Arthur Eddington. Portrait of Sir Arthur Stanley Eddington (1882-1944), British astrophysicist, aged 21. In 1926 he showed that a star's inward gravitational pressure balanced its outward gas and radiation pressure. He realised that there is an upper limit of about 50 solar masses above which this balance cannot be achieved. He discov- ered 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 the deflection of light from stars close to the Sun's rim during an eclipse provided evidence for Einstein's theory of relativity.