We examine the constraints imposed on the numbers and interactions of light particles by our understanding of the late stages of stellar evolution, including red giants, carbon-burning stars and cooling neutron stars. We show that these astrophysical considerations restrict the number of neutrino types to be less than 10+2±1. This result complements the standard constraints from cosmological nucleosynthesis, which was unable to exclude numbers of neutrinos between a few thousand and the best particle physics limit of order 105. We also investigate the constraints on supersymmetric theories with a light photino and gravitino, finding that the supersymmetry breaking scale parameter f>O(100GeV) and the selectron masses are >20 to 40 GeV. Finally, we study energy-loss rates by majoron and invisible axion emission.