Robert Kirshner, an astrophysicist from the Harvard-Smithsonian Center for Astrophysics, answers the question, "What is a supernova?"
"A supernova explosion is something where you see a star that you hadn't seen before suddenly turn billions of times brighter than it was," Kirshner said. "So it's an exploding star, and we think we know the energy for these stars comes from either nuclear energy -- that is thermonuclear fusion like a bomb going off for some supernova -- or from gravity, from the collapse of a star down to form a little nugget of a neutron star or even a black hole.
"Supernova 1987A, we think, is of this second kind. It's the kind that's powered by gravity where the center of a star runs out of fuel, collapses in a fraction of a second and then bounces and blasts its material out into the gas between the stars. So a supernova explosion is an exploding star. It's something that is the end of a star, and it's a tremendously energetic explosion."