A nova is a new star which shines brightly and then fades over a period of months. We have seen that most stars have a companion tied to it by gravity, forming a binary or multiple star. In most binaries one star is larger than the other. The large star evolves quicker than the small one, and becomes first a red giant and then a white dwarf . When the other star eventually becomes a red giant, its outer parts reach out towards the white dwarf.
Gas is pulled by gravity towards the white dwarf, forming an accretion disc around it.
Here it gathers in a layer on the surface of the white dwarf until the temperature and pressure at the base of the layer become high enough for nuclear fusion to start. The energy builds up and the temperature rises until a nuclear explosion is triggered, producing the nova and an expanding shell of gas. Note that two of these nova are called novae, because the word is Latin. In any one galaxy , typically a few tens of novae occur every year. Some novae are occur only once, while others repeat over tens of years.