Short answers, pulled from the story.
A supernova is a powerful and luminous explosion of a star. It occurs during the last evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion. At its peak optical luminosity a supernova can rival an entire galaxy before fading over several weeks or months.
Supernovae in our galaxy are expected on average about once every 61 years, with observations suggesting a rate of roughly 1.6 to 4.6 times per century. The last one observed in the Milky Way was Kepler's Supernova in 1604. Several thousand supernovae are typically seen in distant galaxies every year.
Most supernovae are triggered either by the sudden re-ignition of nuclear fusion in a white dwarf or by the sudden gravitational collapse of a massive star's core. A white dwarf can re-ignite through accretion from a binary companion or a stellar merger. A massive star's core collapses once fusion can no longer counteract its own gravity, which must happen once it begins fusing iron.
The first widely recorded supernova was SN 1006, observed in AD 1006 in the constellation of Lupus. It was described by observers in China, Japan, Iraq, Egypt and Europe. The supernova SN 1054, recorded by Chinese astronomers in AD 1054, produced the Crab Nebula.
Astronomers classify supernovae by their light curves and by the absorption lines in their spectra. A spectrum containing hydrogen lines is classified Type II, while one without hydrogen is Type I. Type Ia shows a strong ionised silicon line, Type Ib shows helium lines, and Type Ic lacks helium.
Supernovae are a major source of elements in the interstellar medium from oxygen through to rubidium. Type Ia supernovae produce mainly silicon and iron-peak elements like nickel and iron, while core collapse supernovae eject larger masses of light alpha elements and elements heavier than zinc. Evidence shows a nearby supernova helped shape the composition of the Solar System 4.5 billion years ago.