Stellar Classification

Astronomers sort stars into the spectral types O, B, A, F, G, K, and M, remembered by the mnemonic "Oh Be A Fine Girl, Kiss Me." O-type stars are the hottest and bluest, above 30,000 Kelvin. M-type red dwarfs are the coolest, under 3,500 Kelvin. The Sun is a G-type star.

Each type splits into subtypes from 0 to 9, plus luminosity classes from I (supergiant) to V (main sequence). So a label like G2V packs a star's temperature, size, and evolutionary stage into just three characters.