Supernova are massive stars that are entering a very violent stage of their lives.
A star is a delicate balance between the radiation pressure generated by nuclear reactions deep inside the star, and the combined gravity of all the material that makes up a star. When the reactions stop, the star collapses onto itself catastrophically. Huge amounts of energy are released, and subsequently the star tears itself apart in a violent explosion. Such a supernova can outshine by itself all the other billions of stars in its galaxy combined. Supernovae may used to estimate the distances to the galaxies in which they are located, as well as providing basic information on how stars evolve.