Harvard Spectral Classification Scheme

"The modern stellar spectral classification scheme (also known as the Harvard Spectral Classification Scheme) was created by Annie Jump Cannon through her examination of spectra from 1918 to 1924. Originally, the scheme used capital letters running alphabetically, but was later reordered to reflect the surface temperatures of stars. In order of decreasing temperature, these types were O, B, A, F, G, K and M. Three additional categories also in the scheme: R, N, and S types, were later realized to represent stars with peculiar heavy-metal abundances. Other types (Q for novae, W for Wolf-Rayet stars, etc) are not encountered frequently. According to astronomical myth, Henry Norris Russell suggested the following mnemonic to assist students in remembering the scheme: Oh Be A Fine Girl, Kiss Me!"