Supernovae. Part I: the events

Since the heroic era of Baade and Zwicky, our understanding of supernovae has advanced in hops and skips rather than steadily. The most recent jump has been into fairly general agreement that observations of Type I's can be interpreted as the manifestation of the decay of about $1M_{\odot }$ of ${\mathrm{Ni}}^{56}$ and observations of Type II's as the manifestation of ≳${10}^{51}$ ergs deposited at the bottom of a supergiant envelope by core bounce as a central neutron star forms. This paper explores the history of these and other ideas of what is going on in supernovae, the presupernova evolution of the parent stars and binary systems, observed properties of the events, and models for them. A later paper (Part II: the aftermath) will address the results of supernovae—their remnants, production of cosmic rays and gamma rays, nucleosynthesis, and galactic evolution—and the future of supernova research.