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M05EPH t1_j6m9uq3 wrote

Some other good answers already, but this is ELI5, so let me anthropomorphise everything for you. Lets swap the concept of energy with the concept of money.

Hydrogen is a very rich atom. So, it has the abillity to do as it pleases. It can afford to fuse with another hydrogen (actually more than one hydrogen atom involved here...) to make helium. The money the hydrogen atoms pay is released into the surroundings, and so the helium atom now cannot afford to become hydrogen again. Helium can still afford to fuse into carbon and oxygen, which can afford to fuse into neon, then silicon, and then iron.

Iron is now the poorest element. It cannot afford the cost to return to silicom, and it cannot afford to fuse to heavier elements. On its own, it's stuck. If iron wanted to change, it is completely reliant on the its environment to provide money for it. Not even the core of a star can afford the cost, but a collapsing star absolutely can, which is how we believe heavier elements are made.

Finally, you may ask "just because helium can afford to fuse, why does it?". The answer is because helium never wants anything, it has no will. If it's possible, then helium has a non-zero probabillity of doing it. Helium cannot spontaneously become hydrogen, but it can fuse to heavier elements. Given enough time, it'll happen.

Hope that helps!

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Martin_RB t1_j6mdhpl wrote

To simplify further: You can think of iron as the ash of atomic reactions.

Why can't you burn ash? Because it's already been burnt and given all the energy it has to give.

The same goes for iron, it is the end result of atomic processes and doing anything with iron requires you putting more energy into it.

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antilos_weorsick t1_j6mgc3g wrote

No offence, but this doesn't actually explain anything. You use a lot of words to say "stars don't fuse iron because they can't".

You even have to throw away your analogy at the end, because it doesn't make sense.

I don't understand why people think ELI5 means they should anthropomorphise stuff. Sometimes that's useful, but sometimes (like now), it just serves to cover the fact that you didn't explain anything. "I don't actually know, little Timmy, but here's a nice story to occupy your mind, so you don't have time to ask any more questions."

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M05EPH t1_j6mgsyv wrote

Useful feedback. Trying to get better at explaining things like this, so thanks.

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remarkablemayonaise t1_j6maana wrote

Or you can just imagine atoms in a valley with iron at the bottom and the fusable nuclei on one side and fissable nuclei on the other side. Supernovae are like skateboards where you can get from the fusable side the the fissable side with a bit of momentum, but once you're at the bottom of the valley (iron) and have no energy source or "momentum", there's no getting back up either side.

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