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

There’s no extra matter, it’s just that their density gets smaller, like blowing a balloon. There’s no new rubber material coming in when you blow a balloon, yet they get bigger right?

If there’s anything their material actually reduced as they swell, since it’s caused by fusion, which is combining 2 or more (usually 2) atoms into one, so their atom count gets smaller and smaller.