In 1938, following a suggestion by C. F. von Weizsäcker, he and the American physicist Charles Louis Critchfield (1910–94) worked out the details of the proton–proton chain, and argued that it would account for energy production in stars less massive than the Sun. The following year, independently of Weizsäcker, he suggested that the carbon–nitrogen cycle, in which hydrogen is converted into helium, is the means by which the Sun and more massive stars generate their energy. For this work he received the 1967 Nobel Prize in Physics. With the German-born theoretical physicist Walter Heinrich Heitler (1904–81) he developed the cascade theory of cosmic rays.