In this paper we describe ASTRA, a computational aid for generating process explanations in nuclear astrophysics. The system operates in two stages, the first using knowledge of quantum theory to produce a set of legal reactions among elements and the second searching for pathways of such reactions that explain the construction of some element from lighter ones. ASTRA has found apparently novel reactions that involve proton and neutron capture, as well as novel fusion reactions that produce neutrons and deuterium. The system has also generated reaction pathways for helium, carbon and oxygen that do not appear in the scientific literature. However, ASTRA also finds many other reaction pathways that are less interesting and that suggest priorities for future research. (C) 2000 Academic Press.