Main-Sequence stars accumulate helium at center, as nuclear fusion turns hydrogen into helium. In stars with average mass, 30% of stars, when helium becomes more than hydrogen, helium fuses to make carbon. This makes star hotter, so hydrogen nuclear fusion becomes faster. Outer hydrogen layer expands. After 10^9 years, outer layer is thousands of times bigger and is cooler, so star {red giant}| is red.
Physical Sciences>Astronomy>Star>Kinds
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Date Modified: 2022.0224