Supernova
A supernova is the violent explosion of a star, either the core collapse of a massive star or the detonation of a white dwarf, that briefly outshines a galaxy.
Definition
A supernova is a gigantic explosion at the death of a star that can briefly outshine an entire galaxy and blasts the star's material into space. It happens in two ways: when a star more than about $8$ times the Sun's mass runs out of fuel and its core collapses, or when a white dwarf gains too much mass from a companion and detonates; the collapse type can leave a neutron star or black hole. In a core-collapse supernova the iron core exceeds the Chandrasekhar mass and falls inward in under a second, and the rebounding shock, driven mainly by neutrino heating, ejects the envelope at speeds near $10{,}000\ \text{km/s}$, releasing about $10^{46}\ \text{J}$, $99\%$ of it as neutrinos. Type Ia supernovae are thermonuclear detonations of white dwarfs near $1.4$ solar masses with nearly uniform peak luminosity, and both types forge elements beyond iron.
Example
In the year 1054, Chinese observers recorded a new star bright enough to see in daylight for weeks; its remains are the Crab Nebula, now about $11$ light-years across, expanding at roughly $1{,}500\ \text{km/s}$, with a pulsar at its center spinning $30$ times per second. Supernova 1987A, in the Large Magellanic Cloud about $168{,}000$ light-years away, was visible to the naked eye and was the closest supernova seen in almost $400$ years.
Key Insight
The gold in jewelry and the iron in your blood were made in supernovae and other stellar deaths long before the Sun formed, so you are made of star material. Supernovae are rare in any single galaxy, only a few per century in the Milky Way, but across billions of galaxies astronomers catch thousands each year. Fusion cannot release energy beyond iron, so an iron core is a dead end; the star explodes because it has run out of ways to hold itself up, and it is the collapse, not fusion, that powers the blast.