A supernova is a massive explosion that occurs upon the death of an old star. When a massive star runs out of nuclear fuel, the star collapses under gravity. The resulting shock wave blows the outer layers of the star into space, possibly leaving a core that becomes either a neutron star or a black hole, depending on the remaining mass. A different kind of supernova occurs (more rarely in our Milky Way Galaxy) when a white dwarf star in a tight binary system swallows material from her companion, eventually becoming too massive and collapsing.

In Star Trek, supernova which occurred 600,000 years ago, during the Age of Makto, was responsible for the fall of the Tkon Empire. (TNG: "The Last Outpost"). In 2366, the star Beta Stromgren exploded as a supernova. The last stages of the star's life were observed by the USS Enterprise-D while taking first contact with the creature Gomtuu. (TNG: "Tin Man")

As of the 24th century, only three Starfleet vessels ever witnessed supernovae firsthand. The record for the closest observation was less than 10 billion kilometers and was set by the USS Voyager in 2373. A number of supernovae occurred within a small region of the Delta Quadrant, as a result of the Q Civil War. These supernovae were actually the result of spatial disruptions within the Q Continuum. The crew of Voyager, acting on information provided by the female Q, used the explosion of one of these supernovae to enter the Continuum themselves, in order to rescue Q and Captain Janeway. (VOY: "The Q and the Grey")

In the Alpha Quadrant, the Changeling posing as Doctor Julian Bashir attempted to trigger a supernova in the Bajoran sun using trilithium, tekasite and protomatter. The Founder was stopped by Kira Nerys, Jadzia Dax and the crew of the USS Defiant. (DS9: "By Inferno's Light").

In the real universe, supernovae are the death explosion of certain types of star, resulting in a sudden, vast increase in brightness followed by a gradual fading. They are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy before fading from view over several weeks or months. During this short interval, a supernova can radiate as much energy as the Sun could emit over its life span.

Several types of supernovae exist that may be triggered in one of two ways, involving either turning off or suddenly turning on the production of energy through nuclear fusion. After the core of an aging massive star ceases to generate energy from nuclear fusion, it may undergo sudden gravitational collapse into a neutron star or black hole, releasing gravitational potential energy that heats and expels the star's outer layers. Alternatively, a white dwarf star may accumulate sufficient material from a stellar companion to raise its core temperature enough to ignite carbon fusion, at which point it undergoes runaway nuclear fusion, completely disrupting it. Stellar cores whose furnaces have permanently gone out collapse when their masses exceed the Chandrasekhar limit, while accreting white dwarfs ignite as they approach this limit (roughly 1.38 times the mass of the sun). White dwarfs are also subject to a different, much smaller type of thermonuclear explosion fueled by hydrogen on their surfaces called a nova. Solitary stars with a mass below approximately nine solar masses, such as the Sun itself, evolve into white dwarfs without ever becoming supernovae.

Supernovae are the primary suppliers of heavy elements for the universe nucleosynthesis. Elements necessary for life, such as carbon and oxygen, as well as heavier elements like iron, are produced by nucleosynthesis within the star. In the explosive death of the star, these elements are thrown out so that they may be recycled by other stars and gases. The amount of heat and pressure released from a supernova explosion may create new regions of star birth by compressing the surrounding interstellar medium. In addition, supernovae are used as light beacons to measure cosmological distances. Important as they are, few supernovae have been observed in our own galaxy. The last one in our galaxy exploded in 1604 and was observed by Johannes Kepler before the invention of the telescope.