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Type II Supernovas
involve massive stars that burn their gases out within a few million years. If the star is massive enough, it will continue to undergo nucleosynthesis after the core has turned to helium and then to carbon. Heavier elements such as phosphorus, aluminum, and sulfur are created in shorter and shorter periods of time until silicon results. It takes less than a day for the silicon to fuse into iron; the iron core gets hotter and hotter and in less than a second the core collapses. Electrons are forced into the nuclei of their atoms, forming neutrons and neutrinos, and the star explodes, throwing as much as 90% of its material into space at speeds exceeding 18,630 mi (30,000 km) per sec. After the supernova explosion, there remains a small, hot neutron star, possibly visible as a pulsar, surrounded by an expanding cloud, such as that seen in the Crab Nebula.
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