Hertzsprung-Russell diagram

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Hertzsprung-Russell diagram [for Ejnar Hertzsprung and H. N. Russell ], graph (see illustration) showing the luminosity of a star as a function of its surface temperature. The luminosity, or absolute magnitude , increases upwards on the vertical axis; the temperature (or some temperature-dependent characteristic such as spectral class or color) decreases to the right on the horizontal axis. It is found that the majority of stars lie on a diagonal band that extends from hot stars of high luminosity in the upper left corner to cool stars of low luminosity in the lower right corner. This band is called the main sequence. Stars called white dwarfs lie sparsely scattered in the lower left corner. The giant stars—stars of great luminosity and size (see red giant )—form a thick, approximately horizontal band that joins the main sequence near the middle of the diagonal band. Above the giant stars, there is another sparse horizontal band consisting of the supergiant stars. The stars in the lower right corner of the main sequence are frequently called red dwarfs, and the stars between the main sequence and the giant branch are called subgiants. The significance of the H-R diagram is that stars are concentrated in certain distinct regions instead of being distributed at random. This regularity is an indication that definite laws govern stellar structure and stellar evolution . In population I regions (see stellar populations ) like the spiral arms of galaxies or open star clusters, the stars fall almost exclusively on the main sequence. In population II regions like the nuclei of galaxies and globular clusters, the stars are older and have evolved significantly. The most luminous stars have evolved furthest, and an H-R diagram of such a region will show the upper end of the main sequence depopulated and will show a well-developed giant branch. In such a diagram it appears that the main sequence has "burned down" from the top like a candle. Thus, the point at which the main sequence terminates and the giant branch begins is an indication of the age of a star cluster. A modified H-R diagram of the stars in a cluster of unknown distance can be used to determine the absolute magnitude, or luminosity, of the stars. Since the apparent magnitude of a star of given absolute magnitude depends only on the star's distance, the observed apparent magnitude of the stars can be used to calculate the distance to the cluster.

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.

Hertzsprung-Russell diagram (H-R diagram) Plot of the absolute magnitude of stars against their spectral type; this is equivalent to plotting their luminosity against their surface temperature or colour index. Brightness increases from bottom to top, and temperature increases from right to left. Henry Norris Russell devised the diagram in 1913, independently of Ejnar Hertzsprung, who had the same idea some years before. The H-R diagram reveals a pattern in which most stars lie on a diagonal band, the main sequence.