Hertzsprung- Russell diagram - (n.)

A diagram on which stars are represented according to their absolute magnitudes (on the vertical axis) and spectral types (on the horizontal axis).Because the physical properties of stars are interrelated, stars do not fall randomly on such a diagram, but instead lie in well-defined regions according to their state of evolution. Very similar diagrams can be constructed that show luminosity instead of absolute magnitude, and temperature or color index in place of spectral type.