A nova is an enormous nuclear explosion caused by the accretion of hydrogen
onto the surface of a white dwarf.
When a white dwarf has a close companion star, the companion will often begin to
have its outer atmosphere drawn away from it by the white dwarf's
gravity as the companion star ages and expands
into a red giant.
A nova must not be confused with the much more energetic supernova, which is the explosion of a whole,
massive star ending its life.