A supernova explosion called SN2006gz discovered last year, was found to be due to two white dwarf stars colliding.
Supernovae are originally thought to be of 2 categories:
1) the explosion of a young, massive star when it dies, or
2) a white dwarf star stealing gas from a companion star until it explodes.
However, in the light of SN 2006gz, we have found, yet another way that supernovae can occur!