A supernova is the explosion of a star that has reached the end of its life. Supernovae are among the largest explosions that take place in space. They can briefly outshine entire galaxies and radiate more energy than our sun will in its entire lifetime.

On average, a supernova will occur about once every 50 years in a galaxy the size of the Milky Way. Put another way, a star explodes every second somewhere in the universe. Exactly how a star dies depends in part on its mass. Our sun, for example, doesn't have enough mass to explode as a supernova. But with the right amount of mass, a star can burn out in a fiery explosion.

What's left after the explosion is either a neutron star, an ultra-dense object that can pack the mass of the sun in a city-sized sphere, or a black hole, an even more dense and mysterious object of which even light cannot escape.