What is a supernova?

There are actually two kinds of supernova (pl. supernovae). The one referred to as a Type II supernova is generally the one people think of when they hear the word “supernova.”

During the “life” of a star, the pressure created by nuclear fusion exerts an outward force that is balanced by the inward force of gravity. When the star runs out of fuel, the pressure is gone and the inward force of gravity causes the star to collapse. As the star collapses it releases gravitational energy that heats and ejects the outer layers of the star. It’s these exploding outer layers that can briefly out shine the entire Galaxy and create what we see as a supernova.