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An H-R diagram (HRD) is a plot of a set of stars' effective temperatures versus their luminosities (or absolute magnitudes, effectively the log of luminosity) on a log-log scale. The modern H-R diagram is an outgrowth of tables and charts produced by Ejnar Hertzsprung and Henry Norris Russell. It is a starting-point to stellar theory, showing key correlations between stars' observable characteristics which stellar theory now associates with the various phases of stellar evolution, such as the main sequence, the red-giant branch, the horizontal branch, the asymptotic giant branch, and white dwarfs.
Main sequence stars generally form a diagonal line in the diagram, from "bright and hot" to "dimmer and cooler" whereas giant stars reside in their own region of the diagram, being both bright and cool. The separation of these regions is known as the Hertzsprung gap (HG), where few stars are found because they evolve relatively quickly from the main sequence to the giant phases. Stellar temperatures range from a few thousand kelvins to around 40,000 (about 3 dex, but ranging cooler if brown dwarfs or even gas planets are included), and stellar luminosities from a thousandth the Sun's to a million times the Sun's (about 9 dex).
H-R diagrams are often plotted of just the stars of a specific open cluster or globular cluster to display the cluster's characteristics. The apparent magnitude can serve as a proxy for the absolute magnitude because the variance in their distances is small compared to the full distances. In this context, the term isochrone (which means "same age") indicates a line (undoubtedly curved) that traces the positions of stars of a specific age. Such H-R diagrams can also be done for the stars of a galaxy.
Similar diagrams can be made such that each point plots an entire galaxy (color-magnitude diagrams), stellar cluster or other grouping of stars.