Daniel and Kelly’s Extraordinary Universe - Why can't stars fuse Iron?

Episode Date: January 31, 2023

Daniel and Jorge talk about how stars fuse lighter elements and what changes when they reach Iron.See omnystudio.com/listener for privacy information.

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bananas drop every Tuesday in the exactly right network. Listen to bananas on the IHeart radio app, Apple Podcasts, or wherever you get your podcasts. science of Marvel Superheroes, right? Yeah, I'm like Thanos. Stap my fingers and make half of your questions disappear. Awesome. Well, I'm wondering why Iron Man is called Iron Man. Isn't his suit made of some super high-tech material like Tony Starkium or something?

I'm not sure that's a thing in the Marvel Universe, but if you look at his origin story, you'll see the first suit he made was made out of iron. So Iron Man is made of iron or what? Yeah, his first suit was made out of iron. and he made it in a cave. So I guess technically he could also be caveman. So when he upgraded his suit to something fancier, they didn't upgrade his name also?

I guess he stuck to the original name. I mean, they're not all literal, you know. Like the Black Widow is not actually a spider. Captain America is not really a captain. He's not really in charge of America? What? Also, they're not real, Daniel. You're saying there is no science in the Marvel universe.

There is science, I'm sure. But, you know, it's all fiction. Well, science will always. be my superhero. And physicists are the villains? Are you saying physicists don't actually do physics? We just kept the original name after we got upgraded from physicist to villain. Hmm. Maybe you then can be called ironic man. H.D. Comics. Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine and I really do my best to avoid being a supervillain. That doesn't sound good enough, Daniel. What do you mean? Don't you know there is no try? There's only do?

You want me to do better than my best? That seems like a big ask. I'm saying trying not to be a supervillain is, you know, not an excuse for being a super villain. I'm saying I'm not trying to create weapons that will allow super villains to destroy the world. What else could you ask for me? To not even think about it. I mean, it seems pretty obvious. But welcome to our podcast, Daniel and Jorge, Explain the Universe, a production of iHeard Radio. In which we try not to destroy the world,

but we do hope to break it apart into tiny little understandable pieces. We want to cast your minds out into this incredible glittering cosmos. Think about all the amazing and mysterious processes going on at the inside of our stars, the inside of our planets, and in the inside of every single object. the tiny little buzzing particles that weave together to make our cosmos. We want to understand all of it. We believe it all somehow makes sense and we want to explain all of it to you.

That's right because it is an amazing, uncanny and marvel us universe out there for us to explore and to learn about. We'll have wonders and amazing processes going on as we speak all the time, everywhere in the universe. And although the universe is mostly hydrogen and has always been mostly hydrogen since the very beginning, It does contain an interesting smattering of other stuff. Heavier objects, helium, lithium, beryllium, oxygen, carbon, even some iron out there to make the universe a little bit spicier. That's right. It would be a little bit boring if the whole universe was just hydrogen. Fortunately, the universe has figured out a way to make other things besides hydrogen, including helium and all of the heavier elements. Don't they say, Daniel, that we're all made out of star stuff?

We are indeed built out of little bits that have been assembled in the inside of stars. think of the whole universe as sort of being on fire. It's taking all that hydrogen fuel and trying to burn it into heavier stuff. We are sort of put together from the ashes of the fires inside stars. Yeah, the universe is pretty fire. As the kids say these days, it's pretty good. And it's a good thing it can make heavier things other than hydrogen because that's kind of where we come from. That's where plants come from. That's where planets come from. That's where all good things to humans seem to come from. That's right. Heavier elements allow us to make much more complicated and much more interesting things. We couldn't have organic life if we didn't have

the backbone of carbon to allow us to make all those complex molecules. We couldn't breathe if oxygen wasn't even a thing. Most of the processes of life rely on biochemistry, which rely on heavier atoms than just hydrogen. And so our very existence, the existence of intelligence and probably even life relies on stars to convert that hydrogen into heavier, interesting stuff. But even the awesome power of stars is limited. That's right. We're all born inside of stars. Not literally, though, right?

Like, it's just the heavier elements other than hydrogen are made inside of stars. And then they're released out into space, into the cosmos, when the stars explode, basically, right? Yeah, there are these fascinating cycles. When we hear that we are made of star stuff, we don't mean that the stuff that you and I are made out of was formed inside our star. We mean that a previous generation of stars, ones that formed and burned for maybe billions of years, created the elements that now make us and the whole solar system up. That star burned and then died and then exploded and spread its ingredients out to see the next generation of stars. So our solar system started out already enriched in these heavier metals.

Of course, our sun is busy making even more of them, but all the heavy metals and all the non-hydrogen stuff that is on Earth came from the, heart of another star, not from our own son. From the corpse of an old star, right? I mean, we're sort of like the bits and pieces that the old star had made. And so we're part of this new generation. Daniel, are we the baby boomers of the solar system life forms or the Gen X? I think that's kind of a dark way to look at it, a corpse of stars.

I prefer to think of them as ashes of like a campfire. The end of the night, you don't say my campfire has died a grisly death. You say that it's burned out, which doesn't seem like such a negative outcome. You say that because you're not the wood, and then you burned to death. Yeah, exactly. I'm the positive outcome of this process. We don't know how many generations of stars have been involved in making our stuff. We think that in general, there have been around three generations of stars.

We call these type 1, type 2, type 3 stars because we notice populations of stars that have been around for a very, very long time and also populations of stars that are sort of younger. But it is possible that there were many, many generations very early on. in the universe when stars were very big and very hot and didn't live for very long. Yeah, it's a fascinating process, the life cycle of stars. But within one life cycle of a star, it is pretty busy making stuff, right? It takes hydrogen and combines it to make helium, and then it takes helium to combine it into heavier elements. And that's how most of the heavier elements are made up to a certain point.

Yeah, even these stellar fusion engines are limited. They can make heavier stuff out of lighter stuff, but not forever. You can't take an element with like 500 protons and fuse it together with another element of 500 protons to make something with 1,000 protons in its stars are not capable of that. As we look around the universe, we notice that there seems to be a lot of like iron and nickel and this kind of heavy stuff. But above that, heavier elements than that are much more rare because stars in their fusion process cannot make them. That's right. Star fusion of materials inside of them is limited up to iron. Right. And so that's the heaviest element that a regular sun or star can make. And so today on the program, we'll be tackling the question.

Why can't stars fuse iron? Or iron men. That is an interesting question. Why can stars fuse iron? They can fuse anything lighter than iron, but anything heavier than iron. They can't do it. Yeah, it provides a clue about what's going on at the heart of these nuclei, how you build them out of protons and neutrons what the physics is of constructing the nucleus of an atom and why something

fundamental changes after you get to iron. Yeah. So as usual, we were wondering how many people had thought about the limitations of stars and why they can't fuse anything heavier than iron. So as usual, Daniel went out there to ask people on the internet this question. Thank you very much to everybody who volunteers for this portion of the podcast. If you'd like to hear your voice for future episodes, please don't be shy. Write to me to questions at danielandhorpe.com and i will set you up here's what people are to say the reason why stars can't fuse iron is because stars are so hot that iron essentially evaporates into something else it's the electromagnetic forces rejecting any additional positively charged protons coming in and there needs to be

more neutrons than that maybe because they are not cool enough maybe the process of fusing iron requires lower temperatures. Well, simply because it will take an insane amount of energy to do that. And well, now even stars that we know of until now are not capable of doing this. I actually think they can, but the nuclear reaction absorbs more neutrons than it emits. And so perpetuating the nuclear reactions within a star doesn't work if it's fusing iron, whereas everything before iron keeps the reaction going. All right, a lot of interesting reasons here.

Are any of these correct? Some of them are sort of close to being correct or in the right direction. Other ones are pretty much dead wrong. But they're entertaining speculation nonetheless. You're the physicists, they are not. So it's an interesting question. Why can stars fuse iron together? And it'd be great if they could, right?

They could make heavier and heavier elements, right? I mean, the universe would definitely be different. You might have like more uranium and more gold in the universe. I don't know if that would be better. You know, it might be like more poisonous for life. If you start tweaking the basic parameters of the universe, you never know what you might end up with. We might all be Superman or we might never have evolved.

We might all be gold men is kind of what I'm hearing. Might all be a lot shinier and blinked out. But then gold would be so common. It wouldn't even be valuable. We'll have to find something else to, you know, overpriced. Rare comic books, perhaps. I'm sure that humanity will find something to argue about. I have that much faith in us.

Well, this is a fun question here. Why can stars fuse iron together? And so I guess maybe step us through kind of the history of how things get fused together, starting from the Big Bang. Yeah, to understand why stars can't fuse iron, we first have to understand what's going on when we fuse lighter elements. And, you know, all of the initial ingredients for all of this fusion came very, very early on in the universe. when the universe cooled down and protons were formed and neutrons were formed and electrons were formed. And they were just flying around on their own until it cooled even further so much so that electrons were moving too slow to escape the electric attraction of those protons. And then they cooled together into neutral hydrogen.

I guess maybe a question is why didn't the Big Bang make heavier things than hydrogen? Like if things could be made heavier than hydrogen, why did most protons stop there if the Big Bang was so? you know, hot and intense, why didn't heavier elements get formed? The short answer is that it just didn't really have enough time. Like, things were cooling pretty rapidly in the Big Bang. And after about 10 to the minus six seconds, we got things like protons and neutrons. And those protons are basically hydrogen. There was a little bit of helium made during the Big Bang.

It was hot enough to fuse that hydrogen together into helium, but not a whole lot. It wasn't hot for very long. Things cooled off very rapidly. And because the next element, lithium, is very unstyed. It doesn't stick around for very long. The universe sort of couldn't build up even further during the Big Bang. So there was this like initial hot flash when hydrogen was made and a little bit of it fused into helium and a tiny little bit was made into lithium. But that lithium sort of falls apart, which doesn't allow you to then fuse lithium together to make heavier stuff.

I guess maybe something some listeners may not know is that a hydrogen atom is basically just one electron orbiting around one proton, right? And that's exactly right. In fact, we call a proton hydrogen. It's like ionized hydrogen, even though it really is just a proton. So in the beginning of the universe, the protons were formed after like 10 to the minus six seconds, like a millionth of a second into the universe. But it took a few hundred thousand years before the universe cooled enough that those protons could grab onto electrons.

But we still consider them to be hydrogen before they got their electrons. Right. Hydrogen is kind of like the OG atom, right? It's like the most basic atom you can have, right? Because if you have just a proton, then that's like a. Yeah, that's a hydrogen atom without an electron is what you're saying. Yeah, that's like ionized hydrogen. And so those protons were flying around in the very early universe.

They were made after like 10 to minus six seconds. And then things were hot enough for like a few minutes. For like three minutes, we think things were hot enough for those protons to fuse together to make helium. But after that, things had cooled too much. You didn't have the conditions necessary anymore to make heavier stuff. So for like the first three minutes of the universe, everything was about as hot and dense as the inside of a star.

Right, but I guess a basic atom of hydrogen is just one proton with one electron. Now, to make heavier elements, you have to fuse hydrogen together because the heavier elements have extra protons at their nuclei and extra electrons floating around them. But it's hard to fuse two protons together, right? Because they're both positively charged

and so they repel each other, right? And so it's hard to make an atom with two protons and it's nucleus. Yeah, it is hard to get these things together. That's why the universe has to be hot, and dense for it to happen. Well, that's why it happens at the inside of stars, for example, and not just like in a balloon filled with hydrogen.

The Hindenburg, which was filled with hydrogen, didn't have fusion going on inside because the protons do repel each other. There's a subtle point there, though, which is the protons repel each other because of their positive charges. Once you get them close enough, if you happen to squeeze them together, then they attract each other because another force takes over the strong force. So at long distances, protons repel each other,

they avoid getting near each other. If you do manage to get them close enough, however, they will stick together to make helium. Right, but it takes a lot to get them really close together because they are repelling each other through the electromagnetic force. And so that's kind of where suns come in, right? If you have a bunch of hydrogen out there in space, gravity pulls it all together, squeezes those protons close enough so close together that eventually the strong force takes over. Two protons merge. And boom, you got a sun. You sort of dot, dot, dot it over a few critical elements there.

Yada, Yada, Yada. Life on Earth. Podcasts. Superheroes. Interesting choice of focus. Yeah. And so this process is called hydrogen burning. And you might imagine that it's just, like you said, two hydrogens come together to make a helium, right? That makes sense. Two protons come together to make a new nucleus with two protons in it. Helium, though, usually has two protons and two neutrons in it. So to actually make a helium nucleus, helium four, we call it, you need four hydrogen nuclei. You need four protons. Two of which convert into neutrons. neutrons along the way. So hydrogen burning is actually a multi-step process. First, you take the two hydrogens, you squeeze them together. You don't immediately get helium. What you get is deuterium. You get an isotope of hydrogen with a proton and a neutron because one of those hydrogens has flipped from a proton to a neutron. So now you have H2. And you take two of those. You squeeze those together and you end up with helium four.

But I guess the question is, why do you need those neutrons to make a stable atom? Why can you just have an atom with two protons? in the middle. So remember, these protons are positively charged and they are pushing against each other. A neutron is neutral, right? It doesn't have any electric charge and so it sort of helps space the protons apart from each other. All these objects have little bits of the strong force. They all stick together using the strong force. The neutrons are there to sort of keep the protons a little bit further from each other.

But you're saying neutrons in our atoms are just filler. They're like what you add to meatloaf to make it fluffier and less dense. I mean, they're sort of like the pallet cleanser. which is an important part of any menu, right? No, no, no. You said spacer, which sounds like filler. They're sort of like the therapist in a marriage, right? They keep everybody happy. You know, the construction of the nucleus is a delicate balance

between the strong force, which is trying to stick everything together and the electromagnetic force, which in the end is pushing things apart. And we'll see as nuclei get larger and larger, the balance of power between these two things changes because the strong force is only powerful over very short distances and the electromagnetic force is powerful over longer distances. So you need the neutrons as filler. What happens if you take them out?

Wouldn't the protons smooge together even more? If you take them out, then the protons get closer and the electrostatic repulsion increases. So helium 2 is not stable. If you don't have the neutrons, it falls apart. You need the glue, which is sort of like the neutrons. All right, well, that's the beginning of merging atoms together to make different materials. And that's what's happening inside of stars. And this goes on and on.

But at some point, it stops at iron. And so let's talk about why that is and what's so special about. Iron. But first, let's take a quick break. Have you ever wished for a change but weren't sure how to make it? Maybe you felt stuck in a job, a place, or even a relationship. I'm Emily Tish Sussman, and on she pivots, I dive into the inspiring pivots of women who have taken big leaps in their lives and careers. I'm Gretchen Whitmer, Jody Sweetie.

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Wait, what? Oh, that's the run right. I'm looking at this thing. Listen to no such thing on the IHeartRadio app, Apple Podcasts, or wherever you get your podcasts. All right, we're talking about Marvel superheroes and DC superheroes. Is that basically what this episode is about? why can't you fuse Ironman and Superman?

All right, well, so we talked about how inside of stars, heavier elements get made from hydrogen, which are the simplest atoms you can have, which is a proton and an electron, and you can make helium out of that, and you can merge those to make heavier elements. But at some point, we stop at iron. Somehow stars are not able to make iron.

Daniel, what's the next step after helium? So after helium, you can try to make heavier stuff. Lithium and beryllium, the next element of the periodic table, are very, very unstable. So you can make them inside a star, they just don't last very long. So they're not good building blocks for the heavier stuff. What do you mean?

You make it and it dissolves right away? Or like you make a nucleus of lithium and it breaks apart right away? They break apart right away and they're also destroyed by other reactions like photo disintegration. Photons made by other reactions tend to break up lithium, beryllium, and boron. So they just don't last very long inside of stars. So they're not good building blocks for heavier stuff. But if they don't last, how do you make stuff that's heavier than them? So you need to skip over them.

So what you do is you take three helium. and you combine them to make carbon, right? Carbon is atomic number six. So three helium can come together to make carbon. It's not an easy thing to do to get the three helium to stick together, right? Getting two protons together to do hydrogen burning is complicated enough. Now you need three things all to dance together to make carbon. Requires a very hot, very dense kind of sun.

Well, I mean, it seems hard to make helium in the first place, right? You said you need four hydrogen atoms to make a helium atom. You do. You need four hydrogen atoms. And so inside the sun, these four things have to be kind of in a collision course with each other? No, the steps are sort of independent. You have two hydrogen atoms come together to make deuterium, which is a proton and a neutron. That bangs together with another hydrogen to make helium three.

And then the helium three is together combined to make helium four and give off some more protons. All those steps are independent and the intermediate pieces are more stable. So it's not as unlikely as requiring four things to all come together at once. But to form carbon, you do need three helium nuclei to come together. together pretty quickly because two helium come together to make beryllium which is really not stable for very long and you need that third helium to come in and turn it into carbon before the beryllium falls apart well so literally inside the sun you need to have three helium atoms by chance just be on a collision cord with each other or inside of stars does this happen like by squeezing

it's a less kind of an explosion it's more like things get squeezed together yeah things are getting squeezed together it's the density right the pressure that's creating the possibility for this to happen you You need like a certain number of helium atoms per cubic centimeter to make the probabilities anything greater than basically zero. Right. So inside of suns, the gravity is squeezing stuff together. It creates these reactions. You make heavier elements and that releases energy because that's kind of how suns work, right? They squeeze them together. Once they pop into place, once they merge, they snap together, a bunch of energy is released. And that's the energy of the sun. Yeah, that's the crucial thing we haven't talked about yet.

When you combine hydrogen together to make helium, you don't just get helium. you also give off photons, give off neutrinos as well. But energy is released when you do this reaction. And that helps create the conditions for the next reaction. It makes the core of the sun very, very hot. And so this is just sort of like a fire, the way like when you start a log burning, it helps create the conditions for the next log to burn because it creates that heat, which will ignite the next log.

So in plasma physics, they call this ignition when the plasma is hot enough to create fusion, and that fusion then maintains the heat of the plasma. The fact that these reactions release, heat is what allows them to continue to go. And also what warms up our summers. Yeah, it's interesting because it's like merging atoms together is what releases the energy, which is I think maybe a little counterintuitive for most people because we're sort of used to like breaking things to release energy, right? Like we're used to associating a huge release of energy with like an explosion, something breaking. But this is the opposite actually. When you put things together, it releases

energy. When you merge them, it's like taking two pieces of clay and somehow when you stick them together, that releases a bunch of light. Yeah, the energy flow is exactly the crucial concept here. Reactions can either release energy when you form something or they can cost energy, right? So you can release energy when you make something, which means that it takes energy to break it up. Like if sticking things together releases energy, then those things are now bound together and it costs energy to break it up. If you like mechanical analogies, you can think of this, they're like inside of a cup together,

where that cup is now an analogy for like the potential energy of this system. And when they fall into the cup, they have to release energy to fall into the cup. And then later, if you want to break it apart again, you have to put energy in. So if you want to break helium into hydrogen, you have to zap it with a laser to break it up. So if it takes energy to break it up, that means that it releases energy when you make it. Yeah, I think it's still a little counterintuitive. I guess I'm not quite wrapping my head around it or how to explain it. Because it feels like the atoms want to be together to some degree, right?

I mean, they stick together because they're attracted to each other. And so why would that release energy if you are sticking them together? In order for them to stick together, they have to give up some of their energy. Sticking together means that they're bound together. They're like together in a potential well. Think about an analogy in terms of like orbit because gravitational binding energy works the same way. If a planet is flying by a sun and it has a huge amount of energy, then the sun is not going to capture it. But if a planet is flying by the sun and it releases some of its energy,

gives up some of its energy into something else, like it bangs into a rock and sends that rock away, it's lost some of its energy and then it can fall into the gravitational well of the sun and be trapped there. Now you've given up some energy by banging into this rock and sending it out to infinity and created this combined state, this planet that's now orbiting the sun. So when something falls into a potential well, it's losing energy. It has to give up that energy somehow. So in the same way, two hydrogen atoms have like a lot of kinetic energy. When they get trapped together into helium, they have to give up that energy to like release photons so they can fall into the potential well of their binding state.

It might help to talk about where this energy comes from, like two hydrogen atoms before they're fused together. You're saying they have a certain amount of energy. And after you fuse them, you're saying they have less energy together as a pair. Where did this energy come from? You just said maybe the kinetic energy of these particles? or is it in like the binding energy inside of their corks? So it's not inside the binding energy of the corks, right? The proton doesn't change.

It comes from the energy of the motion of hydrogen. These hydrogen atoms are a very energetic state. They have to be in order to even get close to each other because otherwise they're getting pushed apart by the electrostatic repulsion because they're both positively charged. So they have a lot of energy. You push them together and then in order to stay together in order to fall into this hole together,

they have to sort of release energy. Think about, for example, if you're minigolfing and you're trying to get a ball into a little hole on the top of a volcano. You've got to give it enough speed so it gets up to the top, right? And then it falls into the hole. Now it's like stuck in that hole. It's got to give up some energy to go into the hole. And so in the same way, the hydrogen atom needs a lot of speed to approach the other hydrogen atom.

Then it has to give up that speed when it falls into the hole of the strong force, which is attracting the other one. Basically, you're saying they had some kinetic energy when they were flying apart together. But once they smash into each other, that energy, that kinetic energy has to go somewhere, and that's basically the energy that powers the sun is when these things smash into each other. Yeah, the reorganization of two protons into helium has less energy than just the two protons by themselves.

Another way to think about it is in terms of the mass. Remember that mass is just a measure of how much energy is stored inside something, not actually the amount of stuff. And the mass of the helium atom is 0.8% less than the mass of the nucleons that make it up. But that's really just another way of saying how much energy is stored in it, has less energy stored inside it, than the nucleons that make it. And so to get into that state, you have to release energy. Just like how an electron, when it moves down an energy level around an atom, it has to give up a photon to move down an energy level, right? Energy is conserved there.

In the same way here, these two protons are moving into another state, which has lower energy. So they have to give up that energy. You might ask like, well, why does it have lower energy? What's lower energy but having two protons stuck together and having two protons fly apart, right? Yeah, I think that's the main question. What's different? There's the mathematical answer and then there's the intuitive answer. Mathematically, whenever you have a force that's attractive, you can think about it in terms of a potential energy difference.

Forces like to push things towards lower potential energy, the way gravity pulls a rock down a hill to lower gravitational potential energy. So pulling something in with an attractive force like the strong force means bringing it to a lower energy state. That's called the binding energy. The way I think about it intuitively is thinking about the reverse process, right? Like if these two things are stuck together, if the strong force is really holding tightly on them, then you've got to zap them. You have to give them energy to push them apart, right? Just the same way, if you want to release the Earth from the sun's orbit, you've got to give it a push.

If you want to break up the helium nucleus into two high. hydrogen, you've got to zap one of them to release them from the pull of the other one. And that costs energy. So if breaking it up costs energy, then the reverse process forming it must release energy. All right. Well, let's maybe move on and talk about what happens after that, which is that you get heavier and heavier elements. But this only works until you get to iron.

So what happens when you try to make iron? First of all, how many steps are there between hydrogen and helium and making iron? So you can keep going for a while. You can combine helium together to make carbon. You can combine carbon together with more helium to make oxygen. You can keep going and make silicon and heavier and heavier stuff. There's multiple steps there. It's not like one single pathway. Now you can have lots of different combinations of things that you can put together. And that happens inside the sun? Like, you know, everything's mixing together with everything and making different heavier elements. It happens inside some stars. In order for those steps to happen, it has to be hotter and hotter because now these nuclei have large. positive charges. So they're pushing against each other even more. So in order to get carbon to fuse together with other carbon or with something else, it requires even more temperature and density. So our star is not hot enough to fuse anything basically but hydrogen into helium. But

other stars out there in the universe are and they can keep fusing stuff all the way up to iron. Oh, our sun cannot make heavier elements than helium. Is that what you're saying? Like our sun is limited to helium? Our sun is limited to helium until the very end of its life. For a few moments near the end of its life, maybe minutes or seconds, there will be a little bit of helium burning. It's actually called a helium flash because it all happens so quickly near the end of its life and it expends an enormous amount of energy during these last moments. But for most of the lifetime of the sun, for the next few billion years, it will not fuse any helium. It's just not massive enough to create the temperature and pressure

at its core necessary to do that. Interesting. I feel like that's something that peser physicists don't talk about often. You know, when they say we're made out of star stuff, really they mean we're made out of some stars stuff. Not all stars make stuff like us, right? You need special stars. Yeah, only the bigger, more massive stars are capable of fusing heavier and heavier elements. Okay, but even those big stars can combine carbon and hydrogen and make these heavier elements, but even the biggest and hottest stars have to stop at iron. And so the question is, why is that? What's so special about iron? So as you move up the periodic table, you're getting more protons, and more neutrons in there, things are getting tighter and tighter.

The binding energy actually increases because now you have more of these things, feeling the strong force and pulling on each other. So as you go up the periodic table, you are releasing more energy because the binding is getting stronger, right? Remember, binding getting stronger means you need more energy to break it up. So it takes a more powerful laser to break up carbon than it does to break up helium. It takes an even more powerful laser to break up heavier elements than carbon. Because as you keep adding nucleons, they like to stick together even more. They're like all working together to make this stuff even stickier.

And that's really the key that the binding energy per nucleon is going up as you do fusion all the way up to iron. The potential well is getting deeper. The atoms are getting stickier. You stick two atoms together, but the combined atom has more than twice the original binding energy. So it's more tightly bound per nucleon, which is why it releases more energy to make that combined atom. But wouldn't that release more energy? Is that kind of like a runaway reaction in a way? Like, you know, a sun starts to make heavier elements.

And when you fuse those together, they release even more energy. It sounds like what you're saying. Merging two carbons together releases a whole bunch of energy. Yes, exactly. And so as you keep going up the periodic table, things get tighter and tighter. And you keep releasing energy. You can keep doing fusion and it keeps releasing energy. until you get to iron.

Iron is a tipping point when the electrostatic force takes over again. And what happens is that the nucleus is now so big that the strong force between protons on a different size of the nucleus can't really do its thing anymore because the protons are so far apart. But the electrostatic force, which is a longer range, much, much longer than the size of the nucleus, can. So the atom becomes a little bit less tightly bound. Instead of adding another proton which sticks everything together more,

you're adding another proton which sticks everything together a little bit less. So you're reducing the binding energy of the nucleus. You're making it easier to break it up than it was before. Alternatively, you're saying it gets harder to fuse things to a really big atom-like iron. Like there are so many protons inside of the nucleus of an iron atom that is just super duper positive. There's a lot of positive charge there in one spot. And so like adding one more proton just gets harder and harder, A, because there's so much

positivity there are repelling you. But also, like, even the strong force that's holding all those protons together gets kind of more diluted. That's exactly right. But the thing that controls whether or not this happens very often in stars is really the energy flow. Because what it means is that to fuse iron together, for example, costs energy rather than releasing energy. Because fusing iron together means sticking it together into a bigger nucleus, which is not as tightly bound. Right. And so remember, if something is really tightly bound, it costs more energy to break it up, which

means it releases energy to make it. If something is less tightly bound, then it doesn't take as much energy to break it up. So it costs energy to make it. And so what happens after iron is now you're making things that are less and less tightly bound. And so it actually absorbs energy. It costs energy to do it. You want to fuse iron together? You can do it, but you take energy away from the star. So in effect, you're like putting out the fire of the star. Instead of fueling it for the next reaction, you're cooling it down. Yeah, I feel like you're making folks here, do some superhero style mental gymnastics here with so many inversions.

I think maybe a simpler way to put it is that when you're fusing something simple like hydrogen, it takes a little bit of energy to get the hydrogen atoms together. But once they fuse, they release much more energy than the energy took to get them together. But as you get into these heavier and heavier and bigger atoms, the energy it takes to like fuse something to them is now more than the energy that gets released when it actually happens. Yeah. Fusing iron together is an energy loser, right? It costs energy to do that. The reason, and we say that a lot in the podcast, that you can't make things heavier than iron because it cools stars. The reason for it, the fundamental reason for why fusing iron together cools a star,

instead of heating a star, the way fusing hydrogen together does come down to this nuclear binding energy, how the nucleus is put together. When you put heavier and heavier nuclei together, they are not as tightly bound. They're easier to break up. So if you transition from a nucleus which is more tightly bound to less tightly bound, then it costs energy to do that. You have to absorb energy to go from more tightly bound to less tightly bound. You're like moving up energy levels. So it costs energy. If you don't like thinking about the binding energy, here's another way to think about it. Fission releases energy because a heavy nucleus like uranium 235 is like a cocked mouse trap. It took energy to squeeze all those protons and neutrons hard enough together to make them barely stick together

using the nuclear force that fights against the natural tendency of all those protons to fly violently apart due to their electrostatic repulsion. So when that heavy nucleus, the uranium 235, is struck by an incoming neutron, for example, it's like a mouse touching the trigger pedal of the trap. Bang! goes the nucleus as it breaks apart. In the case of fusion, the mechanism really is different. The nuclear force between the nucleons is very powerfully attractive, but only kicks in when the particles are so close to each other that they are almost touching. That attraction is not quite enough to stick the two protons together against their electrostatic repulsion, but if you add two neutrons to the recipe, you get enough mutually

attractive nuclear force stickiness to overcome the electrostatics. And the particles fall into each other's potential well, like a ball getting trapped in a cup, giving off energy as they fall in, and that gives a powerful bang. Well, maybe a question I might have is in the sun, there's a lot going on, right? Even in a big powerful sun that can make things like iron, there's still hydrogen beam fuse and the lower elements beam fuse that create a whole bunch of energy. You know, why can't the sun sustain an iron fusing reaction

with the energy from these other reactions? You know, like maybe making iron cost it energy, but it's also, you know, getting a lot of surplus energy from some of these other reactions it has. And so why can't the sun just keep making heavy and heavier elements with its surplus energy? Yeah, that's a good question. Remember that our sun can't even get close to that situation because it can't even make iron. So it's not really in the situation of trying to fuse iron together with heavier stuff.

But imagine some more massive star that's hotter at its core and is capable of fusing all the way up to iron. And you can ask like, why can't you make heavier stuff and use up a little bit of that energy that you're producing with all the other fusion processes? you can. And that does happen a little bit, but it tends to kill the star, right? It tends to cool the star down. And that's the end of the star. That's the death of a star. Like if you put water onto a fire, what happens? Well, you do heat the water up, certainly, but it also cools the fire down. And so then the fire goes out. So in the same way, once you get to the point where you were cooling the star, then the star is dying. So you do make a little bit of stuff heavier than iron. It's not like there's a total wall there, but you just can't make very much of it. I think I still have questions about that. And so let's talk about. that and what happens after you make iron and why you can't even use surplus energy to make it. But first, let's take another quick break. How serious is youth vaping? Irreversible lung damage serious. One in 10 kids vape serious, which warrants a serious conversation from a serious parental figure like yourself. Not the seriously know-it-all sports dad or the seriously smart podcaster. It requires a serious

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See? Listen to no such thing on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. All right, we're having a stellar conversation that's not ironic at all about how stars make iron. And it seems like stars can fuse all of the lighter elements starting from hydrogen right up until carbon, oxygen. But then it gets to iron and it can't do it anymore sustainably. I think maybe that's the footnote that you would have to add here is that stars can make heavier elements than iron. It just can't make them sustainably because it costs energy to make anything heavier than iron. Yeah, the same way like a fire can't make steam sustainably if you just pour water onto the fire.

What if I just had a bigger fire? Like, couldn't you imagine a sun or a star that somehow can make heavier elements in iron sustainably? Like it has so much hydrogen in it, perhaps, that it can keep making heavier elements for a while. I mean, I think that's basically what happens inside stars, but it spells the end of a star, right? You have a huge corporation, for example, and you have money losing divisions that are just growing bigger and bigger and bigger. sapping the profits from the money-making divisions, then your business is not going to last very long before you go bankrupt. So these stars basically start to go bankrupt as soon as they turn over past iron and nickel because they start to use up their own heat instead of producing

more heat. So maybe the answer here is that stars can fuse iron, right? I feel like maybe we lie to our listeners, Daniel, because stars can make iron. It can probably go all the way up to heavier and heavy elements. It just can't do it sustainably, right? Yeah. But it can. can, and they do make heavier elements in iron, but you're saying it kind of marks the point in the sun's balance sheet where it starts to lose energy. But then how much longer after that does a star have before it dies or collapses? Not very long. One of the really cool things about stellar evolution is how the first stages can take a very, very long time, hydrogen burning, and the next stages become much faster. So you can burn hydrogen for millions or billions

of years and then burn helium for like days or minutes. And every step after, that gets faster and faster. Why is that? Because what happens is that the temperature is increasing. And as the temperature increases, fusion happens faster, which then increases the temperature, which makes fusion happen faster. So it's a runaway effect. But then when you get to iron, wouldn't that help cool it down and stabilize it? When you get to iron, that does help cool it down. But now you have a heart of a star, which doesn't have what it needs in order to fuse, right? You have this cold blob of iron at the heart of your star. You have these shells of lighter materials going all the way out. Like the hydrogen has been pushed all the way out to the outside of the star. And

it was only hydrogen burning happening in the edges. And they have a layer of helium, which is burning. And then you have a layer of carbon, which is burning and layers of oxygen and neon, et cetera, all the way down to iron at the heart. So now the core of the star starts to cool down. And that's what triggers this collapse. By the way, iron heart is the name of a superhero as well. Tony Stark's protege who built her own iron suit. So then that's where stars basically collapse and become supernovas, right? Or some stars, I should say. The reason that stars aren't collapsing in the first place is this heat produced from fusion. Fusion is what's pushing back against gravity to keep a star imbalance. That's why it keeps going for billions of years the way that it can. And so once the star

starts to cool and fusion starts to slow down, then that spells the end of it and it starts to collapse. And then you can get a supernova in some cases. You can just get a gravitational collapse, which leads to like a white dwarf or a neutron star or a black hole, all sorts of fun outcomes. Right, because what happens is the star makes heavier and heavier elements. It gets up to iron. Iron causes it to cool down. Then you got all this cold iron in the middle of the star. And then basically that's when gravity kind of wins, right?

Takes all that iron and squeezes it down to like super duper dense materials and which can either stay there or cause the whole star to collapse and explode. Yeah, I think gravity only really wins if you get to a black, hole. Even if you get to something really dense like a neutron star, gravity is still being resisted. There's still some force there that's pushing back to prevent the collapse into a black hole. And so like a white dwarf, for example, it's just like a big hot lump of that metal that was made inside the star. And it's resisting gravity trying to compress it into a black hole. But yeah, there are various stages of retreats sort of against gravity. As gravity gets stronger and stronger

collapses a huge burning star into a white dwarf or into a neutron star or maybe, you know, even into a black hole. Yeah, but some stars collapse, and that collapseation causes kind of like a rebound, right? Because all the star basically collapses in and itself, it like rebounds. And that's one of the kinds of supernova that exists out there, right? That's what causes some supernova. Yeah, if you have a massive enough star to start from, something like eight times the mass of our sun, then this last stage is it puffs out to be a red super giant.

And then it collapses into what we call a type two supernova. It's a gravitational collapse. And then you can get like a black hole or a neutron star at its heart. If you have a lower mass star, like less than eight times the mass of the sun, then you're more likely to get like a white dwarf as an outcome. And if that white dwarf then later gets some more mass added to it, then it can collapse into a type 1a supernova. Right.

But then I think what's interesting is that in these events, like when a star goes supernova, that's then when the heavier elements get made, right? That's when there's so much energy being released, that the shock wave compresses things and there's enough energy there to actually fuse these heavier elements. Yeah. So as we said earlier, there's a little bit of fusion of these heavier elements, things heavier than iron inside the star, though it tends to cool the star. But most of the heavy elements in the universe, the gold and the platinum and the uranium, are not made inside

those stars. They're made either at the end of the star, like during the supernova, when so much energy is released, that you can use some of it up to fuse these even heavier things. And also, much later on when neutron stars, which are the remnants of some of these supernovas, when those collide. And then you can get enormous creation, like entire earth-sized chunks of gold or platinum can be made in those neutron star collisions. Right. So in a way, stars do fuse iron and make heavier elements, right? I mean, they don't just make it at the end of their lives, but also in these new ways,

they do make the heavier elements. That's true. I guess they can get some credit for that as well. I mean, they get all the credit, right? Like, are these heavier elements made any other way? It's almost entirely supernova and neutron star mergers. And I guess you could say the neutron star comes from the original star. And so when you merge it together to make gold or platinum or plutonium or whatever,

that gets credited on the account of the original star. It's a different sort of process, though, right? It's not fusion happening at the heart of the star the way you make carbon and silicon, though it is a product of the star, which is later fusing together to make these heavy elements. Not the same process, but still a fusion. the elements together to make heavier elements. Yeah, it certainly is fusion.

And it costs energy in that case, right, instead of creating energy. And that's kind of why these heavier elements are also so rare, right? Because they're only made at the end of the life of a star and only briefly. And so that's why, for example, here on Earth, there's a ton of iron, but not a lot of gold or titanium. Yeah, that's why the core of the earth is mostly iron and nickel and these kinds of things. And not gold and not platinum and not uranium. That's why these really heavy elements are trace in the universe compared to iron and nickel, which are like the endpoints of stellar nucleosynthesis.

Stars are like factories for turning hydrogen into heavier stuff, but they can only do that sustainably up to about iron, which really controls what life on Earth is made out of, what Earth itself is made out of. Yeah, including us. And Danny, are you saying that gold and titanium are basically made out of iron as well? Because you have to merge iron to make gold and titanium. Yeah, I suppose so. In that same sense, everything is made out of hydrogen, right?

You and I were both just hydrogen. I'm just saying Iron Man is named appropriately because even if it's made out of gold and titanium, it does have iron aluminum. Although technically you're saying he should be maybe called Hydrogen Man. He should be Hygian Man, exactly. Something that's really interesting for me to think about sort of philosophically is to think about the difference between, for example, oxygen and helium, right? In the end, they're really the same components.

Like a single atom of oxygen has 24 upcorks. 24 down quarks and 8 electrons. If you take four atoms of helium, it has the same thing. It's 24 ups, 24 downs and 8 electrons. It's just arranged differently. And so all we're doing in the hearts of stars and in supernova and in neutron star collisions is just moving the pieces around to make different arrangements. And that's what makes oxygen different from helium and different from iron and different from titanium. And so it's fascinating to me to see this happen that the crucial thing is the arrangement of those bits and the the energy needed to put the bits together into these special arrangements to make up me and you and Iron Man. Yeah, that's fascinating. So you're saying, instead of saying we're all made out of star stuff, we should be saying we're all made out of quarks. And all superheroes should just be called quarkman.

Corkman, exactly. Thor, also quarkman. Captain America, Captain Quark. Hey, I'm a reductionist, right? I like to reduce everything to the simplest possible terms. There you go. I just call everything quark. All right.

Well, it's interesting to think about the processes that let, to all of the things that we see around us and the inside of our phones, inside of our bodies and kind of reflect on why we're here. We're here because of these processes that happen inside of previous generations of stars and how the physics of the universe limits that and kind of determines the things that we are made out of and how those bits are arranged together. And even though the universe has been working for billions of years to convert hydrogen into more interesting stuff, it's still got a long way to go. The universe is about 90s. 92% hydrogen still after billions of years.

And most of the rest of that is just helium. The bits that make up me and you and everything else, the interesting bits, the heavy bits are a tiny fraction of 1% of the universe. Yeah. So even when the Avengers assemble, they're still made out of course. All right. Well, we hope you enjoyed that. Thanks for joining us.

See you next time. listening and remember that Daniel and Jorge Explain the Universe is a production of IHeartRadio. For more podcasts from IHeartRadio, visit the IHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. Welcome to one of my favorite people, an incomparable soccer icon, Megan Rapino, to the show, and we had a blast. Take a listen. Sue and I were like riding the lime bikes the other day and we're like, we're like, we're people ride bikes because it's fun. We got more incredible guests like Megan in store, plus news of the day and more. So make sure you listen to Good Game with Sarah Spain on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts.

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