A form of classification used for stars, based on their spectra. The Harvard classification, introduced in 1890 and modified in the 1920s, is based on the seven star types known as O, B, A, F, G, K, M:

• O hottest blue stars; ionized helium lines dominant

• B hot blue stars; neutral helium lines dominant, no ionized helium

• A blue blue-white stars; hydrogen lines dominant

• F white stars; metallic lines strengthen, hydrogen lines weaken

• G yellow stars; ionized calcium lines dominant

• K orange-red stars; neutral metallic lines dominant, some molecular bands

• M coolest red stars; molecular bands dominant