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

It isn’t gaining matter, it is going from fusing hydrogen to fusing helium which releases more energy causing the volume of the Sun to expand with the additional heat as the Sun is just a huge hot ball of gas. https://youtu.be/64L0Dv55_Cw