In stellar astronomy, the Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram) shows the relationship between absolute magnitude, luminosity, stellar classification, and surface temperature. This was devised, c.1910, by Hertzsprung and Russell.
There are two equivalent forms. One is the observer's form which plots the color of the star on one axis and the absolute magnitude on the other axis. These two quantities can be derived from observations.
The theoretician's form plots the temperature of the star on one axis and the luminosity of the star on the other. These two quantities can be calculated from computer models.
The exact transformation from one to the other is not trivial, and depends on the model being used and their parameters (like age and composition).
The H-R diagram is used to define different types of stars, and to match theoretical predictions of stellar evolution using computer models with observations of actual stars.
An examination of the diagram shows that stars tend to fall only into certain regions on the diagram. The most predominant is the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler and less bright), called the main sequence. In the lower-left is where white dwarfs are found, and above the main sequence are the red giants and supergiants.
See also: stellar classification