Hans Bethe

Hans Bethe was a German-American physicist who worked out the sequence of nuclear fusion reactions that generate the enormous energy output of stars, publishing his key results in 1939. Bethe identified two distinct nuclear fusion pathways by which stars convert hydrogen into helium and release energy, the proton-proton chain, dominant in stars around the mass of the Sun, and the carbon-nitrogen-oxygen cycle, dominant in more massive, hotter stars, explaining for the first time in physical detail the process that powers stellar energy production and stellar evolution. He received the Nobel Prize in Physics in 1967 for this contribution to the theory of nuclear reactions in stars. During the Second World War, Bethe led the theoretical division at the Los Alamos laboratory as part of the Manhattan Project developing the first atomic bombs, and after the war he remained an influential voice in American physics and public policy debates over nuclear weapons and arms control for decades, continuing active research well into his later years at Cornell University.