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We classify stars based on their spectra. The sequence of spectral classes in essence tells us the temperature of the star. From hottest to coolest the classes are O, B, A, F, G, K, M.
@evilscientistca So back in the 1920 stars were classified based on the prominence of H-beta Balmer absorption line ― alphabetically, from the most prominent ‘A’ to the least ‘O’.
Annie Jump Cannon projected those A,…,O scale to the temperature based OBAFGKM (missing classes were too close to other classes and were dropped), where the surface temperature smoothly decreases from O to M.
The new scale preserved old names, but at the cost of losing relation between them.
Damn trade-offs!
@evilscientistca and BTW
Only Bad Astronomers Forget Generally Known Mnemonics
@kmicu Though I prefer Anne Jump Cannon's Mnemonic...