A diagram in which the luminosities of stars are plotted against their colours or spectral types. In the conventional way in which this diagram is plotted, luminosity increases logarithmically up the vertical axis and temperature increases from right to left along the horizontal axis. Stars do not occupy all regions of the H-R diagram but form various sequences, the most important being the main sequence, the giant branch and the horizontal branch.
Plotted positions of the stars according to their absolute magnitudes or luminosities on a vertical scale against their spectral class or temperature or color index or a horizontal scale.
is a two-dimensional field of stars where luminosity (total radiation emitted) is the ordinate (dependent variable) and color (surface temperature) is the abscissa (determinant variable). This diagram is used extensively in astronomy to infer properties of stars whose distance makes direct measurement difficult or impossible. In terms of the HR diagram, evolved stars are either overluminous or underluminous for their color, that is, they are above or below the main sequence (q. v.) of the stars.