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?

Question

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?

In: 69

Answer 1

Ok, so a star is a balance of gravity vs fusion pressure. Gravity wants all the matter to pack together, but the fusion pressure prevents it. The fusion pressure increases over time as the star runs out of primary fuel (hydrogen) and starts fusing Helium, the pressure increases as it steps through its fuel and the star bloats.

https://bigthink.com/starts-with-a-bang/big-sun-grow/