The Sun is ~4.6 billion years old. Within ~5 billion years, the hydrogen fuel in its core will run out, and the Sun will begin expanding into a red giant star. In ~6.5 billion years, the helium in its core will ignite and the Sun will settle into a helium-burning phase lasting ~100 million years. In ~7.5 billion years, the helium will be exhausted, and the Sun will enter the asymptotic giant branch, and its envelope will swell to near Earth’s orbit.
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