Abstract
In this chapter addresses the sites and occurrence rates of supernovae. In a few dozen cases, archival presupernova images revealed the immediate stellar progenitors of supernovae, particularly those of Type IIP (SN IIP). The progenitors of SN IIP were red supergiants at the time of explosion; their inferred initial main-sequence masses have ranged from about 8-16 solar masses, with most near the lower end of this range. Supernovae of Types IIP, IIL (prominent linear declines in magnitude), IIn (prominent narrow lines of circumstellar interaction), IIb (only thin outer layers of hydrogen), Ib (little or no hydrogen), and Ic (little or no hydrogen or helium) occur almost exclusively in spiral and irregular galaxies that have ongoing star formation, implicating massive-star progenitors and core collapse. Supernovae of Type Ia (SN Ia) occur in all kinds of galaxies, including ellipticals dominated by old stellar populations, indicating that SN Ia do not arise from core collapse. Determinations of occurrence rates of supernovae in other galaxies suggest that our Milky Way galaxy should produce about 3 supernovae per century, 20% SN Ia, 30% SN IIb, Ib and Ic, and 50% SN IIP. The chapter also discusses the variation of supernova rates with redshift and the SN Ia delay-time distribution.
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