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Mercury, January/February 2005 Table of Contents

Courtesy of NOAO/AURA/NSF.

by Fred Ringwald

In 1896 Annie Jump Cannon began classifying the objective prism spectra of 225,300 stars brighter than 11th magnitude. Her work was for the Henry Draper Catalogue and was conducted at the Harvard College Observatory. Before her, the first cut at classification was to judge a star’s spectrum by the strength of the hydrogen lines present there. Thus a system was born: A-type stars had strong lines, B types strong but less strong than A types, and so on. This system was strictly empirical in that no one knew how stellar spectra form—only that the spectra display certain characteristic dark lines, which varied as a function of the stars' color.

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