# 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?

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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?

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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

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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.

With what other people are saying, but also your question is assuming all volume must contain equal or greater amount of mass.

This is kind of misleading; space itself has an absolutely incomprehensibly huge amount of volume, but not all of it holds the same amount of matter in all points. The Sun, The Earth, Jupiter, etc. are large concentrations of matter in small areas, it doesn’t mean all of space has the same average throughout all of it.

;;

You could also look at it from a perspective of pressure rather than volume & mass – which is what the other posts do more than the above point.

When the sun shifts to fusing hydrogen to fusing helium, it’ll have some fair amount of extra energy to push outward. The *mass* doesn’t change, but the amount of *pressure* does.

We can get a similar kind of idea of what this looks like on Earth by having a balloon, inflated in normal sea level atmosphere, then putting it into a chamber we will vacuum out. The pressure on the inside of the balloon will try to expand, because it is greater than the vacuum/reduced pressure environment around it. A big enough difference in pressure and the balloon itself will probably pop, but only after expanding quite a bit.

In the reverse, is taking something like an vacuum-sealed empty, metal can, diving into the ocean. At only a couple feet, the can itself will probably hold up well against pressure of the rest of the ocean pushing in on it. Go 10 or 20 or 50 or 100 ft down, and that can is no longer strong enough to resist the pressure pushing in on it; it crumples.

Basically here – the contents of the sun isn’t what changes – it’s the amount of pressure it generates to counteract gravity. There isn’t “more matter” or “more mass” just because it has “larger volume”, it’s because it’s generating more pressure (from fusing helium instead of hydrogen), that allows it to balloon outward, against pressure from gravity.

It’s not getting more matter, it’s going to get less dense. Lower density = same amount of stuff spread out over a larger volume.