Who invented the stellar classification system 'OBAFGKM'?
The famous spectral sequence of stellar types was developed by Harvard
astronomer E.C. Pickering based on an original scheme using the
alphabet sequence from A to Q. He used stellar spectra obtained in the 1890's
to distinguish stars that had similar spectra. Then, in the early 1900's
Henry Draper completed the famous 'Henry Draper Catalog' involving the
spectra of over 200,000 stars. This led to the refined Harvard Spectral
Sequence of O B A F G K M, as many of Pickering's original classifications were
found to be redundant. This was replaced in 1943 by the Morgan, Keenan, and
Kellman OBAFGKM sequence with decimal sub-typed such as O4, G5 and M7 based
on precise spectral line strength measurements of certain diagnostic
indicator lines.
The above figure and more details can be seen at Jesse Allen's page over at NASA/Goddard