The Hertzsprung-Russell diagram is a graph of stellar color, temperature, or spectral type against stellar luminosity or absolute magnitude. It was first plotted by Henry Norris Russell in 1913, but was discussed independently by Ejnar Hertzsprung at about the same time.
The Hertzsprung-Russell diagram is dominated by the main sequence (so named by Arthur Eddington), which forms a curved, diagonal band from bright blue stars to faint red ones, and contains stars in their core hydrogen-burning stage, and the giant branch, occupied by red giants. Other conspicuous regions are represented by the supergiants (above the giant branch) and the white dwarfs (below the main sequence).
The HR diagram can be seen as both a snapshot of the state of a large collection of stars, or a generalization of the evolutionary pathways of individual stars.
How the Hertzsprung-Russell diagram came about
The H-R diagram had its origin in an anomaly noticed by Ejnar Hertzsprung of the Potsdam Observatory, of which he published details in 1905. From his studies of red stars Hertzsprung noticed that if near stars were arranged in order from type B (white) to type M (red) their absolute magnitude decreased progressively towards the red, but that distant red stars were much more luminous than near red ones – comparable in luminosity to the white stars. Hertzsprung therefore suggested that there were two distinct types of star – one group, which he termed "dwarf" stars, whose luminosity varied with spectral type, and another group, which he called "giants", whose members were all bright stars and showed no variation of luminosity with spectral type. Hertzsprung was later able to distinguish the two types by means of slight differences in their spectra. Dwarf stars were found to greatly outnumbers giant stars.
The director of Princeton Observatory, Henry Russell, independently discovered the same differentiation during a study of the absolute magnitude of the stars he was observing in a parallax program. Russell, with Harlow Shapley, carried out an ambitious program of stellar density determinations, which also confirmed the existence of the two types of stars. In 1911 Russell demonstrated that the stars in many open clusters showed the same properties, almost all of the stars proving to be dwarf type; some giants were however found in two of the clusters. The original H-R diagram was published by Russell in 1913.