Eli5: How is the Sun going to “swell” to the point that it engulfs Mercury Venus and Earth? Where is all of that extra matter coming from? Has this swelling already started?
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Stars experience an equilibrium between gravity and fusion energy. Gravity pulls all the atoms in the star together which increases pressure and temperature in the core. Under such conditions atoms can fuse together which releases a lot of energy. This energy pushes back against gravity until both forces are equal.
The fusion process turns ligher elements into heavyer elements. First hydrogen is the fuel and helium is the product. If there is not enough hydrogen left, the fusion slows down, gravity pulls everything closer together until helium can be fused. Fusion of Helium takes more energy but also releases more energy then hydrogen fusion so it will push harder against gravity. This goes on until eventually Iron will be the result of fusion which does no longer provide energy in the process and the star dies.
Apart from the different fusion processes a star also loses mass over time (Helium 4 is lighter then 2 Deuterium even though they are made of exactly the same atomic particles for example). The lost mass is emitted as electromagnetic waves like light. But I am not sure if that is a significant amount for the growing of the star. After all stars have a LOT of mass.
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