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.

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.

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!

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.

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.

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.

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."

Does it happen anyway? I understand that it would take energy rather than give, but do the extreme conditions in the core of such a star cause the fusion to happen anyway and steal some of the stars energy?

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

Supernovas do it all the time

That I do understand, but I wasn't sure if heavier elements fuse while the star is still active or if there is absolutely no fusion beyond iron until it blows up

I have no idea why people down-voted you, this is a perfectly legitimate and pretty good question; some here are just jerks...

Yes, but rarely. And mostly at the end when the star is at its hottest in the center. I don't have numbers on how often it actually happens, but it definitely does.

At the most extreme end in particular, when a star goes supernova (not all do) it collapses so hard to its center that this creates extreme pressure and releases absurd amounts of energy. This fuses iron and all the other stuff beyond all limits; the energy is almost irrelevant, we are talking about hundreds of Earth masses(!!!) as pure energy. This is one of the two processes that creates the elements beyond iron in the amounts we find them (the other option are collisions of neutron stars).

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

What happened to the "explain like I'm five" part?

Hi /u/Atharaphelun!

The sidebar states:

>LI5 means friendly, simplified and layperson-accessible explanations - not responses aimed at literal five-year-olds.

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.

Explain it like I'm an astro-physicist...

Oh come on, a worst that was "explain it like I'm in highschool." It's a complicated question but the answer explained simply enough the complex ideas.

That has more to do with the question asked than the answer given

I'm just messing around. Don't stress.

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.

A fun little chart showing the various origins of the different elements is here.

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.

No, for our Sun-like stars, iron is their eventual death.

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.

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.