What exactly is a nova star?
What does it mean when a star goes nova?
If you looked up its definition, you would find that a nova is a star that suddenly shows an increase in its level of brightness but slowly returns to its original state. This means a nova is a star that suddenly gets very bright for a short period of time. However, a nova is a little more complex than just a star that gets very bright. A number of occurrences and events must happen for a star to become a nova.
Before the ability to see deep into space with modern instruments, astronomers would call a new star a nova, which means “new star.” This was slightly incorrect because these stars were already there, but they were just not bright enough to see with the naked eye.
Binary Star System
Commonly, novas start as white dwarf stars that are part of a binary star system. Binary stars are two stars that are close together in the distance and revolve around each other. You can think of these two stars as a pair or companion stars. The white dwarf is small and dense, and it is essentially the insides of a star that lost its outer layers: these are older stars that have lost their shine and fuel.
The white dwarf is most likely paired with a red giant. The white dwarf begins to pull on its companion, the red giant, with gravitational forces. The white dwarf takes fuel and matter, especially hydrogen, from its companion and pulls it towards itself. The matter is quickly hurled towards the surface of the white dwarf.
Layers of material from the red giant gather on the surface of the white dwarf. As the layers are created, the material begins to grow hotter. When the layers become compressed and hot enough, the material begins to combust and explode through thermonuclear reactions. The hydrogen material begins to react specifically with helium. The explosion of the materials causes the white dwarf to suddenly become bright as it throws off some of the material. This is what astronomers were seeing when they thought new stars were appearing in the sky!
The white dwarf could take months to hundreds of years to go through this process and return to its original brightness. After it combusts some of the material, it then calms. The white dwarf survives this explosion and lives to repeat this process, making it a recurrent nova. The speed at which the nova explodes and repeats the process depends on the size and material of both the white dwarf and its companion star.
A classic nova is a star that is dim and then becomes up to a million times brighter than its original brightness. Recurrent novas and dwarf novas are usually less bright at their peak than the classic novas. Despite having different names, all the novas are part of the same grouping and undergo the process of gathering material from its companion star and then exploding.
Remember that novas are different than supernovas. Novas are stars that gather material from a companion star and then become bright for a period as they burn off those materials.