The Hertzsprung–Russell (H–R) diagram was developed around a century ago by the Danish chemist and astronomer Ejnar Hertzsprung (during 1909–1911) and the American astronomer Henry Russell (during 1910–1913). The H–R diagram has been called “arguably the most famous diagram in the history of astronomy.”
The H–R diagram plots stars by their spectral class, colour, or effective surface temperature (horizontally) and their absolute magnitude or luminosity (vertically). The diagram not only revealed some intriguing patterns, but hinted at a theory about the life story of stars. It remains an important teaching and visualisation tool to this day.
This version of the H–R diagram is from Richard Powell at the Atlas of the Universe. It plots data on a set of 23,000 stars.