If a white dwarf star is close enough to a companion star its gravitational field can pull hydrogen the other star. This hydrogen will form an accretion disk and eventually heat up to a high enough temperature for fusion to begin again.
The hot gases that will form a planetary nebula appear as a nova before they have cooled and dimmed.
After the core has begun to change to iron, nuclear fusion ceases. This causes the entire star to collapse and releases tremendous amounts of energy.
Novae are actually produced by objects burning up as they fall through our atmosphere.
As a star