[64] Although such dwarfs have been modelled^[65] and detected within forty light years by the Wide-field Infrared Survey Explorer (WISE)^{[50][66][67][68][69]} there is no well-defined spectral sequence yet with prototypes. Nevertheless, several objects have been assigned spectral classes Y0, Y1, and Y2.^[70] The spectra of these objects display absorption around 1.55 micrometers.^[71] Delorme et al. has suggested that this feature is due to absorption from ammonia and that this should be taken as indicating the T–Y transition, making these objects of type Y0.^[71][72] In fact, this ammonia-absorption feature is the main criterion that has been adopted to define this class.^[70] However, this feature is difficult to distinguish from absorption by water and methane,^[71] and other authors have stated that the assignment of class Y0 is premature.^[73]

• Y: < 600 K, ultra-cool brown dwarfs

Example: WISE 0410+1502, class Y0

Example: WISE 0350-5658, class Y1

Example: WISE 1405+5534, class Y1

The brown dwarf with the latest assigned spectral type, WISE 1828+2650, is a >Y2 dwarf with an effective temperature originally estimated around 300 K, the temperature of the human body.^[66][67][74] Parallax measurements have however since shown that its luminosity is inconsistent with it being colder than ~400 K; the likely coolest Y dwarf currently known is WD 0806−661B with approximately 350 K.^[2]

The mass range for Y dwarfs is 9–25 Jupiter masses, but for young objects might reach below one Jupiter mass, which means that Y class objects straddle the 13 Jupiter mass deuterium-fusion limit that marks the division between brown dwarfs and planets.^[70]

Carbon-related late giant star classes

Carbon-related stars are stars whose spectra indicate production of carbon by helium triple-alpha fusion. With increased carbon abundance, and some parallel s-process heavy element production, the spectra of these stars become increasingly deviant from the usual late spectral classes G, K and M. The giants among those stars are presumed to produce this carbon themselves, but not too few of this class of stars are believed to be double stars whose odd atmosphere once was transferred from a former carbon star companion that is now a white dwarf.

Class C: carbon stars

Originally classified as R and N stars, these are also known as 'carbon stars'. These are red giants, near the end of their lives, in which there is an excess of carbon in the atmosphere. The old R and N classes ran parallel to the normal classification system from roughly mid G to late M. These have more recently been remapped into a unified carbon classifier C, with N0 starting at roughly C6.
Another subset of cool carbon stars are the J-type stars, which are characterized by the strong presence of molecules of ¹³CN in addition to those of ¹²CN.^[75] A few dwarf (that is, main-sequence) carbon stars are known, but the overwhelming majority of known carbon stars are giants or supergiants.

• C: Carbon stars, e.g. R CMi
  • C-R: Formerly a class on its own representing the carbon star equivalent of late G to early K stars. Example: S Camelopardalis
  • C-N: Formerly a class on its own representing the carbon star equivalent of late K to M stars. Example: R Leporis
  • C-J: A subtype of cool C stars with a high content of ¹³C. Example: Y Canum Venaticorum
  • C-H: Population II analogues of the C-R stars. Examples: V Ari, TT CVn^[76]
  • C-Hd: Hydrogen-Deficient Carbon Stars, similar to late G supergiants with CH and C₂ bands added. Examples: HD 137613, R Coronae Borealis

Class S

Class S stars have zirconium monoxide lines in addition to (or, rarely, instead of) those of titanium monoxide, and are in between the class M stars and the carbon stars.^[77] S stars have excess amounts of zirconium and other elements produced by the s-process, and have their carbon and oxygen abundances closer to equal than is the case for M stars. The latter condition results in both carbon and oxygen being locked up almost entirely in carbon monoxide molecules. For stars cool enough for carbon monoxide to form that molecule tends to "eat up" all of whichever element is less abundant, resulting in "leftover oxygen" (which becomes available to form titanium oxide) in stars of normal composition, "leftover carbon" (which becomes available to form the diatomic carbon molecules) in carbon stars, and "leftover nothing" in the S stars. The relation between these stars and the ordinary M stars indicates a continuum of carbon abundance. Like carbon stars, nearly all known S stars are giants or supergiants.

Examples: S Ursae Majoris, BD Camelopardalis

Classes MS and SC: intermediary carbon-related classes

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

Examples: R Serpentis, ST Monocerotis (MS)

Examples: CY Cygni, BH Crucis (SC)

White dwarf classifications

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

The class D (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.

Examples: Sirius B (DA2), Procyon B (DA4), Van Maanen's star (DZ7)^{[78], Table 1}

The white dwarf types are as follows:^[79][80]

• DA: a hydrogen-rich atmosphere or outer layer, indicated by strong Balmer hydrogen spectral lines.
• DB: a helium-rich atmosphere, indicated by neutral helium, He I, spectral lines.
• DO: a helium-rich atmosphere, indicated by ionized helium, He II, spectral lines.
• DQ: a carbon-rich atmosphere, indicated by atomic or molecular carbon lines.
• DZ: a metal-rich atmosphere, indicated by metal spectral lines (a merger of the obsolete white dwarf spectral types, DG, DK and DM).
• DC: no strong spectral lines indicating one of the above categories.
• DX: spectral lines are insufficiently clear to classify into one of the above categories.

The type is followed by a number giving the white dwarf's surface temperature. This number is a rounded form of 50400/T_eff, where T_eff is the effective surface temperature, measured in kelvins.
Originally, this number was rounded to one of the digits 1 through 9, but more recently fractional values have started to be used, as well as values below 1 and above 9.^[79][81]

Two or more of the type letters may be used to indicate a white dwarf which displays more than one of the spectral features above. Also, the letter V is used to indicate a variable white dwarf.^[79]

Extended white dwarf spectral types:^[79]

• DAB: a hydrogen- and helium-rich white dwarf displaying neutral helium lines.
• DAO: a hydrogen- and helium-rich white dwarf displaying ionized helium lines.
• DAZ: a hydrogen-rich metallic white dwarf.
• DBZ: a helium-rich metallic white dwarf.

Variable star designations:

• DAV or ZZ Ceti: a hydrogen-rich pulsating white dwarf.^{[82], pp. 891, 895}
• DBV or V777 Her: a helium-rich pulsating white dwarf.^{[83], p. 3525}
• GW Vir, sometimes divided into DOV and PNNV: a hot helium-rich pulsating white dwarf (or pre-white dwarf.)^{[84], §1.1, 1.2;[85][86]} These stars are generally PG 1159 stars, although some authors also include non-PG 1159 stars in this class.^[84][87]
• DCV or Gliese 86 B: a cool helium-rich white dwarf.

Non-stellar spectral types: Classes P and Q

Finally, the classes P and Q are occasionally used for certain non-stellar objects. Type P objects are planetary nebulae and type Q objects are novae.

Degenerate and exotic stars

These objects are not stars but are stellar remnants. They are much dimmer and if placed on the HR diagram, would be placed further to the lower left-hand corner.^[88]

Stellar classification, habitability, and the search for life

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 Planetary habitability, under the binary systems section).
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 the Sun (spectral type M) are likely to tidally lock planets within their habitable zone, along with other problems (see Habitability of red dwarf systems).^[89] Although there are many problems facing life on red dwarfs, due to their sheer numbers and longevity many astronomers continue to model these systems.

For these reasons NASA's Kepler Mission has been searching for habitable planets mainly at main-sequence stars that are less massive than spectral type A but more massive than type M – namely dwarf stars of types F, G, and K, the most probable stars to host life.^[89]

Variable star classification

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

Photometric classification

Stars can also be classified using photometric data from any photometric system. For example, we can calibrate color index diagrams of U−B and B−V in the UBV system according to spectral and luminosity classes. Nevertheless, this calibration is not straightforward, because many effects are superimposed in such diagrams: interstellar reddening, color changes due to metallicity, and the blending of light from binary and multiple stars.

Photometric systems with more colors and narrower passbands allow a star's class, and hence physical parameters, to be determined more precisely. The most accurate determination comes of course from spectral measurements, but there is not always enough time to get qualitative spectra with high signal-to-noise ratio.