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

1. The gas becomes less dense, so the same amount of matter can take up a much larger amount of space.
2. No, it hasn’t… I think. I seem to remember that it doesn’t swell until the red giant phase, which is several billion years from now.