Hans Bethe (1906-2005, left) and Fritz London (1900-1954, right), German-US physicists. Bethe was awarded the Nobel Prize in Physics in 1967 for his work on energy production in stars. He proposed the carbon cycle as a source of stellar energy. This is a series of nuclear reactions involving carbon, nitrogen and oxygen, that plays a major part in energy production in hot stars. With Alpher and Gamow, he proposed a theory of the formation of heavy elements in the early universe. Fritz London's work included the London dispersion force (1930), superconductivity (1935, with his brother Heinz), and linking Bose-Einstein statistics and superfluidity (1938). Photographed during the Fifth Washington Conference on Theoretical Physics, Washington DC, USA, in January 1939.