Eli5 Why can’t Stars use Iron in nuclear fusion?
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.
It isn’t that they don’t fuse iron it is that the process of fusing energy with iron and heavier elements is that in fusing them uses energy, which is why with heavier elements when you split them apart they release energy (nuclear fission) https://youtu.be/w1GlDVt1Mpk
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!
It takes energy to fuse atoms together, but the act of fusing atoms together also *releases* energy, aka, you have to break a few eggs to make an omelette.
As long as the energy coming out of fusion is higher than the energy needed to do it, a star can exist happily.
For all the elements before iron, this is the case, more energy comes out.
Iron, however, is the first element that takes MORE energy to fuse than it gives back, the star isn’t so happy any more. It now has to use a *lot* of eggs to make a rather sh***y omelette.
In order to explain this fact, we have to understand [binding energy](https://en.wikipedia.org/wiki/Binding_energy):
Probably the most famous equation in physics is E=mc². It tells us that mass is a form of energy and can, therefore, be transformed into other forms of energy (just like p.ex. movement energy can be transformed into thermal energy).
Atomic nuclei are made up of protons and neutrons. Protons hold positive charge and therefore repel each other. The reason why atomic nuclei can nevertheless be stable is, that they are held together by short-range attractive forces called the [strong force](https://en.wikipedia.org/wiki/Strong_interaction) and the [weak force](https://en.wikipedia.org/wiki/Weak_force). If this sound confusing, your take-away should be that there are two kinds of forces in atomic nuclei, one kind is attractive and the other is replant. This pull-and-push game means, that there is one combination of protons and neutrons that form the most tightly bound nucleus: Iron. In iron, the attractive forces win against the repellent force by the largest margin, so to speak, forming the most tightly bound nucleus. All other combinations, i.e. nuclei that have both fewer or more protons and neutrons in the core, are less tightly bound than iron.
Therefore, very light nuclei, which have fewer particles in the nucleus than iron, get *more stable by gaining protons and neutrons*, while nuclei that are larger than iron get *more stable by losing protons or neutrons*. In physics, being stable is always associated with minimizing your potential energy, so the closer nuclei are to iron, the lower is their energy level. As iron is the most tightly bound nucleus, it is the most stable configuration. Therefore, fusing iron into heavier elements *requires considerable energy to be put into the system*, rather than gaining energy through fusion as is the case for lighter elements.