A classification system for stars based on their surface temperature and spectral characteristics, using letters O, B, A, F, G, K, M from hottest to coolest. Each class shows distinctive absorption lines in the star's spectrum that reveal its chemical composition and physical conditions.
Originated from 'spectrum' (Latin 'specere', to look) and 'class' (Latin 'classis', division). Developed at Harvard Observatory in the 1890s by Annie Jump Cannon, who reorganized earlier alphabetical systems into the temperature sequence we use today, creating the mnemonic 'Oh Be A Fine Girl/Guy Kiss Me.'
The original spectral classification used the entire alphabet, but most letters were dropped when astronomers realized they were seeing temperature differences, not different star types! Annie Jump Cannon personally classified over 350,000 stars, more than any other person in history.
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