Type Ia supernovae are some of the brightest explosions in the Universe, and some of the most important, since they help us measure the Universe's expansion. Nevertheless, determining exactly what is going on to produce this type of supernova has been a challenge: though white dwarfs are almost certainly involved, astronomers have yet to identify the exact process that causes one to explode. Recent analysis of Hubble Space Telescope data, performed by Bradley E. Schaefer and Ashley Pagnotta of Louisiana State University, argues strongly that at least some type Ia supernovae are the result of a merger between two white dwarfs.
A white dwarf is the dense core that remains after a relatively low-mass star like our Sun has burned through its lighter elements. The heat of fusion is no longer able to counteract a gravity-driven collapse; instead, it's balanced by quantum degeneracy pressure from the Pauli exclusion principle. If it collapsed any further, electrons would be forced into the same quantum state, which isn't possible. (This is similar to the force that keeps neutron stars from collapsing.)