If the final element to be fused in a star is iron, where does everything after that come from?
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Stars will fuse elements in their core because it is hot and the matter in the core is under high pressure. The high temperature and pressure combine elements, and combining elements smaller than iron produces energy. Releasing energy keeps the core of the star hot and at a high pressure, which lets fusion processes continue. When iron is fused, it actually consumes energy, which causes the star to cool and the core will lose pressure.
The core / middle of the star is where most fusion happens. The majority of a star doesn’t have fusion happening.
The pressure in the star’s core pushes outward on the outer layers of the star, preventing all that matter from falling into the core. Lower pressure means the star will start to shrink because it’s not pushing up anymore.
If enough iron starts to fuse, so much energy is removed from the core that the rest of the star above the core starts crushing everything in the core because the core’s pressure is not high enough. This crushing will release lots of energy which causes iron and other elements to fuse, using this energy to make heavier elements. They require energy to form, instead of releasing energy like the lighter elements, so the star collapses in what is called a supernova (but only if it is big enough), and will no longer fuse elements after that.
This is also why heavy radioactive elements release lots of energy when they split: it took lots of energy to create them in the first place.
(obviously an oversimplification, but it is ELI5)
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