Hertzsprung–Russell diagram

Hertzsprung–Russell diagram (HR diagram) A graph on which a measure of the brightness of stars (usually their absolute magnitude) is plotted against a measure of their temperature (either spectral type or colour index). The diagram shows how the luminosities and surface temperatures of stars are linked. From a star's position on the diagram, astronomers can estimate its mass and the stage of its evolution.

Most stars lie on the main sequence, a strip which runs from the upper left to the lower right of the diagram. A star on the main sequence is burning hydrogen in its core, and during this phase of its life will remain at a point on the diagram that is determined by its mass. Other areas of the HR diagram are populated by stars that are not burning hydrogen in their cores, but may be burning hydrogen in a thin shell. The most prominent of these areas is the giant branch, consisting of stars which have exhausted the hydrogen fuel in their cores. Other features are the strips occupied by supergiants, with luminosities 300 to 100 000 times that of the Sun, and white dwarfs, dying stars with luminosities typically 10 000 times less than the Sun's. Theories of stellar evolution must explain the various features of the HR diagram. It is named after H. N.Russell and E.Hertzsprung, who independently devised it. See also colour–luminosity relation; colour–magnitude relation.