Los Alamos National Laboratory · Attribution

Hans Bethe worked out in 1939 how stars generate their energy, identifying the CNO cycle as the fusion process powering heavier stars — the discovery that won him the 1967 Nobel Prize in Physics. During the war he led the Theoretical Division at Los Alamos, calculating the critical mass for the first atomic bombs and developing the implosion method used in the Trinity test and the bomb dropped on Nagasaki. He later campaigned with Albert Einstein against further nuclear weapons testing and the arms race he had helped build.