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Concept

OBAFGKM

A concentric circle design with the Harvard spectral classification sequence OBAFGKM arranged around it, representing the order of star types from hottest to coolest.

The Harvard spectral classification, developed by Annie Jump Cannon in the early twentieth century, sorting stars by the absorption features in their spectra. The sequence O, B, A, F, G, K, M runs from hottest blue stars above 30,000 kelvin down to cool red dwarfs below 3700 kelvin. The order is a temperature ladder; the strange letters survive from an earlier scheme that was later reorganised.

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