The Russell-Vogt Theorem - that the nature of a star is nearly completely determined by its mass, age, and initial composition - accounts for why the stars fall along such a well-defined line in luminosity and temperature when they are burning hydrogen in their interiors. This tendency is particularly clear since the initial composition has only a modest effect compared with the other two. The upper luminosity limit is a result of the tendency of stars more massive than about 100 times the mass of the sun to blow themselves apart, while the lower luminosity limit results from the inability of objects less than about 0.08 times the mass of the sun to develop sufficient pressure and temperature in their cores to burn hydrogen.