A star's luminosity, color, and surface temperature are correlated. Most red stars are cool and dim, and most white are hot and bright. But there are some important exceptionsg
The Hertzsprung-Russell, or H-R, diagram plots star color against brightness. Its major feature is the "main sequence," the narrow diagonal band where a star spends most of its life. Ninety percen of all stars fall within the sequence, with brighter, stars at the upper left of the band, and fainter, red stars at the lower right. The Sun, a medium-bright star, lies between these two extremes.
There are other stars on the diagram that have left the main sequence and are neanng the end of their lives. Cool, red giant and supergiant stars, in the upper right corner, have a large surface area that makes them very luminous. Hot, white dwarfs, in the lower left corner, are faint because they are only about as big as Earth.
Based on their locations across the H-R diagram, that is, their color and temperature, stars are divided into seven main spectral types. The hot, blue to white stars are type 0, B, or A; moderate, yellow to orange stars are F, G, or K; and cool, red stars are type M.