By 1939, the salient properties of nuclear reactions are sufficiently well understood to explore in detail some fundamental physics questions. In this article, Bethe explains that the reactions of carbon and nitrogen with protons are a main energy source in stars. He illustrates that the reactions are cyclical whereby carbon and nitrogen act as nuclear catalysts. And he explores how these findings might shed light on astrophysical problems. Bethe receives the 1967 Nobel Prize in Physics for this work. (See also Phys. Rev. Focus 21, story 3.)