classification of stars

Pickering Harem Stars are classified according to a system invented by a group of women named “Pickering’s harem”. This American astronomer (1846-1919), who was then Director of the Harvard observatory, asked them to classify the stars. After several attempts, the final stellar classification schema was based on a series of spectral types, represented by the letters O, B, A, F, G, K and M (a mnemonic phrase can be used to remember them: “Oh, be a fine girl kiss me”). In fact, this series of letters corresponds to a decreasing temperature scale. The ‘O’ stars are the most luminous and hottest ones, the ‘M’ are the coldest ones. Massive stars are grouped into the ‘O’ and ‘B’ categories, our Sun is part of category ‘G’.