More than 100 times more luminous than a nova, and capable of outshining all the stars in a typical galaxy, a supernova is the violent, convulsive explosion of a massive star. Two types of explosion can create supernovae. The first occurs in a binary system when a white dwarf siphons off more matter from a companion star than it can support. The dwarf implodes and then rebounds violently from a rigid core in a massive explosion.
The second type of explosion arises when a massive star runs out of fuel. By the end of its life, a star with at least eight times the mass of the Sun has produced iron in its core via nuclear-fusion processes. Since nuclear reactions using i iron consume energy instead of producing it, the star's internal furnace shuts down. Unable to keep the core hot enough, the gas in the core can no longer support the weighty outer layers, and the star caves in on itself. The material crushes in on the core and then rebounds, producing the explosion.
Both types of explosion destroy the original star. Most of the star's outer material is ejected, creating an expanding shell of gas called a supernova remnant. If it survives the blast, the collapsed core of the star may also remain in the form of a spinning neutron star (which might be visible as a pulsar) or a black hole.
Naked-eye supernovae are very rare events—SN 1987A, in the Large Magellanic Cloud, occurred almost 400 years after the previous one!