Supernovae

Abstract

Supernovae are the explosion of a star. This awesome and exciting realization came about at the turn of the century from the following sequence of observations. The sudden flaring up of a star had been recognized many times by astronomers and had been called a nova, but then the pioneering work of Hubble indicated that galaxies or associations of stars of hundreds of billions at a time, called galaxies, populated our Universe. When a star brightens up in a distant galaxy to a luminosity that is comparable to all the stars in the galaxy, then this phenomenon came to be called a supernova. The color temperature of these objects and the fact that the light curve rose to maximum in something like a week are sufficient alone to deduce that something like a solar mass of matter must have expanded at a velocity corresponding to energies of several MeV nucleon^-1 and therefore represented a kinetic energy greater than any binding energy of stars then known. Since the violence of the explosion is so great, it was natural to consider exotic mechanisms for its origin. Fritz Zwicky in his pioneering work on supernovae even made the wild suggestion that the formation of a neutron star could be the source of the energy and indeed I believe he was correct because now we have seen neutron stars (pulsars) at the center of the remnants of supernovae — the Crab and the Vela. In the past, astrophysicists had to grasp almost at science-fiction to find an explanation for supernovae, but now it appears feasible to transport the energy released in the binding of a neutron star to the outer layers of the imploding star by an extraordinarily large neutrino flux, thereby pushing and ejecting the outer layers. Conversely, it is equally plausible that most stars evolving to a carbon-oxygen core will thermonuclearly detonate and destroy themselves. The neutrino transport theory of supernovae has recently been bolstered by the new theoretical considerations concerning the weak interaction force and how this relates to resonant neutrino scattering off larger nuclei.