While novae can occur many times, over and over again, a supernova can happen only once. Instead of only expelling hot gasses from its surface, a supernova is much more powerful, and completely annihilates the star. There are two types of supernovae, one begins with a white dwarf, the other with a supermassive star. In both types, the star collapses in on itself, and then rebounds with an explosion of unimaginable energy.