Hi /u/Drippidy!

In order to explain this fact, we have to understand 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 and the 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.

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.

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 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

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.

Iron is the “kiss of death” for a star. Stars are just giant engines fusing elements - at the center you have a dense mass that gravity is pushing together and fusing elements into heavier elements. Once a star fuses it’s way up the chain and reaches iron - a chain reaction is set into place because there’s just not enough energy in a star to fuse iron. Since fusion can no longer occur this is where the engine shuts down and gravity overcomes the star, collapsing in on itself

I’ll take a stab at simplifying the reasoning

Everything up to that point (hydrogen, helium etc) the gravity / force of a regular star is strong enough to overcome the natural repelling force of the 2 atoms. For iron it isn’t.

Or you could think of burning things - Think of ash in a fire. A regular star has a “temperature” that can burn elements up to iron, when it gets to iron it starts to cool because iron is like ash (eg it’s not hot enough to burn it so temperature drops due to fuel starvation). That’s why we have other elements - certain situations can make special stars that are hot enough to burn that ash into other elements. Easy comparison from home fire / stove burning paper to wood to coal or to a furnace melting metals.

Iron actually takes more energy to fuse than it gives out. It's the first element to do that.

A star is actually inflated by the energy output of the fusion. What happens during a supernova is the star starts fusing iron, all of the sudden the star stops putting out massive amounts of energy. This causes the outer layers of the star to fall inward very quickly under the force of gravity. All of those layers slamming into the core causes all sorts of reactions. At once and the bounce back from that is a supernova. So much energy is released during that process that it can create all the other elements from iron to uranium. They all take more energy to create than their fusion gives, but there's no much energy at play that there's still enough left over in the supernova to continue exploding.

They can, it's just that fusing iron or anything heavier takes more energy than it releases, because of the size and stability of the nucleus.

So usually iron and heavier elements mean the star is in its final stages. There's a LOT of energy in the core of a star, so heavier elements can still fuse, but they're absorbing more than they're producing. Once the star runs low on fuel that produces more energy than it takes to fuse, the total temperature starts to decline, eventually leading to collapse or implosion.

The heaviest stuff is only produced in the most intense parts of supernovas.

They can. They do it during the supernovae. However, it consumes energy rather than producing it, so rather than generating more radiation pressure that stop gravitational collapse, it reduces the amount of radiation pressure and accelerate collapse. Therefore, the supernova happens very fast. Still, all elements heavier than iron also comes from nuclear fusion and nuclear breeding (neutrons being absorbed by nucleus) and there are a lot of them.

They can. They do. Iron is just the threshold where fusion starts to "consume" energy instead of "releasing" it, which is supplied by the gravitational pressure of the, at this point, collapsing star.