Some stars can become white dwarves in a more spectacular way.
A nova is a new star which at first shines brightly and then fades over a period of months. What causes them?
We have seen that most stars have a companion tied to it by gravity, forming a 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 and then a .
When the other star eventually becomes a red giant, its outer parts reach out towards the white dwarf. Gas is pulled by 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 , typically a few tens of novae occur every year.
Some novae shine only once, while others flare up and then die down several times over tens of years.
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