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Henry Norris Russell (1877-1957), the American astronomer. In 1913 he discovered that the absolute magnitude (intrinsic brightness) of stars correlates well with their spectral types, which are indicative of surface temperature, and related to their colour. He plotted these two parameters to make what is now known as the Hertzsprung- Russell diagram. Although its interpretation by Russell as being representative of the path of stellar evolution was quickly abandoned, the diagram has remained an important tool in astro- physics. Russell also used the solar spectrum to study chemical abundances in the Sun, and so was able to make an estimate of its hydrogen content.
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