spectral type

astronomy

Also known as: spectral class

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July 14, 2026

Quick Summary

In astronomy, a spectral type is a classification of a star based primarily on its surface temperature. This system uses letters, ordered from hottest to coolest stars: O, B, A, F, G, K, and M. These are supplemented by R, N (or C-type/carbon stars), and S types, as well as L, T, and Y for brown dwarfs. Our Sun, for example, is classified as a G2 spectral type.123

The spectral sequence is also a color sequence, with O and B stars appearing blue and hottest, while M, R, N, and S stars are red and coolest. This classification helps astronomers understand a star's temperature, chemical composition, and luminosity.2

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Classification of spectral types
Most stars are grouped into a small number of spectral types. The Henry Draper Catalogue and the Bright Star Catalogue list spectral types from the hottest to the coolest stars (see stellar classification). These types are designated, in order of decreasing temperature, by…
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Most stars are grouped into a small number of spectral types. The Henry Draper Catalogue and the Bright Star Catalogue list spectral types from the hottest to the coolest stars (see stellar classification). These types are designated, in order of decreasing temperature, by…
Read More