Stellar classification

Astronomers classify stars by their spectra. 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 (O type) to the coolest (M type).

A common mnemonic for remembering the order of the spectral type letters, from hottest to coolest, is "Oh, Be A Fine Guy/Girl, Kiss Me".