What are supernovae caused by?
When nuclear fuel runs out.
In binary star systems.
A supernova happens when there is a change in the core of the star. A change can occur in two different ways, with the result of a supernova.
The first type of supernova happens in binary star systems. A binary star system is when two stars orbit the same point. One of the stars, a white dwarf, steals matter form its companion star. Eventually, the white dwarf accumulates too much matter. Having too much matter, causes the star to explode, resulting in a supernova.
The second type happens at the end of a star's life cycle. When a star runs out of nuclear fuel, some of its mass flows into the core. Eventually, the core becomes so heavy, that it cannot withstand its own gravitational force. The core collapses and a supernova happens.