Complex sentence: A nova (plural') is a cataclysmic nuclear explosion in a star caused by the accretion of hydrogen on to the surface of a white dwarf star, which ignites and starts nuclear fusion in a runaway manner.
Split into simpler sentences: A nova (plural') is a cataclysmic nuclear explosion in a white dwarf star. It is caused by the accretion of hydrogen on to the surface of the star, which ignites and starts nuclear fusion in a runaway manner.