B5A for stars halfway between types B and A, F2G for stars one fifth of the way from F to G, and so on.

Finally, by 1912, Cannon had changed the types B, A, B5A, F2G, etc. to B0, A0, B5, F2, etc. This is essentially the modern form of the Harvard classification system. This system was developed through the analysis of spectra on photographic plates, which could convert light emanated from stars into a readable spectrum.

Mount Wilson classes

A luminosity classification known as the Mount Wilson system was used to distinguish

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