Stellar classification

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 ==
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 ==
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) 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[citation needed]. Physically, the classes indicate the temperature of the star's atmosphere and are normally listed from hottest to coldest.
Class	Effective temperature	Vega-relative chromaticity	Chromaticity (D65) 	Main-sequence mass
(solar masses)	Main-sequence radius
(solar radii)	Main-sequence luminosity
(bolometric)	Hydrogen
lines	Fraction of all
main-sequence stars
O 	≥ 33,000 K 	blue 	blue 	≥ 16 M☉ 	≥ 6.6 R☉ 	≥ 30,000 L☉ 	Weak 	0.00003%
B 	10,000–33,000 K 	bluish white 	deep bluish white 	2.1–16 M☉ 	1.8–6.6 R☉ 	25–30,000 L☉	Medium 	0.12%
A 	7,300–10,000 K 	white 	bluish white 	1.4–2.1 M☉ 	1.4–1.8 R☉ 	5–25 L☉ 	Strong 	0.61%
F 	6,000–7,300 K 	yellowish white 	white 	1.04–1.4 M☉ 	1.15–1.4 R☉ 	1.5–5 L☉ 	Medium 	3.0%
G 	5,300–6,000 K 	yellow 	yellowish white 	0.8–1.04 M☉ 	0.96–1.15 R☉	0.6–1.5 L☉ 	Weak 	7.6%
K 	3,900–5,300 K 	light orange 	pale yellowish orange	0.45–0.8 M☉ 	0.7–0.96 R☉ 	0.08–0.6 L☉ 	Very weak	12%
M 	2,300–3,900 K 	orangish red 	light orangish red 	0.08–0.45 M☉	≤ 0.7 R☉ 	≤ 0.08 L☉ 	Very weak	76%
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!".
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. The Sun is classified as G2.
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. 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.
Harvard astronomer Cecilia Payne then demonstrated that the O-B-A-F-G-K-M spectral sequence is actually a sequence in temperature. Because the classification sequence predates our understanding that it is a temperature sequence, the placement of a spectrum into a given subtype, such as B3 or A7, depends upon (largely subjective) estimates of the strengths of absorption features in stellar spectra. As a result, these subtypes are not evenly divided into any sort of mathematically representable intervals.
=== Yerkes spectral classification ===
The Yerkes spectral classification, also called the MK, or Morgan-Keenan (alternatively referred to as the MKK, or Morgan-Keenan-Kellman) 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. 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, 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.
Yerkes luminosity classes
Luminosity class 	Description 	Examples
0 or Ia+ 	hypergiants or extremely luminous supergiants	Cygnus OB2#12 – B3-4Ia+
Ia 	luminous supergiants 	Eta Canis Majoris – B5Ia
Iab 	intermediate-size luminous supergiants 	Gamma Cygni – F8Iab
Ib 	less luminous supergiants 	Zeta Persei – B1Ib
II 	bright giants 	Beta Leporis – G0II
III 	normal giants 	Arcturus – K0III
IV 	subgiants 	Gamma Cassiopeiae – B0.5IVpe
V 	main-sequence stars (dwarfs) 	Achernar – B6Vep
sd (prefix) or VI	subdwarfs 	HD 149382 – sdB5 or B5VI
D (prefix) or VII	white dwarfs 	van Maanen 2 – DZ8
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.
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.
=== Spectral peculiarities ===
Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum.
Code 	Spectral peculiarities for stars
: 	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
eq 	Emission lines with P Cygni profile
f 	N III and He II emission
f* 	N IV 4058Å is stronger than the N III 4634Å, 4640Å, & 4642Å lines
f+ 	Si IV 4089Å & 4116Å are emitted, in addition to the N III line
f? 	C III 4647–4650–4652Å emission lines with comparable strength to the N III line
(f) 	N III emission, absence or weak absorption of He II
(f+) 	
((f)) 	Displays strong He II absorption accompanied by weak N III emissions
((f*))	
h 	WR stars with hydrogen emission lines.
ha 	WR stars with hydrogen seen in both absorption and emission.
He wk 	Weak Helium lines
k 	Spectra with interstellar absorption features
m 	Enhanced metal features
n 	Broad ("nebulous") absorption due to spinning
nn 	Very broad absorption features
neb 	A nebula's spectrum mixed in
p 	Unspecified peculiarity, peculiar star.
pq 	Peculiar spectrum, similar to the spectra of novae
q 	P Cygni profiles
s 	Narrow ("sharp") absorption lines
ss 	Very narrow lines
sh 	Shell star features
var 	Variable spectral feature (sometimes abbreviated to "v")
wl 	Weak lines (also "w" & "wk")
Element
symbol	Abnormally strong spectral lines of the specified element(s)
z 	indicating abnormally an strong ionised helium line at 468.6 nm
For example, 59 Cygni is listed as spectral type B1.5Vnne, indicating a spectrum with the general classification B1.5V, as well as very broad absorption lines and certain emission lines.
== 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.
In the late 1890s, this classification began to be superseded by the Harvard classification, which is discussed in the remainder of this article.
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:
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:
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.
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
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.	Classes carried through into the MK system are in bold.	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. 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.
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. 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.
=== 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.
Summary of 1897 Harvard system
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 did not 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:
* (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.
=== 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.
Finally, by 1912, Cannon had changed the types B, A, B5A, F2G, etc. to B0, A0, B5, F2, etc. This is essentially the modern form of the Harvard classification system. This system was developed through the analysis of spectra on photographic plates, which could convert light emanated from stars into a readable spectrum.
=== Mount Wilson classes ===
A luminosity classification known as the Mount Wilson system was used to distinguish between stars of different luminosities. This notation system is still sometimes seen on modern spectra.
* sd: subdwarf
* d: dwarf
* sg: subgiant
* g: giant
* c: supergiant
== 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.
=== "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. The terms "early" and "late" were carried over, beyond the demise of the model they were based on.