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Gnonthgol t1_j6m8bdi wrote

Iron have the lowest energy in its nucleus of all the elements. If you fuse hydrogen atoms into helium the resulting helium atom have less energy then the hydrogen atoms you started with so the rest of the energy is released into the star. As you fuse together elements up the periodic table you always end up with more energy left over until you get to iron. If you fuse iron and hydrogen for example the resulting cobalt require more energy to hold its nucleus together then the iron and hydrogen atom combined, so you need to put more energy into the fusion then you gain from it. It is still possible to do this, in fact that is how all the cobalt and other heavy elements are made. But it reduces the amount of energy in the star instead of increase it so this is a very short lived period in a stars life.

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HunterDHunter t1_j6m8ui6 wrote

Heavy elements are also created in supernova explosions. I think that's where most of them come from as opposed to fusion within a star.

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tyler1128 t1_j6pa6s5 wrote

Slight correction, but that's generally correct. Iron has the lowest binding energy per nucleon, and adding a proton adds the energy to add that proton in to the nucleus as well as energy needed to keep all the existing nucleons together with it, which ends up being more than just adding a proton and keeping all else equal.

The reason for this is quantum mechanical in nature and involves the strong force. It has to do with the strong force being strongest at certain distances before falling off rapidly, and protons don't naturally want to be near each other because they are both positively charged. It also involves spin and nuclear orbitals and other fun things, but the intuitive not really but kind of right idea is that the nucleus is getting large enough that the nucleus is getting large and complex enough that it takes more energy to keep them all together happily and not wanting to change to a new configuration.

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