Hertzsprung-Russell diagram

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A plot of stellar luminosity against surface temperature, devised independently by Ejnar Hertzsprung around 1911 and Henry Norris Russell in 1913, and the instrument that turned stellar evolution from speculation into a readable pattern. Stars do not scatter across it: they fall into a diagonal main sequence, a giant branch, a supergiant region and a faint white dwarf sequence. Because no star visibly changes in a human lifetime, the diagram is the only way to see the stages - a population of stars of different masses and ages is a snapshot of one life history sampled at many points. Hertzsprung was first to distinguish giants from dwarfs; Russell's prestige meant the plot was initially called the Russell diagram, and the name was corrected later. Gaia parallaxes have since replaced schematic versions with an empirical diagram containing over a billion stars. Not a place.

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