---
title: Stellar classification
summary: How astronomers sort stars by their spectra. A letter (O, B, A, F, G, K, M, and L, T, Y for the coolest objects) and a number give the surface temperature; a Roman numeral gives the size and brightness. The Sun is a G2V star.
science_status: [observed, sim]
categories: [Stars, Stellar classification, Classification schemes]
aliases: [Spectral classification, Spectral type, Spectral class, Stellar spectral type, Harvard classification, Morgan-Keenan classification, MK system, MKK system, Luminosity class, OBAFGKM, Stellar class, Star types, Star classification]
infobox:
  type: star_class
  name: Stellar classification
  image: File:Gaia_HR_diagram_ESA.jpg
  caption: "Observation: the Hertzsprung-Russell diagram of about four million nearby stars measured by Gaia. Credit: ESA/Gaia/DPAC"
  code: "56 sim stellar types; 32 galaxy-map display classes"
  spectral_types_covered: "O, B, A, F, G, K, M; L, T, Y (brown dwarfs); subclasses 0 to 9 (sometimes with decimals)"
  luminosity_class: "0 (hypergiants), Ia, Iab, Ib (supergiants), II (bright giants), III (giants), IV (subgiants), V (main sequence, dwarfs), VI (subdwarfs), VII (white dwarfs, rarely used)"
  effective_temperature: {value: "about 2,350 (M9.5V) to about 41,000 (O5V) on the main sequence; about 250 (Y4) for the coolest brown dwarfs", unit: K, source: observed}
  mass: {value: "0.079 (M9V) to about 43 (O5V) on the main sequence", unit: M_Sun, source: observed}
  radius: {value: "0.10 (M9V) to about 11 (O5V) on the main sequence", unit: R_Sun, source: observed}
  luminosity: {value: "about 0.0003 (M9V) to about 350,000 (O5V) on the main sequence", unit: L_Sun, source: observed}
  bv_colour: {value: "-0.32 (O5V) to 2.17 (M9V)", unit: mag, source: observed}
  absolute_magnitude_v: {value: "-5.35 (O5V) to 19.4 (M9V)", unit: mag, source: observed}
  perceived_colour: "From blue-white (O, B) through white (A, F) and faintly yellowish white (G) to pale orange (K) and orange (M). No star looks green or purple; to the eye most look nearly white"
  share_of_stars: "Within 10 pc: M about 74%, K 11%, G 5%, F 2%, A 1%, white dwarfs 6% (derived from Reylé et al. 2021)"
  subtypes: "[[Red dwarf]] (M), [[K-type main-sequence star]] (K), [[G-type main-sequence star]] (G), [[Red giant]], [[Supergiant]], [[Brown dwarf]] (L, T, Y)"
  real_examples: "The Sun (G2V), Sirius A (A1V), Rigel (B8Ia), Betelgeuse (M1-M2Ia-Iab), Proxima Centauri (M5.5Ve), Arcturus (K1.5III)"
  physics_engine: "Stellar parameter derivation and class assignment modules; 44 per-class physics engines"
  science_doc: "The sim's stellar classification reference (56 types, XXX-YY-ZZ subtype codes)"
  last_verified: "2026-09-27, writer B"
sim:
  entity: classification.stellar
refs:
  - id: pm13
    type: article-journal
    author: [{family: Pecaut, given: Mark J.}, {family: Mamajek, given: Eric E.}]
    title: "Intrinsic Colors, Temperatures, and Bolometric Corrections of Pre-main-sequence Stars"
    container-title: The Astrophysical Journal Supplement Series
    volume: 208
    page: 9
    issued: 2013
    DOI: 10.1088/0067-0049/208/1/9
  - id: eem22
    type: dataset
    author: [{family: Mamajek, given: Eric E.}]
    title: "A Modern Mean Dwarf Stellar Color and Effective Temperature Sequence (version 2022.04.16)"
    URL: https://www.pas.rochester.edu/~emamajek/EEM_dwarf_UBVIJHK_colors_Teff.txt
    accessed: 2026-09-27
  - id: pc1897
    type: article-journal
    author: [{family: Pickering, given: E. C.}, {family: Cannon, given: A. J.}]
    title: "Spectra of bright southern stars"
    container-title: The Astrophysical Journal
    volume: 6
    page: 349
    issued: 1897
    DOI: 10.1086/140407
  - id: hdcat
    type: dataset
    author: [{family: Cannon, given: A. J.}, {family: Pickering, given: E. C.}]
    title: "Henry Draper Catalogue and Extension (Annals of Harvard College Observatory 91 to 100), VizieR III/135A"
    URL: https://vizier.cds.unistra.fr/viz-bin/VizieR?-source=III/135A
    accessed: 2026-09-27
  - id: struve1926
    type: article-journal
    author: [{family: Struve, given: Otto}]
    title: "Review: Stellar Atmospheres. A Contribution to the Study of High Temperature Ionization in the Reversing Layers of Stars, by Cecilia H. Payne"
    container-title: The Astrophysical Journal
    volume: 64
    page: 204
    issued: 1926
    DOI: 10.1086/143003
  - id: mkk43
    type: book
    author: [{family: Morgan, given: W. W.}, {family: Keenan, given: Philip C.}, {family: Kellman, given: Edith}]
    title: "An Atlas of Stellar Spectra, with an Outline of Spectral Classification"
    publisher: University of Chicago Press
    issued: 1943
  - id: mk73
    type: article-journal
    author: [{family: Morgan, given: W. W.}, {family: Keenan, given: P. C.}]
    title: "Spectral Classification"
    container-title: Annual Review of Astronomy and Astrophysics
    volume: 11
    page: 29-50
    issued: 1973
    DOI: 10.1146/annurev.aa.11.090173.000333
  - id: kk89
    type: article-journal
    author: [{family: Keenan, given: Philip C.}, {family: McNeil, given: Raymond C.}]
    title: "The Perkins catalog of revised MK types for the cooler stars"
    container-title: The Astrophysical Journal Supplement Series
    volume: 71
    page: 245
    issued: 1989
    DOI: 10.1086/191373
  - id: gc09
    type: book
    author: [{family: Gray, given: Richard O.}, {family: Corbally, given: Christopher J.}]
    title: "Stellar Spectral Classification"
    publisher: Princeton University Press
    issued: 2009
    DOI: 10.1515/9781400833368
  - id: kirk99
    type: article-journal
    author: [{family: Kirkpatrick, given: J. Davy}, {family: Reid, given: I. Neill}, {family: Liebert, given: James}, {literal: "et al."}]
    title: "Dwarfs Cooler than \"M\": The Definition of Spectral Type \"L\" Using Discoveries from the 2 Micron All-Sky Survey (2MASS)"
    container-title: The Astrophysical Journal
    volume: 519
    page: 802-833
    issued: 1999
    DOI: 10.1086/307414
  - id: kirk05
    type: article-journal
    author: [{family: Kirkpatrick, given: J. Davy}]
    title: "New Spectral Types L and T"
    container-title: Annual Review of Astronomy and Astrophysics
    volume: 43
    page: 195-245
    issued: 2005
    DOI: 10.1146/annurev.astro.42.053102.134017
  - id: cush11
    type: article-journal
    author: [{family: Cushing, given: Michael C.}, {family: Kirkpatrick, given: J. Davy}, {family: Gelino, given: Christopher R.}, {literal: "et al."}]
    title: "The Discovery of Y Dwarfs Using Data from the Wide-field Infrared Survey Explorer (WISE)"
    container-title: The Astrophysical Journal
    volume: 743
    page: 50
    issued: 2011
    DOI: 10.1088/0004-637X/743/1/50
  - id: hh21
    type: article-journal
    author: [{family: Harre, given: Jan-Vincent}, {family: Heller, given: René}]
    title: "Digital color codes of stars"
    container-title: Astronomische Nachrichten
    volume: 342
    page: 578-587
    issued: 2021
    DOI: 10.1002/asna.202113868
  - id: gaiahrd18
    type: article-journal
    author: [{literal: "Gaia Collaboration"}, {family: Babusiaux, given: C.}, {family: van Leeuwen, given: F.}, {literal: "et al."}]
    title: "Gaia Data Release 2: Observational Hertzsprung-Russell diagrams"
    container-title: Astronomy & Astrophysics
    volume: 616
    page: A10
    issued: 2018
    DOI: 10.1051/0004-6361/201832843
  - id: reyle21
    type: article-journal
    author: [{family: Reylé, given: C.}, {family: Jardine, given: K.}, {family: Fouqué, given: P.}, {literal: "et al."}]
    title: "The 10 parsec sample in the Gaia era"
    container-title: Astronomy & Astrophysics
    volume: 650
    page: A201
    issued: 2021
    DOI: 10.1051/0004-6361/202140985
  - id: henry18
    type: article-journal
    author: [{family: Henry, given: Todd J.}, {family: Jao, given: Wei-Chun}, {family: Winters, given: Jennifer G.}, {literal: "et al."}]
    title: "The Solar Neighborhood XLIV: RECONS Discoveries within 10 parsecs"
    container-title: The Astronomical Journal
    volume: 155
    page: 265
    issued: 2018
    DOI: 10.3847/1538-3881/aac262
  - id: prsa16
    type: article-journal
    author: [{family: Prša, given: Andrej}, {family: Harmanec, given: Petr}, {family: Torres, given: Guillermo}, {literal: "et al."}]
    title: "Nominal Values for Selected Solar and Planetary Quantities: IAU 2015 Resolution B3"
    container-title: The Astronomical Journal
    volume: 152
    page: 41
    issued: 2016
    DOI: 10.3847/0004-6256/152/2/41
images_wanted:
  - file: File:Gaia_HR_diagram_ESA.jpg
    subject: "Gaia's Hertzsprung-Russell diagram of about four million stars within 5,000 light years: colour against brightness, the main sequence, giant branch and white dwarf sequence clearly separated"
    source: other
    page_url: https://www.esa.int/ESA_Multimedia/Images/2018/04/Gaia_s_Hertzsprung-Russell_diagram
    credit: "ESA/Gaia/DPAC"
    licence: "CC BY-SA 3.0 IGO (the page offers CC BY-SA 3.0 IGO or the ESA Standard Licence; use CC BY-SA 3.0 IGO). Acknowledge Gaia DPAC and Carine Babusiaux as the page asks."
    note: "esa.int image allowed only because it is marked CC BY-SA 3.0 IGO. Do not use 'Gaia's Stellar Family Portrait' or 'Gaia's first Hertzsprung-Russell diagram' (ESA Standard Licence only)."
  - file: File:Spectral_class_colour_sequence_diagram.svg
    subject: "Diagram: a row of discs for O5V, B0V, A0V, F0V, G2V, K0V, K5V, M0V, M5V and M9V, each filled with its perceived colour from Harre and Heller (2021), sized by radius on a log scale, labelled with temperature and mass from the table on this page"
    source: other
    note: "shot list: to be drawn as an SVG; colours from Harre and Heller 2021 (AN 342, 578), not from a blackbody"
  - file: File:Jewel_Box_cluster_ESO.jpg
    subject: "The Jewel Box star cluster in true colour, blue-white young stars around a few bright orange-red supergiants"
    source: eso
    page_url: https://www.eso.org/public/images/eso0940a/
    credit: "ESO/Y. Beletsky"
    licence: "CC BY 4.0 (ESO usage terms)"
---

**Stellar classification** is the sorting of stars by their spectra, the patterns of dark lines that atoms and molecules in a star's atmosphere cut into its light. Each star gets a letter, O, B, A, F, G, K or M, running from the hottest to the coolest, with L, T and Y added for the still cooler [[Brown dwarf|brown dwarfs]]; a number from 0 to 9 that divides each letter into finer steps; and a Roman numeral, the luminosity class, that says whether the star is a compact dwarf or a swollen giant. The Sun is a **G2V** star: a yellow-white, hydrogen-burning main-sequence star with a surface temperature of 5,772 K :cite[prsa16]. A spectral type sums up a star's temperature, size and stage of life in three characters, and it is the first thing astronomers look up about a star.

## The spectral sequence

### From Harvard's letters to temperatures

The letters come from the Harvard College Observatory, where in the 1890s Edward Pickering's team photographed and sorted the spectra of hundreds of thousands of stars. Annie Jump Cannon refined the system while classifying the southern sky :cite[pc1897], and the resulting Henry Draper Catalogue, published between 1918 and 1924, gave spectral types for 225,300 stars down to about ninth magnitude :cite[hdcat]. The letters were first assigned alphabetically by the strength of the hydrogen lines, then reordered into the sequence OBAFGKM once it became clear that the sequence was one of temperature. Cecilia Payne's 1925 thesis applied the new physics of atomic ionisation to show how temperature, rather than differences in composition, controls which lines a star shows :cite[struve1926].

Temperature matters because it decides which atoms are ionised and which molecules survive. In the hottest stars even helium loses an electron; in the coolest, molecules such as titanium oxide can form. The hallmarks of each class are these :cite[mk73] :cite[gc09]:

| Class | Surface temperature (main sequence) | Hallmark lines | Example |
|---|---|---|---|
| O | above about 31,500 K | Ionised helium | Zeta Ophiuchi |
| B | about 10,000 to 31,500 K | Neutral helium; hydrogen strengthening | Rigel, Spica |
| A | about 7,300 to 10,000 K | Hydrogen strongest | Sirius A, Vega |
| F | about 6,000 to 7,300 K | Hydrogen weakening; ionised calcium and metals | Procyon A |
| G | about 5,300 to 6,000 K | Ionised calcium very strong; many metal lines | The Sun, Alpha Centauri A |
| K | about 3,900 to 5,300 K | Neutral metals; molecular bands appear | Alpha Centauri B, Arcturus |
| M | about 2,350 to 3,900 K | Titanium oxide bands | Proxima Centauri, Betelgeuse |

The boundaries in the table are the temperatures between the last subtype of one class and the first of the next in Eric Mamajek's compilation of main-sequence stars, which builds on Pecaut and Mamajek (2013) :cite[eem22] :cite[pm13]. Giants and supergiants of the same spectral type are slightly cooler.

### Beyond M

Surveys in the infrared found objects cooler than any M star. Class L was defined in 1999 from 2MASS discoveries, for objects around 1,300 to 2,300 K whose spectra show metal hydrides and alkali metals :cite[kirk99]. Class T, with strong methane absorption, followed :cite[kirk05], and the first Y dwarfs, below about 500 K, were found with the WISE satellite in 2011 :cite[cush11]. Most L dwarfs and all T and Y dwarfs are brown dwarfs, objects too small to sustain hydrogen fusion.

Special classes cover stars whose chemistry, not temperature, sets their spectra: W for the hot, stripped Wolf-Rayet stars, C for carbon stars, S for giants rich in zirconium oxide, and D for white dwarfs.

## Luminosity classes

Two stars of the same temperature can differ in size by a factor of a thousand, and the spectrum shows which is which. A giant's atmosphere is thin and its surface gravity weak, so its absorption lines are narrower than those of a dwarf, whose denser gas broadens them. In 1943 William Morgan, Philip Keenan and Edith Kellman at Yerkes Observatory published the atlas that added this second dimension, now called the MK (or MKK) system :cite[mkk43] :cite[mk73]:

| Class | Meaning | Example |
|---|---|---|
| 0 or Ia+ | Hypergiant | Rho Cassiopeiae |
| Ia, Iab, Ib | Luminous, intermediate and less luminous supergiants | Rigel (B8Ia), Betelgeuse |
| II | Bright giant | |
| III | Giant | Arcturus (K1.5III), Aldebaran |
| IV | Subgiant | |
| V | Main-sequence star ("dwarf") | The Sun (G2V) |
| VI | Subdwarf | |
| VII | White dwarf (rarely used; white dwarfs have their own D classes) | |

Classifiers add finer notes: "e" for emission lines, "p" for peculiar, and so on. The standard stars that define the classes are listed in catalogues such as the Perkins catalogue of cooler stars :cite[kk89].

## The main sequence in numbers

Most stars spend most of their lives on the main sequence, fusing hydrogen in their cores, and along it spectral type is tied to mass, size and brightness. Representative values :cite[eem22] :cite[pm13]:

| Type | Temperature (K) | Mass (Sun = 1) | Radius (Sun = 1) | Luminosity (Sun = 1) | B-V colour | Absolute magnitude M_V |
|---|---|---|---|---|---|---|
| O5V | 41,400 | 43 | 11.5 | about 350,000 | -0.32 | -5.35 |
| B0V | 31,400 | 17.7 | 7.2 | about 45,000 | -0.30 | -3.90 |
| A0V | 9,700 | 2.18 | 2.19 | 38 | 0.00 | 0.99 |
| F0V | 7,220 | 1.61 | 1.73 | 7.2 | 0.30 | 2.57 |
| G2V | 5,770 | 1.00 | 1.01 | 1.02 | 0.65 | 4.80 |
| K0V | 5,270 | 0.88 | 0.81 | 0.46 | 0.82 | 5.78 |
| K5V | 4,440 | 0.70 | 0.70 | 0.17 | 1.15 | 7.28 |
| M0V | 3,850 | 0.57 | 0.59 | 0.069 | 1.42 | 8.80 |
| M5V | 3,060 | 0.162 | 0.196 | 0.0030 | 1.83 | 14.15 |
| M9V | 2,380 | 0.079 | 0.102 | 0.0003 | 2.17 | 19.40 |

Luminosity rises steeply with mass: an O5 star is about 40 times the Sun's mass but hundreds of thousands of times as bright, and burns out in a few million years, while an M dwarf will outlive the present age of the universe many times over. The compiler notes that the mean masses are tentative, and the O5V row and the absolute magnitudes come from the online table rather than the 2013 paper :cite[eem22].

## The Hertzsprung-Russell diagram

Plotting stars' temperature (or colour, or spectral type) against their luminosity gives the Hertzsprung-Russell diagram, in which the luminosity classes separate into bands: the main sequence running diagonally, the giants and supergiants above it, and the white dwarfs below. The European Space Agency's Gaia mission has measured distances to so many stars that its diagram of the solar neighbourhood shows these sequences with unprecedented sharpness, including fine structure within the main sequence and the white dwarf sequence :cite[gaiahrd18].

::figure{src="File:Gaia_HR_diagram_ESA.jpg" size=wide alt="A dense scatter plot of millions of stars: a broad diagonal main sequence, a giant branch rising to the upper right, and a thin white dwarf sequence at lower left" caption="Observation: Gaia's Hertzsprung-Russell diagram of about four million stars within 5,000 light years. Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO."}

## What colour are stars?

Stellar colours are subtler than the letters suggest. Folding model stellar spectra through the colour response of the human eye shows that there are no green or purple stars. M dwarfs look orange rather than red, the Sun seen from space is white with the faintest yellow tint, and the whitest stars are late F types :cite[hh21]. To the naked eye most stars look white, because at low light levels the eye sees little colour.

::figure{src="File:Spectral_class_colour_sequence_diagram.svg" size=wide alt="A row of coloured discs from blue-white O and B stars through white A and F stars to pale yellow G, pale orange K and orange M stars, sized by radius" caption="Diagram: main-sequence stars from O5 to M9, coloured as the eye would see them from space."}

## How common is each class?

The brightest stars in the sky are a poor guide, because luminous stars are seen from far away. A census of every star within 10 parsecs (about 33 light years) finds 249 M dwarfs, 38 K stars, 18 G stars, 8 F stars, 4 A stars and 20 white dwarfs, with no giants at all :cite[reyle21]. That is roughly three quarters M dwarfs, one in nine K dwarfs, one in twenty G stars like the Sun, and hardly any A, B or O stars. The RECONS census of the same volume gives nearly identical shares :cite[henry18].

:::callout{type=sim title="In Pax Abyssi"}
The sim's stellar taxonomy has 56 entries: seven main-sequence classes named by colour (blue, blue-white, white, yellow-white, yellow, orange and red for O to M), 28 evolved classes (each of the seven colours as a subgiant, giant, supergiant and hypergiant), white dwarfs, three brown dwarf classes (L, T and Y), seven exotic classes (Wolf-Rayet, carbon, S-type, subdwarf, protostar, T Tauri and Herbig Ae/Be stars), four kinds of neutron star, three sizes of black hole, and three keys for multiple systems. Each star's class comes from its catalogue spectral type: the letter gives the colour and the luminosity class gives the stage, so a K giant such as Arcturus is an "orange giant". Most classes have their own physics engine and three-level subtype codes. The galaxy map groups them into 32 display classes.

The 119,626 stars of the game's catalogue are real stars from the HYG database, corrected with XHIP spectral types and Gaia distances. Because a catalogue of visible stars favours luminous ones, it holds 33,053 K stars, 26,288 F, 23,382 G, 18,226 A, 11,102 B, 6,530 M and 326 O, and most of its K stars are giants; the procedural galaxy that fills in the rest draws its stars from the true mix, dominated by M dwarfs. Star colours are drawn from each star's measured colour index, converted to temperature and then to a blackbody tint, with saturation raised by design so that star colours read clearly on screen. That is an artistic choice: real stars look paler.
:::

## See also

- [[Red dwarf]]
- [[K-type main-sequence star]]
- [[G-type main-sequence star]]
- [[Red giant]]
- [[Supergiant]]
- [[Brown dwarf]]
- [[White dwarf]]
- [[Neutron star]]
- [[Star catalogue]]