The British theoretician Arthur Stanley Eddington (1882–1944) was the most distinguished astrophysicist of his time. He was the first scientist to propose that the tremendous heat production at a star’s core is what prevents a star from collapsing under its own gravity. His seminal book, The Internal Constitution of the Stars, helped launch the modern theoretical study of stellar evolution. When astronomers puzzled over the nature of Sirius B, Eddington suggested the explanation that turned out to be correct: The matter of Sirius B is in a state called electron degeneracy—a special condition that is not found anywhere on Earth.