A Nova – Hydrogen Ignition on a WD
Hydrogen can accumulate on the surface of the white
dwarf. If enough builds up, it can re-experience the hydrogen shell burning
phase. But there is no upper star to hold the fusion in. An explosive
burning phase engulfs the surface, and there is a huge explosion as material
is ejected. The body of the white dwarf barely cares, and settles back down
to receiving
more hydrogen (so novae can re-occur many times).