Stellar 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 system using the letters [|O], [|B], [|A], F, G, K, and [|M], a sequence from the hottest to the coolest. Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest. The sequence has been expanded with three classes for other stars that do not fit in the classical system: W, S and C. Some stellar remnants or objects of deviating mass have also been assigned letters: D for white dwarfs and L, T and Y for brown dwarfs.
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 for subdwarfs, and class D 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 classification. Each star is assigned a spectral class 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.

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. 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 – in particular, newly-formed white dwarfs – can have surface 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	Vega-relative chromaticity	Chromaticity	Mainsequence mass	Mainsequence radius	Mainsequence luminosity	Hydrogen lines	Percentage of all mainsequence stars
O	≥ 33,000 K	blue	blue	≥	≥	≥	Weak	0.00003%
B	10,000–33,000 K	bluish white	deep bluish white				Medium	0.12%
A	7,300–10,000 K	white	bluish white				Strong	0.61%
F	6,000–7,300 K	yellowish white	white				Medium	3.0%
G	5,300–6,000 K	yellow	yellowish white				Weak	7.6%
K	3,900–5,300 K	light orange	pale yellowish orange				Very weak	12%
M	2,300–3,900 K	light orangish red	orangish red		≤	≤	Very weak	76%

The traditional mnemonic for remembering the order of the spectral type letters, from hottest to coolest, is "Oh, Be A Fine Guy/Girl: Kiss Me!". Many alternative mnemonics have been proposed, in contests held by astronomy courses and organizations, but the traditional mnemonic remains the most popular.
The spectral classes O through M, as well as other more specialized classes discussed later, are subdivided by Arabic numerals, 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 was not fully understood until after its development, though by the time the first Hertzsprung–Russell diagram was formulated, 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 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.

Morgan–Keenan classification

The Yerkes spectral classification, also called the MK, or Morgan-Keenan 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 classification scheme is based on spectral lines sensitive to stellar temperature and surface gravity, which is related to luminosity. 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.

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 – G5II
III	normal giants	Arcturus – K0III
IV	subgiants	Gamma Cassiopeiae – B0.5IVpe
V	main-sequence stars	Achernar – B6Vpe
sd or VI	subdwarfs	HD 149382 – sdB5 or B5VI
D 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 between the two luminosity classes:

A slash means that a star is either one class or the other.
A hyphen 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.
Nominal luminosity class VII 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.