spectral classification

In astronomy, classification of stars according to their surface temperature and luminosity, as determined from their spectra. Stars are assigned a spectral type (or class) denoted by the letters O, B, A, F, G, K, and M, where O stars (about 40,000 K/39,700°C/71,500°F) are the hottest and M stars (about 3,000 K/2,700°C/5,000°F) are the coolest.

Each letter may be further divided into ten subtypes, B0, B1, B2, and so on. Stars are also assigned a luminosity class denoted by a Roman numeral attached to the spectral type: I (supergiants), II (bright giants), III (giants), IV (subgiants), V (main sequence), VI (subdwarfs), or VII (white dwarfs). The Sun is classified as type G2V. See also Hertzsprung-Russell diagram.