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.
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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.
Stars experience an equilibrium between gravity and fusion energy. Gravity pulls all the atoms in the star together which increases pressure and temperature in the core. Under such conditions atoms can fuse together which releases a lot of energy. This energy pushes back against gravity until both forces are equal.
The fusion process turns ligher elements into heavyer elements. First hydrogen is the fuel and helium is the product. If there is not enough hydrogen left, the fusion slows down, gravity pulls everything closer together until helium can be fused. Fusion of Helium takes more energy but also releases more energy then hydrogen fusion so it will push harder against gravity. This goes on until eventually Iron will be the result of fusion which does no longer provide energy in the process and the star dies.
Apart from the different fusion processes a star also loses mass over time (Helium 4 is lighter then 2 Deuterium even though they are made of exactly the same atomic particles for example). The lost mass is emitted as electromagnetic waves like light. But I am not sure if that is a significant amount for the growing of the star. After all stars have a LOT of mass.
Imagine a money pit, the kind where people try to grab as much cash as they can before their time runs out, blowing dollars off the floor. Imagine it is weak and only blowing the money as high as your knees. This is the power level of the Sun fusing hydrogen into helium. In a few billion years from now, that hydrogen will have effectively run out. The Sun will shrink until the density and pressure is high enough to start fusing the helium. That gives more energy than the hydrogen fusing did, and in our imaginary money pit, it’s like the blower got cranked really high and now the money is reaching all the way to the top of the money pit tube. It’s still the same amount of dollar bills, but it’s getting floofed by all the extra energy, floofed so far that it reaches past Venus and possibly all the way to Earth.
Ok, so a star is a balance of gravity vs fusion pressure. Gravity wants all the matter to pack together, but the fusion pressure prevents it. The fusion pressure increases over time as the star runs out of primary fuel (hydrogen) and starts fusing Helium, the pressure increases as it steps through its fuel and the star bloats.
https://bigthink.com/starts-with-a-bang/big-sun-grow/
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.
Go shake a can of soda really hard. Before you open it, take note of the volume of the contents (the size of the can). Now open the can, and take note of the volume of the contents. Same amount of matter, larger volume because it is no longer constrained and higher pressure was able to equalize outward.
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