After a star leaves the Main Sequence, its core contracts and heats up. Hydrogen burning ignites in a shell around the core, causing the envelope to expand, but as it does, it cools. The cooler but bigger star becomes redder but more luminous.
Eventually, the temperature in the core reaches 100 million degrees and helium burning ignites in the core. The star at this point has a surface temperature of 3500 degrees K and a radius equal to the orbit of Mercury. It is now a red giant.
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