The Hertzsprung-Russell diagram is a “two-dimensional” graph of the mathematical relationship between the absolute magnitude, luminosity, stellar classification, and surface temperature of stars—all resulting in a diagram of the stellar life cycle. It was plotted by Danish astronomer Ejnar Hertzsprung (1873–1967) in 1911 and independently by American astronomer Henry Norris Russell (1877–1957) in 1913.