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Red giant (upper right) seen from a planet in orbit around it, computer artwork. A red giant is one of the final stages in the life of a Sun-like star. When a star runs out of hydrogen fuel at its core, the core collapses until it gets hot and dense enough for hydrogen burning to begin again. This time the reactions occur in a shell around the old dense core. This causes the outer layers of the star to expand dramatically, and the expansion also cools the gas, making it redder. The Sun will become a red giant in around five billion years.
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