Solar and Supernova Constraints on Cosmologically Interesting Neutrinos

The sun and core-collapse supernovae produce neutrino spectra that are sensitive to the effects of masses and mixing. Current results from solar neutrino experiments provide perhaps our best evidence for such new neutrino physics, beyond the standard electroweak model. I discuss this evidence as well as the limited possibilities for more conventional explanations. If the resolution of the solar neutrino problem is $\nu_e \to \nu_\mu$ oscillations, standard seesaw estimates of $m_{\nu_\tau}$ suggest a cosmologically interesting third-generation neutrino. I discuss recent nucleosynthesis arguments that lead to an important constraint on this possibility.