Transcript: Mass is the fundamental quantity that controls stellar evolution. The main sequence on an HR diagram is a correlation between the properties of luminosity and effective temperature. The main sequence runs from high luminosity hot stars down to low luminosity cool stars, but the underlying variable on the main sequence is the mass of the star. High luminosity hot stars are high mass and low luminosity cool stars are low mass. The principles of stellar evolution were first worked out by the English theorist Arthur Eddington in the 1920s. He used the idea of pressure force balancing the gravity inward force in a star to deduce the way that stars evolved. This hydrostatic equilibrium that applies at every point within a star creates a stable situation where the rate of evolution is governed by mass.