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Danish astronomer who specialized in photographing the stars and introduced the concept of absolute magnitude. Hertzsprung also demonstrated the relationship between the surface temperature of stars and their absolute magnitude, but his work was ignored until Henry Russell independently developed a similar correlation, which is now named after both of them.
Danish astronomer who classified types of stars by relating their colour to their absolute brightness-an accomplishment of fundamental importance to modern astronomy. The Hertzsprung-Russell diagram of stellar types was named (in part) for him. In 1913 he established the luminosity scale of Cepheid variable stars, a tool for measurement of intergalactic distances.