What is a Supernova? A supernova is simply defined as the explosion of a star in space.
How do they occur? Any time the center of a star is significantly altered, a supernova takes place. However, this can happen in multiple ways. First: A binary system consists of two stars that orbit the same point. The white dwarf in the system will steal matter from its companion star. After a while, it will have stolen too much matter, grown too large, and explode. Second: At the end of a stars life, it begins to run out of nuclear fuel. Therefore, some of the stars mass flows into its core. Eventually the core becomes so massive that it can't hold itself and collapses. The explosion results in a supernova.