Asked by Tim Baldwin
When stars run out of fuel they begin to collapse rapidly under their own weight. Some stars that are large enough naturally end their lives by exploding in a supernova.
Throughout their lives, stars are fighting against the crushing force of their own gravity. Inside, nuclear reactions fuse together smaller elements, like hydrogen, to create bigger ones and release energy. Stars must burn through fuel and release energy to prevent them from collapsing in on themselves, but this cannot go on forever. Eventually the star will run out of its essential fuel entirely, resulting in its explosive end.
The compression of a star many times larger than our Sun can be so much that a rebounding shockwave is created. This can cause a final expansion that releases the layers of a star in a supernova.
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