Textbook in PDF format This book presents the story of an experiment with historical significance that, for the first time, uncovered the mechanisms responsible for solar energy production and those generating energy in massive stars. The results were obtained by directly measuring the neutrino fluxes produced by individual fusion reactions in the Sun and providing the initial evidence of nuclear reactions occurring in massive stars via the CNO cycle. The CNO cycle is theorized to be the primary channel for hydrogen burning in stars more massive than the Sun and is, in fact, the primary channel for hydrogen burning in the Universe. These groundbreaking discoveries stem from a singular 32-year experiment, spanning from 1990 to 2021