Novae are not to be confused with their more explosive supernovae brothers. The latter are the result of very large stars such as red supergiants collapsing, or most of the mass of a white dwarf explosing. A supernova will typically eject more than 1.38 solar masses (the mass of our Sun) of material. A nova, by comparison, ejects just 1/10,000th of a solar mass.
Novae occur in binary systems where two stars are orbiting one another. One of these will typically be a small, white dwarf star and the other a red giant. As the red giant expands it moves into the gravitational influence of its small companion. A white dwarf has a very strong gravitational field and therefore rips matter from the red giant. Once the white dwarf has absorbed so much matter that it can no longer support itself, it suddenly explodes as a nova and ejects its hot surface gas. However, the central white dwarf star survives, unlike a type 1 supernova where the majority of the white dwarf’s mass explodes. It immediately begins consuming matter from the red giant again and the nova process repeats within a period of 100,000 years.
Image credit: NASA/JPL-Caltech