 were largely determined by the star's mass and chemical composition, which became known as the Vogt-Russell theorem
(including Heinrich Vogt who independently discovered the result).Since a star's chemical composition 
gradually changes with age (usually in a non-homogeneous fashion), stellar evolution results.Russell dissuaded Cecilia Payne-Gaposchkin from concluding that the composition of the Sun is different from that of the Earth in her thesis, as it contradicted the accepted wisdom at the time.