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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The size of a star is based on two things:
1. The force of gravity trying to squeeze it and make it small. this is based on the mass of the star, and it doesn’t really change.
2. The pressure created by the fusion reactions happening inside. this pressure pushes outwards and makes the star expand.
Since gravity is pretty much constant, the size is mostly dependent on the strength of the nuclear reaction happening in the star. the stronger the force of the nuclear reaction inside the star, the larger the star will grow, until the two forces become balanced.
The sun has layers. The only reason that the sun hasn’t collapsed in on itself is due to the fusion of hydrogen into helium in the core itself. The fusion pushes everything outward while gravity tries to push everything inward. Right now, everything is in equilibrium, meaning the outward ppush is equal to the inward push.
No other layers are fusing elements right now, just the core.
When the core reaches the end of its fuel reserve,it will begin to shrink. This process produces a LOT of heat. The layer above the core (which is still mostly hydrogen) begins to heat up. It will eventually reach a critical temperature where hydrogen begins to fuse in the layer above the core. Since the core is running out of fuel, this layer will be fusing more than the core, so the outward force from this layer will be stronger than the core’s.
Because of this new outward force further away from the center, the equilibrium is broken, and there’s now more outward force than inward, so the sun swells until equilibrium is reachieved.
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