Sir Arthur Stanley Eddington (1882-1944), British astrophysicist who pioneered the study of internal stellar structure. In 1926 he showed that in order to remain in equilibrium, a star's inward gravit- ational pressure must balance its outward gas and radiation pressure. He realised that there is an upper limit on the mass of a star above which this balance can't be achieved. He discovered the mass- luminosity relationship, which shows that the more massive a star the greater its luminosity, and which allows the mass of a star to be determined from its intrinsic brightness. His observations of stars close to the Sun's rim during an eclipse provided evidence for the theory of relativity.