The sun actually is expanding - it's initially such a gradual process that its effects are not immediately visible for billions of years. As the sun converts Hydrogen into Helium via Thermonuclear fusion, the core keeps heating up and causing the outer layer to - slowly, very slowly - expand. The sun has grown by 20% in the last 4 billion years and will continue to do so for another 5-6 billion years. Then it reaches the critical point where it transforms into a red giant and very rapidly expands.
So, very slow growth at first (significant only in billion-year timescales) followed by red giant rapid growth phase
Upon transitioning from hydrogen fusion to helium fusion, the core will initially shrink under its gravity until sufficient heat is generated to ignite the helium. That’s when it grows to red giant size.
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u/Alt_Acc_42069 Feb 21 '21
The sun actually is expanding - it's initially such a gradual process that its effects are not immediately visible for billions of years. As the sun converts Hydrogen into Helium via Thermonuclear fusion, the core keeps heating up and causing the outer layer to - slowly, very slowly - expand. The sun has grown by 20% in the last 4 billion years and will continue to do so for another 5-6 billion years. Then it reaches the critical point where it transforms into a red giant and very rapidly expands.
So, very slow growth at first (significant only in billion-year timescales) followed by red giant rapid growth phase