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

The size of a star is based on two things:

1. The force of gravity trying to squeeze it and make it small. this is based on the mass of the star, and it doesn’t really change.
2. The pressure created by the fusion reactions happening inside. this pressure pushes outwards and makes the star expand.

Since gravity is pretty much constant, the size is mostly dependent on the strength of the nuclear reaction happening in the star. the stronger the force of the nuclear reaction inside the star, the larger the star will grow, until the two forces become balanced.