What is the mnemonic for the Harvard spectral classification sequence O, B, A, F, G, K, M?
Answer
Oh, Be A Fine Girl/Guy, Kiss Me
The Harvard spectral classification system orders stars based on decreasing surface temperature, using the letters O, B, A, F, G, K, and M. The standard, convenient way to memorize this progression from the hottest class ($ ext{O}$) down to the coolest class ($ ext{M}$) is by using the phrase 'Oh, Be A Fine Girl/Guy, Kiss Me'. For context, $ ext{O}$ stars can exceed $33,000 ext{ K}$, while $ ext{M}$ stars are between $2,000$ and $3,500 ext{ K}$, with the Sun being a $ ext{G}$ type star.
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