The Star Types Hertzsprung-Russell diagram. A schematic diagram showing the main sequence and areas occupied by stars at different stages of evolution. Absolute magnitude is plotted against color index in this example. The dashed lines link stars of different luminosities with the same spectral types but different color indices. Color, temperature or some other comparable quantity may be substituted for spectral type as the quantity plotted along the horizontal axis.
Temperature conventionally decreases towards the right.
Either magnitude or luminosity relative to the Sun are frequently used for the vertical scale. What is now known as a Hertzsprung-Russell diagram was first plotted by Henry Norris Russell in 1913. It was later recognized that Ejnar Hertzsprung had independently put forward similar ideas at around the same time. Any star whose spectral type and luminosity are known may be plotted as a single point on the HR diagram, but the diagram acquires particular significance when plotted for a related group of stars, such as a star cluster. It arises because the most significant factor determining a star's spectral type and luminosity is its mass - the main sequence is in effect a mass sequence.