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Violent explosion of a star.  The star is destroyed, with a remnant forming a neutron star (pulsar) or black hole (collapsar) depending on the residual mass.  There are generally two forms of supernova.  One results at the end of the life of a star with at least about eight solar masses, which in a series of progressively shorter-lived and less efficient thermonuclear reactions generates ever heavier chemical elements in layers about the core.  Effectively this is a star that lives fast and dies young.  When each step in the process chokes up the core with nuclear "ash", contractions follow under gravity, driving up temperatures until they are sufficient to synthesise the next heavy element from this waste product.  When the core fills with iron, the end point is reached; it takes more energy to fuse iron into anything heavier than the fusion reaction produces.  At this point the star collapses, driving temperatures into perhaps twelve figures Kelvin and triggering an explosion that blows most of the star to smithereens.  A second type occurs in close orbiting binary systems where one star A, being more massive, evolves more quickly to the red giant phase and develops a thin outer envelope and a core rich in carbon.  Its companion B skims off the outer layer, grows in mass and itself evolves to the red giant stage.  At this point the carbon-rich star A begins reclaiming the hydrogen; when the gas accreting onto it drives its mass over 1.4 solar masses a huge nuclear reaction ensues and A blows itself apart.  Supernova explosions can briefly outshine the combined output of all the stars in at least a modest galaxy.  They are also responsible for seeding the Universe with the heavy chemical elements of which the Earth and our very bodies are made.

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