Spectral classes

A classification system for star spectra was developed, arranging stars by surface temperature using a system of letters: The spectral classes of the main sequence, in order of decreasing temperature, turned out to be O, B, A, F, G, K, and M, and each class is divided into ten numbers. Sun is a G2 star, Sirius is A1, Rigel is B5, Arcturus is K2, Betelgeuse is M2, etc.
 Annie Jump Cannon United States 1918 Spectral classes

A classification system for star spectra was developed, arranging stars by surface temperature using a system of letters: The spectral classes of the main sequence, in order of decreasing temperature, turned out to be O, B, A, F, G, K, and M, and each class is divided into ten numbers. Sun is a G2 star, Sirius is A1, Rigel is B5, Arcturus is K2, Betelgeuse is M2, etc.