Between 1911-13, Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell independently developed what is now known as the Hertzsprung-Russell diagram. Used by astronomers, a Hertzsprung-Russell diagram is a two-dimensional graph or plot used to depict various types of stars and stages of the stellar life cycle. Absolute magnitude (or intrinsic luminosity on a logarithmic scale) on the vertical axis is plotted against stellar temperatures recorded on the horizontal axis.

Because the spectral appearance of star depends on temperature of their photosphere, spectral classification is a grading of stars according to their surface temperature (i.e., on the horizontal axis of a Hertzsprung-Russell diagram) and does not directly reflect the star's chemical composition. Spectral classes group stars according to similarities in violet, blue, and green portions of visible spectrum. Stars are subdivided into seven temperature groups called spectral classes (designated as O, B...