shows 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.
