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What is the origin of the letters used in star classification? Why didn't astronomers use ABCD?

J. HALDIMAN, CHICAGO
Stellar spectral types originally followed alphabetical order. But as astronomers observed more stars and obtained more detailed spectra, they consolidated some categories and re-ordered the remaining ones. This left seven basic stellar types, designated OBAFGKM, which generations of astronomers and science-fiction readers have learned through the mnemonic "Oh, Be A Fine Girl (or Guy), Kiss Me."

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