Transcript: There’s a simple formula to give an approximation for the lifetime of a main sequence star. The lifetime is the mass divided by the luminosity of the star, both in solar units, times nine billion years. However the lifetime does not depend exclusively on mass. As shown by theorists Russell and Vogt in the 1920s the final state of a main sequence star depends both on its mass and its chemical composition. For example a one solar mass star of three quarter hydrogen one quarter helium by mass composition can only form one stable configuration. However it will be a slightly different configuration from a one solar mass star made of half helium and half hydrogen. They will have different positions on the HR diagram. The Russell-Vogt theorem allows astronomers to calculate stellar evolution more accurately because as a star evolves it is continually changing its chemical composition by the conversion of hydrogen into helium, and thus the stable configuration of the star is also evolving with time.