A Nova is not a new star that appears all of a sudden in the sky, as its name suggests. It's actually a sudden brightness that appears on the surface of an existing white dwarf star in a binary system with another star.The White dwarf star's gravity starts pulling off the material that lies on its binary neighbor, when it is close enough. This material that got accumulated on the surface of the white dwarf mostly contains hydrogen atoms. and occasionally, they got hot enough to start a nuclear fusion and the process begins suddenly. The hydrogen atoms on the surface of the white dwarf gets fused in to the helium atoms and in turn makes the star shine brightly.This process continues until the other star gets completely devoured of matter, and it ranges from a few days to almost thousands of years.Nova should not be confused with Supernova, which completely is a different process. Earlier it was thought that Supernova is a kind of very bright Nova and hence the name. Nova happens very often in the space, not like a Supernova, which appears very rare.