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Runaway Universe
Birth of a Supernova
Type 1a
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Type 1a (1)

Most of the heat that a star generates comes about through nuclear reactions in its core. The reactions are triggered when the high pressure and temperature within the star cause hydrogen atoms to fuse together to form helium atoms. When a star this size runs out of hydrogen, its core begins to contract and its outer layers expand. The star becomes a red giant.

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