Why can't stars fuse Iron?

Daniel and Jorge talk about how stars fuse lighter elements and what changes when they reach Iron.

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Episode Transcript

Speaker 1 (00:08):
Hey, or hey, you're the go to expert on the
science of Marvel superheroes. Right, yeah, I'm like Thanos and
stab my fingers and make half of your questions disappear. 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 the thing

(00:29):
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? His first suit was made out of iron?
He made it in a cave, so I guess technically
he could also be cave Man. So when he upgraded
his suit to something fancier, they didn't upgrade his name. Also,

(00:49):
they stuck to the original name. I mean, they're 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 a Erica 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, the science will always

(01:10):
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 physicists to villain.
Maybe you then can be called ironic Man. Hi am

(01:38):
or Hammack cartoonists and the creator of PhD comics. Hi.
I'm Daniel. I'm a particle of physicist and a professor
at U c 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's 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

(02:01):
an excuse for being a supervillain. I'm saying, I'm not
trying to create weapons that will allow supervillains to destroy
the world. What else could you ask for me to
not think about it? I mean it seems pretty obvious,
But welcome to our podcast Daniel and Jorge Explain the Universe,
a production of I Heart Radio in which we try
not to destroy the world, but we do hope to

(02:22):
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, and the inside of
our planets, and in the inside of every single object,
the tiny little buzzing particles that we've together to make

(02:42):
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 love
wonders and amazing processes going on as we speak all
the time. Everywhere in the universe, and all of the

(03:03):
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,

(03:25):
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 on the inside of stars. You
could think of the whole universe is sort of being
on fire. It's taking all that hydrogen fuel and trying
to burn it into heavier stuff. We're sort of put

(03:47):
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.

(04:10):
We couldn't have organic life if we didn't have the
backbone of carbon to allow us to make all of
those complex molecules. We couldn't breathe. That oxygen wasn't even anything.
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,

(04:30):
relies on stars to convert that hydrogen into heavier interesting stuff.
But even the awesome power of stars is limited. That's
the right. We're all born inside of stars, not literally though,
right like it. 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,

(04:52):
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,

(05:13):
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

(05:34):
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. The corps of stars. I
prefer to think of him as ashes of like a
campfire the end of the night. You don't say my
campfire has died a grizzly death. You say that it's

(05:56):
burned out, which doesn't seem like such a negative outcome.
You say it because you're not the wood and then
you burn 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, they have been around three generations of stars.
We call these type one, type two, type three stars

(06:19):
because we noticed 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.

(06:42):
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 that you can make heavier stuff out
of lighter stuff, but not forever. You can't take an
element with like five protons and fuse it together with

(07:02):
another element of five protons to make something with a
thousand protons, and 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. It's right, star fusion

(07:24):
of materials inside of them is limited up to iron,
and so that's the heaviest element that a regular sun
or star can make. And so to be on the
program will be tackling the question why can't stars fuse
iron or iron? Men? That is an interesting question. Why

(07:45):
can't stars fuse iron? They confuse anything lighter than iron,
but anything heavier than iron they can't do. 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 construct rocking the nucleus of
an atom, and why something fundamental changes after you get
to iron. Yeah. So, as usually, we were wondering how

(08:07):
many people have thought about the limitations of stars and
why they can't fuse anything heavier than iron. So as
Usial 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 our future episodes, please don't
be shy. Right to me two questions at Angele and
Jorge dot com and I will set you up. Here's

(08:29):
what people had 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 electro magnetic forces
rejecting getting 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

(08:52):
requires lower temperatures well, simply because it will take an
insane amount of energy to do it at the well,
novitabile stars that we know until no are not capable
of doing this. I actually think they can. But the
nuclear reaction absorbs more neutrons than it are than it admits,

(09:15):
and so perpetuating the nuclear actions within a star doesn't
work if it's fusing iron, whereas everything before iron keeps
the reaction going. Alright, a lot of interesting reasons here
or 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,

(09:37):
you're the physicists, they are not. So it's an interesting question.
Why can't stars fuse aren't together? And they'd be great.
They could write, They could make heavier and heavier elements, right,
m 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

(09:59):
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 blink out. But
then gold would be so common it wouldn't even be valuable.
We'll have to find something else to you know, overprice
rare comic books. I'm sure the humanity will find something

(10:22):
to argue about. I have that much faith in us. Well,
this is a fun question here. Why can't stars fuse?
Aren't together? And so I guess maybe step us through
kind of the history of how things get fused together,
starting from the big bank. You have 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 the initial ingredients for all of this fusion came

(10:44):
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

(11:07):
didn't the Big Bang make heavier things than hydrogen? Like,
if if things could be made heavier than the hydrogen,
why did most protons stopped 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 ten to
the minus six seconds we got things like protons and neutrons,

(11:29):
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. Wasn't hot for very long. Things
cooled off very rapidly, and because the next element, lithium
is very unstable, it doesn't stick around for very long.
The universe sort of couldn't build up even further during

(11:51):
the Big Bang. So there's this like initial hot flash
when hydrogen was made in 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 that 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.

(12:15):
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 ten 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 electron. But we still consider them
to be hydrogen before they got their electrons, right. Hydrogen

(12:38):
is kind of like the O G atom, Right, It's
like the most basic atom you can have, right, because
if you have just the proton, then that's like, 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 ten the minus six seconds, and then things were
hot enough for like a few minutes. For like three minutes,

(13:01):
we think things were hot enough for those protons to
fuse together to make helium. But after that things that
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 the basic atom of hydrogen, it's just one proton
with one electron. And now to make heavier elements, you

(13:23):
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

(13:44):
hot and dense for it to happen. But that's why
it happens at the inside of stars, for example, and
not just like in a balloon filled with hydrogen. Right.
The Hindenburg, which was filled with hydrogen, didn't have fusion
going on inside, because the protons do repel each Other's
a subtle point there, though, which is the protons appel
each other because of their positive charges. Once you get
them close enough, if you happen to squeeze them together,

(14:05):
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

(14:25):
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 son. You sort of dot dot dotted
over a few critical elements there, yeada, YadA YadA, Life
on Earth podcasts, superheroes. Interesting choice of focus. Yeah, and

(14:48):
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. Lilium, 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.

(15:09):
You need four protons, two of which convert into 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, which 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

(15:31):
have H two and you take two of those, you
squeeze those together, and you end up with helium four M.
But I guess the question is why do you need
those neutrons to make a stable atom. Why can't you
just have an atom with two protons in the middle. Remember,
these protons are positively charged, and they're pushing against each other.
A neutron is neutral, right, doesn't have any electric charge,

(15:51):
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.
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 meat loaf to make it fluffier and less dense.
I mean, there's sort of like the palate cleanser, which

(16:13):
is an important part of any menu. Right. No, no, no,
you said spacer, which we sounds like filler. They're sort
of like the therapist in the 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

(16:34):
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 is filler?
What happens if you take them out when the protons
much together? Even more if you take them out, then
the protons get closer and the electrostatic repulsion increases. So
helium two is not stable if you don't have the neutrons,

(16:55):
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 ed 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. All Right,

(17:27):
we're talking about Marvel superheroes and DC superheroes. Is that
basically what this episode is about. Why can't you use
Iron Man and Superman? All right? Well, so we talked
about how inside of Stars heavier elements get me from hydrogen,
which are the simplest atoms. You can have just a
proton and an electron and you can make helium out
of that and you can merge those to make heavier elements.

(17:47):
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 billium, the next elements of
the periodic table are very very unst able, so you
can make them inside stars. 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

(18:09):
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? Do you need to skip over them?

(18:30):
So what you do is you take three helium and
you combine them to make carbon. Right, carbon his 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

(18:51):
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 lilium,
You do, you need four hydrogen atoms, and so inside
the sun, these four things have to be kind of
a collision course with each other. Now, the steps are
sort of independent. You have two hydrogen atoms come together
to make deuterium, which is a proton and a neutron

(19:12):
that bangs together with another hydrogen to make helium three,
and then the helium three's 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 nuclear to come together pretty quickly, because

(19:33):
two helium come together to make berrillium, which is really
not stable for very long, and you need that third
helium to come in and turn it into carbon before
the berrillium falls apart. WHOA. So literally, inside the sun
you need to have three helium atoms like chance, just
beyond a collision courts with each other, or inside of stars.
Does this happen like by squeezing. It's a less kind

(19:54):
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
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 squeezing stuff together, it
creates these reactions. You make heavier elements, and that releases energy,

(20:16):
because that's kind of how suns work, right, They squeeze
in together. Once they pop into place, once they merge,
they snap together, a bunch of energies 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

(20:38):
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 look nite 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 alasthma. The fact that these

(21:01):
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 to associating
a huge release of energy with like an explosion something breaking,

(21:23):
But it's this is the opposite. Actually, when you put
things together, it releases energy when you immerge them. It's
like taking two piece 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

(21:44):
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 like up 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

(22:06):
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

(22:29):
be together to some degree, right, I mean, they stick
together because they 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

(22:51):
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
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

(23:12):
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, 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 a helium, they have
to give up that energy to like release photons so
they can fall into the potential well of their binding state.

(23:35):
It might help to talk about where this energy comes from.
Like two hydrogen atoms before their fuse 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 that

(23:56):
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 like
fall into this hole together, they have to sort of
release energy. Think about, for example, if you're mini golfing

(24:19):
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 speeds 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

(24:39):
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

(24:59):
than just to 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 zero point
eight percent less than the mass of the nucleons that
make it up. But that's really just another way of

(25:21):
saying how much energy is stored in It has less
energy stored inside it than the nucleons that make it, right,
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,

(25:44):
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 than 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

(26:05):
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

(26:25):
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 forces 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 hydrogen, You've gotta zap one of them

(26:48):
to release them from the pull of the other one,
and that cost energy. So if breaking it up cost 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 are So what
happens when you try to make iron? First of all,

(27:10):
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

(27:31):
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, has to be hotter and hotter,
because now these nuclei have larger positive charges, so they're
pushing against each other even more so. In order to
get carbon diffuse together with other carbon or with something else,

(27:52):
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. Our son cannot make heavier elements than helium.
Is that what you're saying, Like, our son is limited
to helium. Our son is limited to helium until the

(28:14):
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 expands 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

(28:35):
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 testifysist don't talk about often, you know,
when they say we're made out of star stuff. Really,
they mean we were 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

(28:58):
those big stars can combine carbon and hydrogen and make
these heavier elements. But even the biggest and hottest stars
have to stop at ore. And so the question is
why is that what's so special about? All Right? 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

(29:18):
have more of these things feeling the strong force and
pulling on each other. So as you go up the
periodic table, you're releasing more energy because the binding is
getting stronger. 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. Takes an even more powerful laser to
break up heavier elements than carbon, because as you keep

(29:41):
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 stick year. You stick two atoms together,

(30:02):
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 then release more energy? Is that kind of
like a runaway reaction in a way, Like you know,
Sun starts to make heavier elements and when you fuse
those together they release even more energy. It sounds like

(30:25):
what you're saying, Merging two carbons together releases a whole
bunch of energy, Yes, exactly. And so as you keep
going up to 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 the 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

(30:47):
like different sides 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.

(31:08):
So you're reducing the binding energy of the nucleus. You're
making it easier to break it up than it was before. Alternately,
you're saying it gets harder to fuse things to a
really big atom like art, 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

(31:29):
more proton just gets harder and harder a because there's
so much positivity they're repelling you. But also like even
the strongforce 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

(31:50):
than releasing energy, because fusing iron together I means sticking
it together into a bigger nucleus which is not as
tightly bound. Right, And so remember if something it's 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

(32:12):
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

(32:33):
folks here do some superhero style mental gymnastics here. With
so many inversions, I think maybe it's simpiliary to put it.
Is that when you're fusing something simple like hydrogen, it
takes a little bit of energy to get the hydrogen's
atoms together, but once they fused, 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,

(32:53):
the energy it takes to like fuse something to them
is 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 loot on the podcast, that you can't
make things heavy than iron because it cool stars. The
reason for the fundamental reason for why fusing iron together

(33:16):
cools star instead of heating a star the way fusing
hydrogen together does calm 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,

(33:36):
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 two thirty five is
like a cocked mouse trap. It took energy to squeeze

(33:58):
all those protons and neutral is 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 two thirty five, is struck
by an incoming neutron, for example, it's like a mouse

(34:19):
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

(34:42):
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,

(35:02):
there's a lot going on, right Even in a big,
powerful sun that can make things like iron, there's still
hydrogen being fused and the lower elements being fused 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 energy, but it's also you know, getting a lot

(35:24):
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. And 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

(35:45):
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,

(36:06):
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

(36:27):
use surplus energy to make it as far as let's
take another quick break. 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

(36:49):
lighter elements, starting from hydrogen right up until carbon oxygen,
and but then it gets to iron and it can't
do it anymore sustainably. I think maybe that's the footnote
you would have to add here, is that stars can
make heavier elements and iron, it just can't make them
sustainably because it costs energy to make anything heavier than iron. Yeah,

(37:10):
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, can you imagine
a sun or a star that somehow can make heavier
elements and iron sustainably, Like it has so much hyngen
in it, perhaps that it can keep making heavier elements
for a while. I mean, I think that's basically what

(37:31):
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 and 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

(37:52):
heat instead of producing more heat. So maybe the answer
here is that stars can fuse iron. Right, I feel
like maybe we like or Innerstanuel. Because stars can make iron,
it can probably go all the way up to heavier
and every amassie. It just can't do it sustainably, right,
But it can, and they do make heavier elements in iron.

(38:12):
But you're saying, it's 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 the 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

(38:35):
and then burn helium for like days or minutes, and
every step after that gets faster and faster. The wise
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,
when that help cool it down and stabilize it. When

(38:56):
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,
this cold blob of iron at the heart of your star.
You know, 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 there
was only hydrogen burning happening on the edges. And they

(39:16):
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 starts protege who
built her own iron suit. So then then that's where
stars basically collapse and becomes super nervous. Right for some stars,

(39:41):
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
in balance. 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

(40:01):
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 that the star makes heavier and
heavier elements. It gets up to iron iron causes it
to cool down. Then you've 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

(40:22):
and squishes it down to like super duper dense materials
and which can either stay there or cost 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.

(40:44):
And so like a white dwarf, for example, is 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, into a neutron star, or maybe even

(41:04):
into a black hole. Yeah, but some stars collapse, and
that collapsation causes kind of like a rebound, right, because
there's all the star basic collapses in on itself with
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

(41:27):
this last stages, it puffs out to be a red
supergiant and then it collapses until we call it 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

(41:47):
some more mass added to it, then it can collapse
into a type one a 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. Yes, so, as we said earlier,

(42:10):
there's a little bit of fusion of these heavier elements,
things heavier than iron inside the star that 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

(42:32):
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 doop
use iron and make heavier elements, right, I mean, they
don't just make it at the end of their lives.

(42:54):
But also in these new ways they do make the
heavier elements, that's true. I guess they can get some
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

(43:14):
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 fusing the elements together to make
heavier elements. Yeah, it certainly is fusion, and it costs

(43:37):
energy in that case, right, instead of creating energy. And
that's kind of why these heavier elements are also so rarer, 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

(44:00):
platinum and not uranium. That's why these really heavy elements
are traced 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

(44:23):
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 golden titanium, it does have ironum.
Although technically you're saying you should be maybe called hydrogen man,

(44:44):
should hyrogen 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 twenty four up corks, twenty four
down corks, and eight electrons. If you take four atoms
of helium, it has the same thing. It's twenty four ups,

(45:06):
twenty four downs, and eight 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 happened. That the crucial thing is the

(45:28):
arrangement of those bits and the energy needed to put
the bits together into these special arrangements to make up
me and you and Iron Man. Yeah, it'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 quirks, and all superheroes should just be called Corkman exactly,
or also Corkman, Captain America, Captain cork Hey. I'm a reductionist, right,

(45:50):
I'd like to reduce everything to the simplest possible terms.
There you go, just call everything cork. All right. Well,
it's interesting to think about the processes that lead 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,

(46:12):
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 ago. Universe is about nine two percent hydrogen
still after billions of years, and most of the rest

(46:34):
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 one percent of
the universe. Yeah, so even when the Avengers assembled, they're
still made out of court. Alright, Well, we hope you
enjoyed that. Thanks for joining us, See you next time.

(46:59):
Thanks you're listening, and remember that Daniel and Jorge Explain
the Universe is a production of I heart Radio. For
more podcast from my heart Radio, visit the i heart
Radio app, Apple Podcasts, or wherever you listen to your
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