Stellar Classification is the systematic organisation of stars according to observable and inferred physical properties, especially their spectra, temperatures, luminosities, chemical compositions and evolutionary states. The most familiar spectral classification sequence is O, B, A, F, G, K, M, ordered primarily by decreasing surface temperature rather than by alphabetic order. The sequence originated from historical classification schemes based on spectral appearance and was reorganised after astronomers recognised the physical relationship between spectral features and stellar temperature. Subclasses numbered from 0 through 9 provide finer temperature distinctions, so a star classified as A0 is hotter than one classified as A9. Spectral classification is therefore a physical classification system rather than merely a system for naming stars.
The O type contains the hottest ordinary stellar spectra and is characterised by strong ionised helium features and very high surface temperatures. B type stars are also extremely hot and luminous, with prominent neutral helium features in appropriate subclasses. A type spectra are particularly associated with strong hydrogen Balmer absorption lines. F type stars are cooler and show increasingly prominent metallic features, while G type stars have still cooler atmospheres and include the Sun, classified as G2 V. K type stars are orange and cooler, while M type stars are generally the coolest common main sequence stars and often show strong molecular absorption features. These broad categories correspond to major changes in atmospheric ionisation, molecular formation and spectral line strength.
The luminosity class is an additional component of the MK classification system. Roman numeral V denotes main sequence stars, IV denotes subgiants, III denotes giants, II denotes bright giants and I denotes supergiants, with further subdivisions used where necessary. The luminosity class reflects differences in surface gravity and atmospheric structure rather than simply apparent brightness. A giant and a dwarf with similar surface temperatures can have significantly different radii, luminosities, spectral line widths and atmospheric densities. A complete classification can therefore contain both spectral type and luminosity class, allowing astronomers to infer considerably more about a star than temperature alone.
Stellar classification relies heavily on spectroscopy because atoms and molecules produce characteristic absorption and emission features. The presence and strength of spectral lines depend on temperature, ionisation state, electron density, chemical abundance, pressure and other atmospheric conditions. Hydrogen lines are strongest in a particular temperature range rather than in the hottest stars because hydrogen becomes increasingly ionised at higher temperatures. Metal lines become more prominent in cooler atmospheres where the relevant ions and atoms can exist. Molecular bands are especially important in cool stars because molecules become sufficiently stable to leave strong signatures in their spectra.
Classification also includes chemical abundance information. Metallicity in astronomy refers broadly to the abundance of elements heavier than helium, and it can provide clues about a star's origin and evolutionary environment. Population I stars generally have higher metallicity and are often associated with younger stellar populations, whereas Population II stars tend to be older and metal poor. Extremely metal poor stars provide information about early generations of stellar nucleosynthesis and the chemical enrichment history of galaxies. Spectroscopic analysis can measure abundance ratios for many elements, although accurate interpretation requires detailed atmospheric models and careful treatment of temperature, gravity, turbulence and line formation.
Stellar classification is closely related to the Hertzsprung Russell Diagram because spectral type provides a temperature coordinate while luminosity class helps distinguish main sequence, giant and supergiant populations. A star's position on the diagram can therefore reveal aspects of its evolutionary state. Main sequence stars occupy a broad diagonal sequence, while giants and supergiants occupy regions of substantially greater luminosity. White dwarfs occupy a distinct hot but faint region. Classification is consequently both descriptive and diagnostic. It helps astronomers connect observations with stellar structure and evolutionary models.
Classification can become complicated for unusual stars. White dwarfs use specialised spectral notation because their high surface gravity produces distinctive spectral features. Wolf Rayet stars exhibit unusually strong broad emission features caused by powerful stellar winds. Carbon stars have spectra dominated by carbon bearing molecular bands, while chemically peculiar stars can show unusual abundance patterns. Rapid rotation, magnetic fields, stellar winds, binarity and circumstellar material can also affect observed spectra. Astronomers therefore use specialised classifications when the standard O through M sequence cannot describe the physical properties adequately.
Stellar classification is important because it converts complex observational information into a structured system that can support comparison across enormous stellar populations. Classification does not by itself provide an exact measurement of every stellar property, and spectral type should not be treated as a direct substitute for mass, age or chemical composition. Nevertheless, with photometry, spectroscopy, parallax and theoretical models, classification provides a foundation for estimating stellar temperature, luminosity, radius, evolutionary status and population membership. It is one of the central observational frameworks through which astronomers interpret individual stars and the stellar populations of galaxies.
