Hertzsprung-Russell diagram

or H-R diagram

Graph in which the absolute magnitudes of stars are plotted against their colours (a measure of their temperatures). Of great importance to theories of stellar evolution, it evolved from charts begun independently in 1911 by the Danish astronomer Ejnar Hertzsprung (1873–1967) and the U.S. astronomer Henry Norris Russell (1877–1957). On the diagram, stars are ranked from bottom to top in order of increasing brightness and from right to left by increasing temperature. Stars tend to cluster in certain parts of the diagram, especially along a diagonal line, called the main sequence, which is the locus of hydrogen-burning stars of different masses.

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