A two-dimensional diagram used extensively in astronomy, developed independently by Ejnar Hertzsprung in 1911 and Henry Norris Russell in 1913. In its original form, the Hertzsprung-Russell (H-R) diagram was a plot of absolute visual magnitude versus spectral type (O, B, A, and so on). Variants are now commonly used, avoiding requirements of and uncertainties due to spectral classification. The vertical axis of the diagram is some suitable measure of the power output of the star, while the horizontal axis indicates the temperature (or color) of the star's visible surface, or the corresponding spectral type. Each point in the plot represents a nearby star of known distance. In any of its forms, the diagram reveals the most fundamental correlation among observed stellar properties discovered to date.