56Fe. These two processes are responsible for the electromagnetic radiation from type Ia supernovae. In combination with the changing transparency of the ejected material, they produce the rapidly declining light curve. Core collapse supernovae are on average visually fainter than type Ia supernovae, but the total energy released is far higher, as outlined in the following table. In some core collapse supernovae, fallback onto a black hole drives relativistic jets which may produce a brief energetic and directional burst of gamma rays and also transfers substantial further energy into the ejected material. This is one scenario for producing high-luminosity supernovae and is thought to be the cause of type Ic hypernovae and long-duration gamma-ray bursts. If the relativistic jets are too brief and fail to penetrate the stellar envelope then a low luminosity gamma-ray burst may be produced and the supernova may be sub-luminous. When a supernova occurs inside a small dense cloud of circumstellar material, it will produce a shock wave that can efficiently convert a high fraction of the kinetic energy into electromagnetic radiation. Even though the initial energy was entirely normal the resulting supernova will have high luminosity and extended duration since it does not rely on exponential radioactive decay. This type of event may cause type IIn hypernovae. Although pair-instability supernovae are core collapse supernovae with spectra and light curves similar to type II-P, the nature after core collapse is more like that of a giant type Ia with runaway fusion of carbon, oxygen, and silicon. The total energy released by the highest-mass events is comparable to other core collapse supernovae but neutrino production is thought to be very low, hence the kinetic and electromagnetic energy released is very high. The cores of these stars are much larger than any white dwarf and the amount of radioactive nickel and other heavy elements ejected from their cores can be orders of magnitude higher, with consequently high visual luminosity. Progenitor The supernova classification type is closely tied to the type of star at the time of the collapse. The occurrence of each type of supernova depends on the progenitor star's metallicity, since this affect's the strength of the stellar wind and thereby the rate at which the star loses mass. Type Ia supernovae are produced from white dwarf stars in binary star systems and occur in all galaxy types. Core collapse supernovae are only found in galaxies undergoing