In stellar astronomy, the Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram) shows the mathematical relationship between absolute magnitude, luminosity, stellar classification, and surface temperature. This was devised, c.1910, by Ejnar Hertzsprung and Henry Norris Russell.
There are two equivalent forms. One is the observer's form which plots the
color index 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).
See Sekiguchi and Fukugita, for example, for a transformation between B-V
color index and temperature.
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. The Sun
is found on the main sequence at luminosity 1 (magnitude approx. 5).
See also
References