Hertzsprung, Ejnar [key], 1873–1967, Danish astronomer. Although trained as a chemical engineer, Hertzsprung made his career in astronomy, specializing in exacting photographic observations of stars. In 1905 he discovered high-luminosity, or giant, stars. In 1913 he calculated the distance to the Small Magellanic Cloud by a method still used for measuring galactic and intergalactic distances. His 1922 catalog of star colors and luminosities disclosed the absence of bright stars of intermediate color, called the Hertzsprung gap. Working independently, both Hertzsprung and the American astronomer H. N. Russell developed a graph in which the luminosity of a star is plotted against its surface temperature. Such a graph is now called a Hertzsprung-Russell diagram and is the fundamental piece of observational evidence that the theory of stellar evolution must explain.
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