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A supernova (abbreviated SN, plural SNe after supernovae) is a stellar explosion that is more energetic than a nova. It has a tremendous power of washing out a whole galaxy.
A supernova is essentially an exploding star. A supernova is formed from two possible outcomes:
Type I supernova: star accumulates matter from a nearby neighbor until a runaway nuclear reaction ignites.
Type II supernova: star runs out of nuclear fuel and collapses under its own gravity.
Supernovas can briefly outshine entire galaxies and radiate more energy than our sun will in its entire lifetime.
In astronomy, the term "supernova" refers to a particular type of exploding star. The name comes from the Latin word for "new" (nova), since a star that is too distant from the Earth to be seen with the naked eye can suddenly become visible when it explodes, appearing as a new star in the sky.
A star is essentially a gigantic ball of hot gas, which forms when a cloud of gas (mostly hydrogen gas) contracts under the force of its own gravity. In a large gas cloud, gravity pulls atoms together, so the cloud will contract and become progressively smaller, denser, and hotter. As it contracts, it assumes a spherical shape and starts rotating around its own axis. At this stage, it is called a proto-star. The contraction of the proto-star only stops when the gas in its center becomes so dense and hot that the hydrogen atoms start fusing (combining together) to form helium atoms. This nuclear reaction releases a large amount of energy, which further increases the gas temperature and pressure, thus halting the initial contraction. This marks the "birth" of the new star. The energy produced by the hydrogen fusion in the star's core flows toward the surface and is released in the form of electromagnetic radiation (visible light, ultraviolet light, x-rays, infra-red light). The star will then remain in this state until most of the nuclear "fuel" in its core is spent. A star with a mass (i.e., size) similar to that of the Sun, or smaller, will never explode. It will slowly expel its outer layers, and its core will cool and fade. But a star with a mass ten times larger than the Sun's (or larger) will have a dramatic death. Once it runs out of nuclear fuel, its core suddenly collapses under the enormous weight of the star, releasing a huge amount of energy, which rips through the star's outer layers, blowing them out into space. This explosion is called a supernova (a Type II supernova, to be more specific), and it can be as bright as a hundred million stars. A supernova can outshine a whole galaxy. What is left from the explosion is an expanding cloud of gas (called the supernova remnant) and the high-density, fast-rotating core, in which all atoms have been crushed together into neutrons. This object is known as a neutron star, or pulsar. When extremely massive stars explode as supernova, their core can turn into a black hole.
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