The H-R (Hertzsprung-Russell) diagram plots the luminosity or magnitude of stars on the vertical axis and the photospheric temperature, color, or spectral type of those same stars on the horizontal axis. In a typical population of stars, the vast majority of stars will appear on a narrow diagonal band called the main sequence; the sequence runs from hot-and-luminous stars to cool-and-dim stars. Stars that are cool yet luminous, usually red giant stars, are not on the main sequence; star-like objects that are hot yet dim, such as white dwarfs, are also not on the main sequence. Stars not on the main sequence are generally in the end of their life cycles, and their locations on the diagram indicate the stage of life cycle they are in.

Among the many ways to analyze the H-R diagram, looking at the bright and dim limits of the main sequence can help determine the age of the population; the number of different kinds of non-main sequence stars can help determine the evolutionary history of the population; and an extra band of stars parallel to the main sequence could indicate the presence of a second population of star mixed in with the first. Almost every detail of where the data points are on the H-R diagram can provide a valuable piece of evidence about the nature of a complex stellar population.