3.1 The H-R diagram
The Hertzsprung-Russell (H-R) diagram displays the photospheric temperatures and luminosities of the stars. The corresponding radii are obtained from Equation A. The H-R diagram is a very useful aid to our understanding of the stars and their evolution.
The stars tend to concentrate into certain regions of the H-R diagram, and so some combinations of temperature and luminosity occur far more commonly than others. These concentrations define various classes of stars, the main classes being main sequence stars (about 90% of observed stars), red giants, supergiants, and white dwarfs.
We can explain the concentrations on the H-R diagram as places where stars spend comparatively large fractions of their lives, the main sequence phase accounting for the largest fraction.
Different types of variable stars help our understanding of stellar evolution. The supergiant phase ends in a Type II supernova - a huge explosion that destroys the star. The T Tauri stars (one type of irregular variable) seem to be on the threshold of joining the main sequence, approaching it from above on the H-R diagram. Regular variables, such as the Cepheids, give us clues about some of the processes that are of importance in stellar evolution. The novae (another type of irregular variable) help us to understand disturbances to the normal course of evolution that occur in binary systems, and this aids our understanding of the normal course itself.