He also, along with Williamina Fleming designed a stellar classification system based on an alphabetic system for spectral classes that was first known as the Harvard Stellar Classification and became the basis for the Henry Draper Catalog.
In 1888, Fleming discovered the Horsehead Nebula on Harvard plate B2312, describing the bright nebula (later known as IC 434) as having "a semicircular indentation 5 minutes in diameter 30 minutes south of Zeta Orionis."
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