A nova is nuclear explosion caused by hydrogen on the surface of a white dwarf star, which ignites and starts nuclear fusion in an unstable manner.
Many stars travel in couples called binary star systems.
When one of the stars becomes a white dwarf, its force of gravity sometimes attracts matter from its companion star.
This causes the white dwarf to flare up briefly but brilliantly as a nova, but there is no lasting effect.
The nuclear fusion reaction occurs on the surface of the white dwarf and can reach a temperature of some 20 million Kelvin.
Astronomers estimate that the Milky Way experiences about 30 to 60 novae per year.
The astronomer Tycho Brahe observed the supernova SN 1572 in the constellation Cassiopeia, and described it in his book de stella nova.
