Henry Norris Russell and Ejnar Hertzsprung independently demonstrated that the color of a star was related to its surface temperature. By plotting stars of the different spectral types on a graph with luminosity on the vertical axis and spectral class on the horizontal axis, an astounding pattern emerged. Most of the stars fell in a diagonal strip with blue stars in the upper left and red stars in the lower right. Hertzsprung referred to this strip as the main sequence, an area we now know to be populated by young to middle-aged stars. Main sequence stars have a direct relationship between their mass and luminosity, and temperature – the more massive the star, the hotter and more luminous it is [4].

The stars to the lower left of the main sequence, white dwarfs, are stellar corpses that have consumed all of their available fuel and can no longer undergo nuclear fusion. The initial mass of a star determines whether it will end its life quietly as a white dwarf, or become a red giant, whose ultimate fate is to annihilate itself in a violent supernova explosion leaving behind an extremely dense, rapidly rotating neutron star or possibly a black hole.

Figure 6
This diagram plots the luminosity (absolute magnitude) of stars against their spectral class (temperature). Courtesy of Cornell University.