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

• 0:08
  Speaker 1: Hey, or hey, you're the go to expert on the
• 0:11
  Speaker 1: science of Marvel superheroes. Right, yeah, I'm like Thanos and
• 0:15
  Speaker 1: stab my fingers and make half of your questions disappear. Well,
• 0:19
  Speaker 1: I'm wondering why iron Man is called iron Man. Isn't
• 0:23
  Speaker 1: his suit made of some super high tech material like
• 0:26
  Speaker 1: Tony Starkium or something. I'm not sure that's the thing
• 0:29
  Speaker 1: in the Marvel universe, but if you look at his
• 0:32
  Speaker 1: origin story, you'll see the first suit he made was
• 0:34
  Speaker 1: made out of iron. So iron Man is made of
• 0:37
  Speaker 1: iron or what? His first suit was made out of iron?
• 0:40
  Speaker 1: He made it in a cave, so I guess technically
• 0:42
  Speaker 1: he could also be cave Man. So when he upgraded
• 0:45
  Speaker 1: his suit to something fancier, they didn't upgrade his name. Also,
• 0:49
  Speaker 1: they stuck to the original name. I mean, they're they're
• 0:51
  Speaker 1: not all literal, you know, like the Black Widow is
• 0:54
  Speaker 1: not actually a spider Captain America is not really a captain.
• 0:58
  Speaker 1: He's not really in charge of a Erica what. Also,
• 1:01
  Speaker 1: they're not real. Daniel, you're saying there is no science
• 1:05
  Speaker 1: in the Marvel universe. There is science, I'm sure, but
• 1:07
  Speaker 1: you know it's all fiction. Well, the science will always
• 1:10
  Speaker 1: be my superhero and physicists are the villains. Are you
• 1:13
  Speaker 1: saying physicists don't actually do physics? We just kept the
• 1:16
  Speaker 1: original name after we got upgraded from physicists to villain.
• 1:20
  Speaker 1: Maybe you then can be called ironic Man. Hi am
• 1:38
  Speaker 1: or Hammack cartoonists and the creator of PhD comics. Hi.
• 1:41
  Speaker 1: I'm Daniel. I'm a particle of physicist and a professor
• 1:44
  Speaker 1: at U c Irvine, and I really do my best
• 1:46
  Speaker 1: to avoid being a supervillain. That doesn't sound good enough, Daniel,
• 1:50
  Speaker 1: What do you mean? Don't you know there's no try,
• 1:52
  Speaker 1: there's only do you want me to do better than
• 1:54
  Speaker 1: my best? That seems like a big ask. I'm saying
• 1:57
  Speaker 1: trying not to be a supervillain? Is you know, not
• 2:01
  Speaker 1: an excuse for being a supervillain. I'm saying, I'm not
• 2:04
  Speaker 1: trying to create weapons that will allow supervillains to destroy
• 2:07
  Speaker 1: the world. What else could you ask for me to
• 2:10
  Speaker 1: not think about it? I mean it seems pretty obvious,
• 2:14
  Speaker 1: But welcome to our podcast Daniel and Jorge Explain the Universe,
• 2:17
  Speaker 1: a production of I Heart Radio in which we try
• 2:19
  Speaker 1: not to destroy the world, but we do hope to
• 2:22
  Speaker 1: break it. Apart into tiny, little understandable pieces. We want
• 2:26
  Speaker 1: to cast your minds out into this incredible, glittering cosmos.
• 2:30
  Speaker 1: Think about all the amazing and mysterious processes going on
• 2:33
  Speaker 1: at the inside of our stars, and the inside of
• 2:36
  Speaker 1: our planets, and in the inside of every single object,
• 2:39
  Speaker 1: the tiny little buzzing particles that we've together to make
• 2:42
  Speaker 1: our cosmos. We want to understand all of it. We
• 2:45
  Speaker 1: believe it all somehow makes sense, and we want to
• 2:48
  Speaker 1: explain all of it to you. That's right, because it
• 2:50
  Speaker 1: is an amazing, uncanny and marvel us universe out there
• 2:55
  Speaker 1: for us to explore and to learn about. We love
• 2:57
  Speaker 1: wonders and amazing processes going on as we speak all
• 3:01
  Speaker 1: the time. Everywhere in the universe, and all of the
• 3:03
  Speaker 1: universe is mostly hydrogen, and has always been mostly hydrogen
• 3:07
  Speaker 1: since the very beginning. It does contain an interesting smattering
• 3:12
  Speaker 1: of other stuff, heavier objects, helium, lithium, beryllium, oxygen, carbon,
• 3:17
  Speaker 1: even some iron out there to make the universe a
• 3:20
  Speaker 1: little bit spicier. That's right. It would be a little
• 3:22
  Speaker 1: bit boring if the whole universe was just hydrogen. Fortunately,
• 3:25
  Speaker 1: the universe has figured out a way to make other
• 3:27
  Speaker 1: things besides hydrogen, including helium and all of the heavier elements.
• 3:31
  Speaker 1: Don't they say, Daniel, that we're all made out of
• 3:33
  Speaker 1: star stuff. We are indeed built out of little bits
• 3:36
  Speaker 1: that have been assembled on the inside of stars. You
• 3:38
  Speaker 1: could think of the whole universe is sort of being
• 3:41
  Speaker 1: on fire. It's taking all that hydrogen fuel and trying
• 3:44
  Speaker 1: to burn it into heavier stuff. We're sort of put
• 3:47
  Speaker 1: together from the ashes of the fires inside stars. Yeah,
• 3:51
  Speaker 1: the universe is pretty fire. As the kids say these days,
• 3:54
  Speaker 1: it's pretty good. And it's a good thing. It can
• 3:56
  Speaker 1: make heavier things other than hydrogen, because that's kind of
• 3:59
  Speaker 1: where we come from. That's where plants come from, that's
• 4:01
  Speaker 1: where planets come from, that's where all good things to
• 4:04
  Speaker 1: humans seem to come from. That's right. Heavier elements allow
• 4:07
  Speaker 1: us to make much more complicated and much more interesting things.
• 4:10
  Speaker 1: We couldn't have organic life if we didn't have the
• 4:13
  Speaker 1: backbone of carbon to allow us to make all of
• 4:15
  Speaker 1: those complex molecules. We couldn't breathe. That oxygen wasn't even anything.
• 4:20
  Speaker 1: Most of the processes of life rely on biochemistry, which
• 4:23
  Speaker 1: rely on heavier atoms than just hydrogen, and so our
• 4:27
  Speaker 1: very existence, the existence of intelligence and probably even life,
• 4:30
  Speaker 1: relies on stars to convert that hydrogen into heavier interesting stuff.
• 4:36
  Speaker 1: But even the awesome power of stars is limited. That's
• 4:39
  Speaker 1: the right. We're all born inside of stars, not literally though,
• 4:42
  Speaker 1: right like it. Just the heavier elements other than hydrogen
• 4:45
  Speaker 1: are made inside of stars, and then they're released out
• 4:47
  Speaker 1: into space, into the cosmos when the stars explode. Basically, right, yeah,
• 4:52
  Speaker 1: there are these fascinating cycles. When we hear that we
• 4:55
  Speaker 1: are made of star stuff, we don't mean that the
• 4:56
  Speaker 1: stuff that you and I are made out of was
• 4:59
  Speaker 1: formed inside our star. We mean that a previous generation
• 5:03
  Speaker 1: of stars, ones that formed and burned for maybe billions
• 5:06
  Speaker 1: of years, created the elements that now make us and
• 5:10
  Speaker 1: the whole Solar System up. That star burned and then died,
• 5:13
  Speaker 1: and then exploded and spread its ingredients out to see
• 5:17
  Speaker 1: the next generation of stars. So our Solar system started
• 5:20
  Speaker 1: out already enriched in these heavier metals. Of course, our
• 5:23
  Speaker 1: sun is busy making even more of them. But all
• 5:26
  Speaker 1: the heavy metals and all the non hydrogen stuff that
• 5:29
  Speaker 1: is on Earth came from the heart of another star,
• 5:32
  Speaker 1: not from our own son, from the corpse of an
• 5:34
  Speaker 1: old star, right. I mean, we're sort of like the
• 5:37
  Speaker 1: bits and pieces that the old star had made, and
• 5:40
  Speaker 1: so we're part of this new generation, Daniel, Are we
• 5:42
  Speaker 1: the baby boomers of the Solar system life forms or
• 5:45
  Speaker 1: the gen x? I think that's kind of a dark
• 5:47
  Speaker 1: way to look at it. The corps of stars. I
• 5:49
  Speaker 1: prefer to think of him as ashes of like a
• 5:51
  Speaker 1: campfire the end of the night. You don't say my
• 5:53
  Speaker 1: campfire has died a grizzly death. You say that it's
• 5:56
  Speaker 1: burned out, which doesn't seem like such a negative outcome.
• 5:59
  Speaker 1: You say it because you're not the wood and then
• 6:02
  Speaker 1: you burn to death. Yeah, exactly, I'm the positive outcome
• 6:05
  Speaker 1: of this process. We don't know how many generations of
• 6:08
  Speaker 1: stars have been involved in making our stuff. We think that,
• 6:12
  Speaker 1: in general, they have been around three generations of stars.
• 6:16
  Speaker 1: We call these type one, type two, type three stars
• 6:19
  Speaker 1: because we noticed populations of stars that have been around
• 6:22
  Speaker 1: for a very very long time, and also populations of
• 6:24
  Speaker 1: stars that are sort of younger. But it is possible
• 6:26
  Speaker 1: that there were many many generations very early on in
• 6:29
  Speaker 1: the universe, when stars were very big and very hot
• 6:32
  Speaker 1: and didn't live for very long. Yeah, it's a fascinating
• 6:35
  Speaker 1: process the life cycle of stars, but within one life
• 6:38
  Speaker 1: cycle of a star, it is pretty busy making stuff. Right.
• 6:42
  Speaker 1: It takes hydrogen and combines it to make helium, and
• 6:44
  Speaker 1: then it takes helium to combine it into heavier elements,
• 6:47
  Speaker 1: and that's how most of the heavier elements are made
• 6:50
  Speaker 1: up to a certain point. Yeah, even these stellar fusion
• 6:54
  Speaker 1: engines are limited that you can make heavier stuff out
• 6:57
  Speaker 1: of lighter stuff, but not forever. You can't take an
• 7:00
  Speaker 1: element with like five protons and fuse it together with
• 7:02
  Speaker 1: another element of five protons to make something with a
• 7:05
  Speaker 1: thousand protons, and its stars are not capable of that.
• 7:08
  Speaker 1: As we look around the universe, we notice that there
• 7:10
  Speaker 1: seems to be a lot of like iron and nickel
• 7:13
  Speaker 1: and this kind of heavy stuff. But above that, heavier
• 7:16
  Speaker 1: elements than that are much more rare because stars in
• 7:20
  Speaker 1: their fusion process cannot make them. It's right, star fusion
• 7:24
  Speaker 1: of materials inside of them is limited up to iron,
• 7:27
  Speaker 1: and so that's the heaviest element that a regular sun
• 7:30
  Speaker 1: or star can make. And so to be on the
• 7:32
  Speaker 1: program will be tackling the question why can't stars fuse
• 7:41
  Speaker 1: iron or iron? Men? That is an interesting question. Why
• 7:45
  Speaker 1: can't stars fuse iron? They confuse anything lighter than iron,
• 7:49
  Speaker 1: but anything heavier than iron they can't do. It provides
• 7:52
  Speaker 1: a clue about what's going on at the heart of
• 7:54
  Speaker 1: these nuclei, how you build them out of protons and neutrons,
• 7:58
  Speaker 1: what the physics is of construct rocking the nucleus of
• 8:01
  Speaker 1: an atom, and why something fundamental changes after you get
• 8:05
  Speaker 1: to iron. Yeah. So, as usually, we were wondering how
• 8:07
  Speaker 1: many people have thought about the limitations of stars and
• 8:10
  Speaker 1: why they can't fuse anything heavier than iron. So as
• 8:13
  Speaker 1: Usial Daniel went out there to ask people on the
• 8:16
  Speaker 1: internet this question. Thank you very much to everybody who
• 8:18
  Speaker 1: volunteers for this portion of the podcast. If you'd like
• 8:21
  Speaker 1: to hear your voice for our future episodes, please don't
• 8:24
  Speaker 1: be shy. Right to me two questions at Angele and
• 8:27
  Speaker 1: Jorge dot com and I will set you up. Here's
• 8:29
  Speaker 1: what people had to say. The reason why stars can't
• 8:31
  Speaker 1: fuse iron is because stars are so hot that iron
• 8:35
  Speaker 1: essentially evaporates into something else. It's the electro magnetic forces
• 8:40
  Speaker 1: rejecting getting additional positively charged protons coming in, and there
• 8:45
  Speaker 1: needs to be more neutrons than that maybe because they
• 8:49
  Speaker 1: are not cool enough. Maybe the process of fusing iron
• 8:52
  Speaker 1: requires lower temperatures well, simply because it will take an
• 8:57
  Speaker 1: insane amount of energy to do it at the well,
• 9:01
  Speaker 1: novitabile stars that we know until no are not capable
• 9:06
  Speaker 1: of doing this. I actually think they can. But the
• 9:10
  Speaker 1: nuclear reaction absorbs more neutrons than it are than it admits,
• 9:15
  Speaker 1: and so perpetuating the nuclear actions within a star doesn't
• 9:18
  Speaker 1: work if it's fusing iron, whereas everything before iron keeps
• 9:22
  Speaker 1: the reaction going. Alright, a lot of interesting reasons here
• 9:25
  Speaker 1: or any of these correct. Some of them are sort
• 9:27
  Speaker 1: of close to being correct or in the right direction.
• 9:30
  Speaker 1: Other ones are pretty much dead wrong, but they're entertaining speculation. Nonetheless,
• 9:37
  Speaker 1: you're the physicists, they are not. So it's an interesting question.
• 9:40
  Speaker 1: Why can't stars fuse aren't together? And they'd be great.
• 9:44
  Speaker 1: They could write, They could make heavier and heavier elements, right,
• 9:47
  Speaker 1: m I mean the universe would definitely be different. You
• 9:49
  Speaker 1: might have like more uranium and more gold in the universe.
• 9:52
  Speaker 1: I don't know if that would be better. You know,
• 9:54
  Speaker 1: it might be like more poisonous for life. If you
• 9:56
  Speaker 1: start tweaking the basic parameters of the universe. You never
• 9:59
  Speaker 1: know what you might end up with. We might all
• 10:01
  Speaker 1: be Superman, or we might never have evolved. We might
• 10:04
  Speaker 1: all be gold men, is kind of what I'm hearing.
• 10:07
  Speaker 1: Might all be a lot shinier and blink out. But
• 10:09
  Speaker 1: then gold would be so common it wouldn't even be valuable.
• 10:13
  Speaker 1: We'll have to find something else to you know, overprice
• 10:18
  Speaker 1: rare comic books. I'm sure the humanity will find something
• 10:22
  Speaker 1: to argue about. I have that much faith in us. Well,
• 10:24
  Speaker 1: this is a fun question here. Why can't stars fuse?
• 10:27
  Speaker 1: Aren't together? And so I guess maybe step us through
• 10:30
  Speaker 1: kind of the history of how things get fused together,
• 10:32
  Speaker 1: starting from the big bank. You have to understand why
• 10:35
  Speaker 1: stars can't fuse iron. We first have to understand what's
• 10:38
  Speaker 1: going on when we fuse lighter elements. And you know,
• 10:41
  Speaker 1: all the initial ingredients for all of this fusion came
• 10:44
  Speaker 1: very very early on in the universe, when the universe
• 10:47
  Speaker 1: cooled down and protons were formed, and neutrons were formed,
• 10:51
  Speaker 1: and electrons were formed, and they were just flying around
• 10:54
  Speaker 1: on their own until it cooled even further, so much
• 10:57
  Speaker 1: so that electrons were moving too slow to escape the
• 11:00
  Speaker 1: electric attraction of those protons and then they cooled together
• 11:03
  Speaker 1: into neutral hydrogen. I guess maybe a question is why
• 11:07
  Speaker 1: didn't the Big Bang make heavier things than hydrogen? Like,
• 11:10
  Speaker 1: if if things could be made heavier than the hydrogen,
• 11:13
  Speaker 1: why did most protons stopped there? If the Big Bang
• 11:15
  Speaker 1: was so you know, hot and intense, why didn't heavier
• 11:18
  Speaker 1: elements get formed. The short answer is that it just
• 11:20
  Speaker 1: didn't really have enough time. Like things were cooling pretty
• 11:23
  Speaker 1: rapidly in the Big Bang, and after about ten to
• 11:25
  Speaker 1: the minus six seconds we got things like protons and neutrons,
• 11:29
  Speaker 1: and those protons are basically hydrogen. There was a little
• 11:32
  Speaker 1: bit of helium made during the Big Bang. It was
• 11:35
  Speaker 1: hot enough to fuse that hydrogen together into helium, but
• 11:39
  Speaker 1: not a whole lot. Wasn't hot for very long. Things
• 11:41
  Speaker 1: cooled off very rapidly, and because the next element, lithium
• 11:45
  Speaker 1: is very unstable, it doesn't stick around for very long.
• 11:48
  Speaker 1: The universe sort of couldn't build up even further during
• 11:51
  Speaker 1: the Big Bang. So there's this like initial hot flash
• 11:53
  Speaker 1: when hydrogen was made in a little bit of it
• 11:56
  Speaker 1: fused into helium, and a tiny little bit was made
• 11:59
  Speaker 1: into lithium. But that lithium sort of falls apart, which
• 12:01
  Speaker 1: doesn't allow you to then fuse lithium together to make
• 12:04
  Speaker 1: heavier stuff. I guess maybe something that some listeners may
• 12:07
  Speaker 1: not know is that a hydrogen atom is basically just
• 12:10
  Speaker 1: one electron orbiting around one proton, right, And that's exactly right.
• 12:15
  Speaker 1: In fact, we call a proton hydrogen. It's like ionized hydrogen,
• 12:19
  Speaker 1: even though it really is just a proton. So in
• 12:21
  Speaker 1: the beginning of the universe, the protons were formed after
• 12:23
  Speaker 1: like ten the minus six seconds, like a millionth of
• 12:26
  Speaker 1: a second into the universe, but it took a few
• 12:28
  Speaker 1: hundred thousand years before the universe cooled enough that those
• 12:31
  Speaker 1: protons could grab onto electron. But we still consider them
• 12:34
  Speaker 1: to be hydrogen before they got their electrons, right. Hydrogen
• 12:38
  Speaker 1: is kind of like the O G atom, Right, It's
• 12:40
  Speaker 1: like the most basic atom you can have, right, because
• 12:43
  Speaker 1: if you have just the proton, then that's like, that's
• 12:45
  Speaker 1: a hydrogen atom without an electron, is what you're saying. Yeah,
• 12:48
  Speaker 1: that's like ionized hydrogen. And so those protons were flying
• 12:51
  Speaker 1: around in the very early universe. They were made after
• 12:54
  Speaker 1: like ten the minus six seconds, and then things were
• 12:57
  Speaker 1: hot enough for like a few minutes. For like three minutes,
• 13:01
  Speaker 1: we think things were hot enough for those protons to
• 13:03
  Speaker 1: fuse together to make helium. But after that things that
• 13:06
  Speaker 1: cooled too much. You didn't have the conditions necessary anymore
• 13:09
  Speaker 1: to make heavier stuff. So for like the first three
• 13:12
  Speaker 1: minutes of the universe, everything was about as hot and
• 13:14
  Speaker 1: dense as the inside of a star. Right, But I
• 13:16
  Speaker 1: guess the basic atom of hydrogen, it's just one proton
• 13:20
  Speaker 1: with one electron. And now to make heavier elements, you
• 13:23
  Speaker 1: have to fuse hydrogen together because the heavier elements have
• 13:26
  Speaker 1: extra protons at their nuclei and extra electrons floating around them.
• 13:30
  Speaker 1: But it's hard to fuse two protons together, right, because
• 13:33
  Speaker 1: they're both positively charged, and so they repel each other, right,
• 13:37
  Speaker 1: And so it's hard to make an atom with two
• 13:39
  Speaker 1: protons and it's nucleus. Yeah, it is hard to get
• 13:42
  Speaker 1: these things together. That's why the universe has to be
• 13:44
  Speaker 1: hot and dense for it to happen. But that's why
• 13:46
  Speaker 1: it happens at the inside of stars, for example, and
• 13:49
  Speaker 1: not just like in a balloon filled with hydrogen. Right.
• 13:51
  Speaker 1: The Hindenburg, which was filled with hydrogen, didn't have fusion
• 13:54
  Speaker 1: going on inside, because the protons do repel each Other's
• 13:57
  Speaker 1: a subtle point there, though, which is the protons appel
• 14:00
  Speaker 1: each other because of their positive charges. Once you get
• 14:02
  Speaker 1: them close enough, if you happen to squeeze them together,
• 14:05
  Speaker 1: then they attract each other because another force takes over,
• 14:08
  Speaker 1: the strong force. So at long distances, protons repel each other,
• 14:12
  Speaker 1: they avoid getting near each other. If you do manage
• 14:14
  Speaker 1: to get them close enough, however, they will stick together
• 14:17
  Speaker 1: to make helium. Right, But it takes a lot to
• 14:20
  Speaker 1: get them really close together because they are repelling each
• 14:23
  Speaker 1: other through the electromagnetic force. And so that's kind of
• 14:25
  Speaker 1: where suns come in. Right. If you have a bunch
• 14:28
  Speaker 1: of hydrogen out there in space, gravity pulls it all together,
• 14:30
  Speaker 1: squeezes those protons close enough, so close together that eventually
• 14:34
  Speaker 1: the strong force takes over. Two protons merge, and boom,
• 14:38
  Speaker 1: you got a son. You sort of dot dot dotted
• 14:40
  Speaker 1: over a few critical elements there, yeada, YadA YadA, Life
• 14:44
  Speaker 1: on Earth podcasts, superheroes. Interesting choice of focus. Yeah, and
• 14:48
  Speaker 1: so this process is called hydrogen burning. And you might
• 14:51
  Speaker 1: imagine that it's just like you said, two hydrogens come
• 14:53
  Speaker 1: together to make a helium. Right, that makes sense. Two
• 14:56
  Speaker 1: protons come together to make a new nucleus with two
• 14:58
  Speaker 1: protons in it. Lilium, though, usually has two protons and
• 15:02
  Speaker 1: two neutrons in it. So to actually make a helium nucleus,
• 15:06
  Speaker 1: helium four we call it, you need four hydrogen nuclei.
• 15:09
  Speaker 1: You need four protons, two of which convert into neutrons
• 15:13
  Speaker 1: along the way. So hydrogen burning is actually a multi
• 15:16
  Speaker 1: step process. First, you take the two hydrogens, you squeeze
• 15:19
  Speaker 1: them together. You don't immediately get helium, which you get
• 15:22
  Speaker 1: is deuterium. You get an isotope of hydrogen with a
• 15:25
  Speaker 1: proton and a neutron because one of those hydrogens has
• 15:28
  Speaker 1: flipped from a proton to a neutron. So now you
• 15:31
  Speaker 1: have H two and you take two of those, you
• 15:33
  Speaker 1: squeeze those together, and you end up with helium four M.
• 15:37
  Speaker 1: But I guess the question is why do you need
• 15:39
  Speaker 1: those neutrons to make a stable atom. Why can't you
• 15:41
  Speaker 1: just have an atom with two protons in the middle. Remember,
• 15:44
  Speaker 1: these protons are positively charged, and they're pushing against each other.
• 15:47
  Speaker 1: A neutron is neutral, right, doesn't have any electric charge,
• 15:51
  Speaker 1: and so it sort of helps space the protons apart
• 15:54
  Speaker 1: from each other. All these objects have little bits of
• 15:56
  Speaker 1: the strong force. They all stick together using the strong force.
• 16:00
  Speaker 1: Neutrons are there to sort of keep the protons a
• 16:02
  Speaker 1: little bit further from each other. But you're saying neutrons
• 16:04
  Speaker 1: in our atoms are just filler. They're like what you
• 16:07
  Speaker 1: add to meat loaf to make it fluffier and less dense.
• 16:11
  Speaker 1: I mean, there's sort of like the palate cleanser, which
• 16:13
  Speaker 1: is an important part of any menu. Right. No, no, no,
• 16:15
  Speaker 1: you said spacer, which we sounds like filler. They're sort
• 16:18
  Speaker 1: of like the therapist in the marriage, right, they keep
• 16:20
  Speaker 1: everybody happy. You know. The construction of the nucleus is
• 16:23
  Speaker 1: a delicate balance between the strong force, which is trying
• 16:25
  Speaker 1: to stick everything together and the electromagnetic force, which in
• 16:29
  Speaker 1: the end is pushing things apart. And we'll see as
• 16:31
  Speaker 1: nuclei get larger and larger, the balance of power between
• 16:34
  Speaker 1: these two things changes because the strong force is only
• 16:37
  Speaker 1: powerful over very short distances and the electromagnetic force is
• 16:40
  Speaker 1: powerful over longer distances. So you need the neutrons is filler?
• 16:44
  Speaker 1: What happens if you take them out when the protons
• 16:46
  Speaker 1: much together? Even more if you take them out, then
• 16:48
  Speaker 1: the protons get closer and the electrostatic repulsion increases. So
• 16:52
  Speaker 1: helium two is not stable if you don't have the neutrons,
• 16:55
  Speaker 1: it falls apart. You need the glue, which is sort
• 16:57
  Speaker 1: of like the neutrons. All right, Well, that's the beginning
• 17:00
  Speaker 1: of merging atoms together to make different materials, and that's
• 17:03
  Speaker 1: what's happening inside of Stars. And this goes on and on,
• 17:06
  Speaker 1: but at some point it stops ed iron, and so
• 17:09
  Speaker 1: let's talk about why that is and what's so special
• 17:11
  Speaker 1: about iron. But first let's take a quick break. All Right,
• 17:27
  Speaker 1: we're talking about Marvel superheroes and DC superheroes. Is that
• 17:30
  Speaker 1: basically what this episode is about. Why can't you use
• 17:33
  Speaker 1: Iron Man and Superman? All right? Well, so we talked
• 17:36
  Speaker 1: about how inside of Stars heavier elements get me from hydrogen,
• 17:40
  Speaker 1: which are the simplest atoms. You can have just a
• 17:42
  Speaker 1: proton and an electron and you can make helium out
• 17:44
  Speaker 1: of that and you can merge those to make heavier elements.
• 17:47
  Speaker 1: But at some point we stop at iron. Somehow stars
• 17:49
  Speaker 1: are not able to make iron, Daniel, what's the next
• 17:52
  Speaker 1: step after helium? So after helium, you can try to
• 17:54
  Speaker 1: make heavier stuff. Lithium and billium, the next elements of
• 17:58
  Speaker 1: the periodic table are very very unst able, so you
• 18:00
  Speaker 1: can make them inside stars. They just don't last very long,
• 18:03
  Speaker 1: so they're not good building blocks for the heavier stuff.
• 18:05
  Speaker 1: What do you mean you make it and it dissolves
• 18:07
  Speaker 1: right away, or like you make a nucleus of lithium
• 18:09
  Speaker 1: and it breaks apart right away. They break apart right away,
• 18:12
  Speaker 1: and they're also destroyed by other reactions like photo disintegration.
• 18:15
  Speaker 1: Photons made by other reactions tend to break up lithium, beryllium,
• 18:19
  Speaker 1: and boron, So they just don't last very long inside
• 18:22
  Speaker 1: of stars, so they're not good building blocks for heavier stuff.
• 18:26
  Speaker 1: But if they don't last, how do you make stuff
• 18:27
  Speaker 1: that's heavier than them? Do you need to skip over them?
• 18:30
  Speaker 1: So what you do is you take three helium and
• 18:32
  Speaker 1: you combine them to make carbon. Right, carbon his atomic
• 18:36
  Speaker 1: number six, So three helium can come together to make carbon.
• 18:39
  Speaker 1: It's not an easy thing to do to get the
• 18:41
  Speaker 1: three helium to stick together, right, Getting two protons together
• 18:45
  Speaker 1: to do hydrogen burning is complicated enough. Now you need
• 18:48
  Speaker 1: three things all to dance together. To make carbon requires
• 18:51
  Speaker 1: a very hot, very dense kind of sun. Well, I
• 18:54
  Speaker 1: mean it seems hard to make helium in the first place. Right,
• 18:57
  Speaker 1: you said you need four hydrogen atoms to make a lilium,
• 19:00
  Speaker 1: You do, you need four hydrogen atoms, and so inside
• 19:02
  Speaker 1: the sun, these four things have to be kind of
• 19:05
  Speaker 1: a collision course with each other. Now, the steps are
• 19:07
  Speaker 1: sort of independent. You have two hydrogen atoms come together
• 19:10
  Speaker 1: to make deuterium, which is a proton and a neutron
• 19:12
  Speaker 1: that bangs together with another hydrogen to make helium three,
• 19:16
  Speaker 1: and then the helium three's together combined to make helium
• 19:19
  Speaker 1: four and give off some more protons. All those steps
• 19:22
  Speaker 1: are independent, and the intermediate pieces are more stable, so
• 19:25
  Speaker 1: it's not as unlikely as requiring four things to all
• 19:28
  Speaker 1: come together at once. But to form carbon, you do
• 19:30
  Speaker 1: need three helium nuclear to come together pretty quickly, because
• 19:33
  Speaker 1: two helium come together to make berrillium, which is really
• 19:36
  Speaker 1: not stable for very long, and you need that third
• 19:38
  Speaker 1: helium to come in and turn it into carbon before
• 19:41
  Speaker 1: the berrillium falls apart. WHOA. So literally, inside the sun
• 19:45
  Speaker 1: you need to have three helium atoms like chance, just
• 19:48
  Speaker 1: beyond a collision courts with each other, or inside of stars.
• 19:51
  Speaker 1: Does this happen like by squeezing. It's a less kind
• 19:54
  Speaker 1: of an explosion. It's more like things get squeezed together. Yeah,
• 19:57
  Speaker 1: things are getting squeezed together. It's the density, right, the
• 19:59
  Speaker 1: pressure that's creating the possibility for this to happen. You
• 20:02
  Speaker 1: need like a certain number of helium atoms per cubic
• 20:05
  Speaker 1: centimeter to make the probabilities anything greater than basically zero. Right.
• 20:09
  Speaker 1: So inside of suns, the gravity squeezing stuff together, it
• 20:11
  Speaker 1: creates these reactions. You make heavier elements, and that releases energy,
• 20:16
  Speaker 1: because that's kind of how suns work, right, They squeeze
• 20:18
  Speaker 1: in together. Once they pop into place, once they merge,
• 20:21
  Speaker 1: they snap together, a bunch of energies released, and that's
• 20:24
  Speaker 1: the energy of the sun. Yeah, that's the crucial thing
• 20:27
  Speaker 1: we haven't talked about yet. When you combine hydrogen together
• 20:29
  Speaker 1: to make helium, you don't just get helium. You also
• 20:32
  Speaker 1: give off photons, give off neutrinos as well. But energy
• 20:36
  Speaker 1: is released when you do this reaction, and that helps
• 20:38
  Speaker 1: create the conditions for the next reaction. It makes the
• 20:41
  Speaker 1: core of the sun very, very hot, and so this
• 20:43
  Speaker 1: is just sort of like a fire the way like
• 20:45
  Speaker 1: when you start a log burning, it helps create the
• 20:47
  Speaker 1: conditions for the next log to burn because it creates
• 20:49
  Speaker 1: that heat which will look nite the next log. So
• 20:52
  Speaker 1: in plasma physics they call this ignition when the plasma
• 20:55
  Speaker 1: is hot enough to create fusion, and that fusion then
• 20:58
  Speaker 1: maintains the heat of the alasthma. The fact that these
• 21:01
  Speaker 1: reactions release heat is what allows them to continue to
• 21:04
  Speaker 1: go and also what warms up our summers. Yeah, it's
• 21:06
  Speaker 1: interesting because it's like merging atoms together is what releases
• 21:11
  Speaker 1: the energy, which is i think maybe a little counterintuitive
• 21:13
  Speaker 1: for most people because we're sort of used to like
• 21:16
  Speaker 1: breaking things to release energy, right, Like we're to associating
• 21:20
  Speaker 1: a huge release of energy with like an explosion something breaking,
• 21:23
  Speaker 1: But it's this is the opposite. Actually, when you put
• 21:25
  Speaker 1: things together, it releases energy when you immerge them. It's
• 21:28
  Speaker 1: like taking two piece of clay and somehow when you
• 21:30
  Speaker 1: stick them together that releases a bunch of light. Yeah,
• 21:33
  Speaker 1: the energy flow is exactly the crucial concept here. Reactions
• 21:37
  Speaker 1: can either release energy when you form something or they
• 21:41
  Speaker 1: can cost energy. Right, So you can release energy when
• 21:44
  Speaker 1: you make something, which means that it takes energy to
• 21:47
  Speaker 1: break it up. Like if sticking things together releases energy,
• 21:51
  Speaker 1: then those things are now bound together and it costs
• 21:54
  Speaker 1: energy to break it up. If you like mechanical analogies,
• 21:57
  Speaker 1: you can think of this. They're like inside of a
• 21:58
  Speaker 1: cup together like up is now an analogy for like
• 22:01
  Speaker 1: the potential energy of this system, and when they fall
• 22:04
  Speaker 1: into the cup, they have to release energy to fall
• 22:06
  Speaker 1: into the cup. And then later if you want to
• 22:08
  Speaker 1: break it apart again, you have to put energy in.
• 22:11
  Speaker 1: So if you want to break helium into hydrogen, you
• 22:14
  Speaker 1: have to zap it with a laser to break it up.
• 22:16
  Speaker 1: So if it takes energy to break it up, that
• 22:18
  Speaker 1: means that it releases energy when you make it. Yeah,
• 22:21
  Speaker 1: I think it's still a little counterintuitive. I guess I'm
• 22:23
  Speaker 1: not quite wrapping my head around it or how to
• 22:26
  Speaker 1: explain it, because it feels like the atoms want to
• 22:29
  Speaker 1: be together to some degree, right, I mean, they stick
• 22:31
  Speaker 1: together because they attracted to each other, and so why
• 22:33
  Speaker 1: would that release energy if you are sticking them together.
• 22:37
  Speaker 1: In order for them to stick together, they have to
• 22:39
  Speaker 1: give up some of their energy. Sticking together means that
• 22:41
  Speaker 1: they're bound together. They're like together in a potential Well
• 22:44
  Speaker 1: think about an analogy in terms of like orbit, because
• 22:47
  Speaker 1: gravitational binding energy works the same way. If a planet
• 22:51
  Speaker 1: is flying by a sun and it has a huge
• 22:53
  Speaker 1: amount of energy, then the Sun is not going to
• 22:55
  Speaker 1: capture it. But if a planet is flying by the
• 22:57
  Speaker 1: Sun and it releases some of its energy, gives up
• 23:00
  Speaker 1: of its energy into something else, Like it bangs into
• 23:02
  Speaker 1: a rock and sends that rock away, it's lost some
• 23:05
  Speaker 1: of its energy, and then it can fall into the
• 23:07
  Speaker 1: gravitational well of the Sun and be trapped there. Now
• 23:09
  Speaker 1: you've given up some energy by banging into this rock
• 23:12
  Speaker 1: and sending it out to infinity and created this combined
• 23:15
  Speaker 1: state this planet that's now orbiting the Sun. So when
• 23:18
  Speaker 1: something falls into a potential well, it's losing energy, has
• 23:22
  Speaker 1: to give up that energy somehow. So in the same way,
• 23:24
  Speaker 1: two hydrogen atoms have like a lot of kinetic energy.
• 23:27
  Speaker 1: When they get trapped together into a helium, they have
• 23:29
  Speaker 1: to give up that energy to like release photons so
• 23:32
  Speaker 1: they can fall into the potential well of their binding state.
• 23:35
  Speaker 1: It might help to talk about where this energy comes from.
• 23:38
  Speaker 1: Like two hydrogen atoms before their fuse together, you're saying
• 23:41
  Speaker 1: they have a certain amount of energy and after you
• 23:43
  Speaker 1: fuse them, you're saying they have less energy together as
• 23:46
  Speaker 1: a pair. Where did this energy come from? You just said,
• 23:48
  Speaker 1: maybe the kinetic energy of these particles, or is it
• 23:51
  Speaker 1: in like the binding energy inside of their corks. So
• 23:54
  Speaker 1: it's not inside the binding energy of the corks that
• 23:56
  Speaker 1: the proton doesn't change. It comes from the energy of
• 23:59
  Speaker 1: the motion of hydrogen. These hydrogen atoms are a very
• 24:02
  Speaker 1: energetic state. They have to be in order to even
• 24:04
  Speaker 1: get close to each other because otherwise they're getting pushed
• 24:06
  Speaker 1: apart by the electrostatic repulsion. Because they're both positively charged,
• 24:10
  Speaker 1: so they have a lot of energy. You push them together,
• 24:12
  Speaker 1: and then in order to stay together in order like
• 24:14
  Speaker 1: fall into this hole together, they have to sort of
• 24:16
  Speaker 1: release energy. Think about, for example, if you're mini golfing
• 24:19
  Speaker 1: and you're trying to get a ball into a little
• 24:21
  Speaker 1: hole on the top of a volcano. You've got to
• 24:23
  Speaker 1: give it enough speeds so it gets up to the
• 24:25
  Speaker 1: top right and then it falls into the hole. Now
• 24:28
  Speaker 1: it's like stuck in that hole. It's got to give
• 24:30
  Speaker 1: up some energy to go into the hole. And so
• 24:32
  Speaker 1: in the same way, the hydrogen atom needs a lot
• 24:34
  Speaker 1: of speed to approach the other hydrogen atom. Then it
• 24:36
  Speaker 1: has to give up that speed when it falls into
• 24:39
  Speaker 1: the hole of the strong force which is attracting the
• 24:41
  Speaker 1: other one. Basically, you're saying they had some kinetic energy
• 24:44
  Speaker 1: when they were flying apart together, but once they smash
• 24:47
  Speaker 1: into each other, that energy, that kinetic energy has to
• 24:50
  Speaker 1: go somewhere, and that's basically the energy that powers the
• 24:53
  Speaker 1: sun is when these things smash into each other. Yeah,
• 24:55
  Speaker 1: the reorganization of two protons into helium has less energy
• 24:59
  Speaker 1: than just to two protons by themselves. Another way to
• 25:02
  Speaker 1: think about it is in terms of the mass. Remember
• 25:05
  Speaker 1: that mass is just a measure of how much energy
• 25:08
  Speaker 1: is stored inside something, not actually the amount of stuff,
• 25:11
  Speaker 1: and the mass of the helium atom is zero point
• 25:14
  Speaker 1: eight percent less than the mass of the nucleons that
• 25:18
  Speaker 1: make it up. But that's really just another way of
• 25:21
  Speaker 1: saying how much energy is stored in It has less
• 25:24
  Speaker 1: energy stored inside it than the nucleons that make it, right,
• 25:28
  Speaker 1: and so to get into that state you have to
• 25:30
  Speaker 1: release energy, just like how an electron when it moves
• 25:33
  Speaker 1: down an energy level around an atom, it has to
• 25:35
  Speaker 1: give up a photon to move down an energy level. Right,
• 25:38
  Speaker 1: energy is conserved there. In the same way here, these
• 25:40
  Speaker 1: two protons are moving into another state which has lower energy,
• 25:44
  Speaker 1: so they have to give up that energy. You might
• 25:46
  Speaker 1: ask like, well, why does it have lower energy? What's
• 25:48
  Speaker 1: lower energy? But having two protons stuck together than having
• 25:52
  Speaker 1: two protons fly apart, right, Yeah, I think that's the
• 25:55
  Speaker 1: main question. What's different. There's the mathematical answer, and then
• 25:58
  Speaker 1: there's the intuitive answer. Mathematically, whenever you have a force
• 26:02
  Speaker 1: that's attractive, you can think about it in terms of
• 26:05
  Speaker 1: a potential energy difference. Forces like to push things towards
• 26:10
  Speaker 1: lower potential energy the way gravity pulls a rock down
• 26:14
  Speaker 1: a hill to lower gravitational potential energy. So pulling something
• 26:18
  Speaker 1: in with an attractive force like the strong force means
• 26:22
  Speaker 1: bringing it to a lower energy state that's called the
• 26:25
  Speaker 1: binding energy. The way I think about it intuitively is
• 26:28
  Speaker 1: thinking about the reverse process, right, Like, if these two
• 26:31
  Speaker 1: things are stuck together, if the strong forces really holding
• 26:34
  Speaker 1: tightly on them, then you've got to zap them. You
• 26:36
  Speaker 1: have to give them energy to push them apart, right,
• 26:39
  Speaker 1: Just the same way if you want to release the
• 26:41
  Speaker 1: Earth from the Sun's orbit, you've got to give it
• 26:43
  Speaker 1: a push. If you want to break up the helium
• 26:45
  Speaker 1: nucleus into two hydrogen, You've gotta zap one of them
• 26:48
  Speaker 1: to release them from the pull of the other one,
• 26:51
  Speaker 1: and that cost energy. So if breaking it up cost energy,
• 26:54
  Speaker 1: then the reverse process forming it must release energy. All right, Well,
• 26:59
  Speaker 1: let's maybe move on and talk about what happens after that,
• 27:03
  Speaker 1: which is that you get heavier and heavier elements. But
• 27:05
  Speaker 1: this only works until you get to are So what
• 27:08
  Speaker 1: happens when you try to make iron? First of all,
• 27:10
  Speaker 1: how many steps are there between hydrogen and helium and
• 27:13
  Speaker 1: making iron? So you can keep going for a while.
• 27:16
  Speaker 1: You can combine helium together to make carbon, you can
• 27:19
  Speaker 1: combine carbon together with more helium to make oxygen. You
• 27:22
  Speaker 1: can keep going and make silicon and heavier and heavier stuff.
• 27:25
  Speaker 1: There's multiple steps there. It's not like one single pathway.
• 27:29
  Speaker 1: Now you can have lots of different combinations of things
• 27:31
  Speaker 1: that you can put together. And that happens inside the sun,
• 27:34
  Speaker 1: like you know, everything's mixing together with everything and making
• 27:37
  Speaker 1: different heavier elements. It happens inside some stars. In order
• 27:40
  Speaker 1: for those steps to happen, has to be hotter and hotter,
• 27:42
  Speaker 1: because now these nuclei have larger positive charges, so they're
• 27:46
  Speaker 1: pushing against each other even more so. In order to
• 27:49
  Speaker 1: get carbon diffuse together with other carbon or with something else,
• 27:52
  Speaker 1: it requires even more temperature and density. So our star
• 27:56
  Speaker 1: is not hot enough to fuse anything basically but hydrogen
• 27:59
  Speaker 1: into helium. But other stars out there in the universe are,
• 28:02
  Speaker 1: and they can keep fusing stuff all the way up
• 28:04
  Speaker 1: to iron. Our son cannot make heavier elements than helium.
• 28:09
  Speaker 1: Is that what you're saying, Like, our son is limited
• 28:11
  Speaker 1: to helium. Our son is limited to helium until the
• 28:14
  Speaker 1: very end of its life. For a few moments, near
• 28:17
  Speaker 1: the end of its life, maybe minutes or seconds, there
• 28:19
  Speaker 1: will be a little bit of helium burning. It's actually
• 28:22
  Speaker 1: called a helium flash because it all happens so quickly
• 28:25
  Speaker 1: near the end of its life, and it expands an
• 28:27
  Speaker 1: enormous amount of energy during these last moments. But for
• 28:31
  Speaker 1: most of the lifetime of the Sun, for the next
• 28:32
  Speaker 1: few billion years, it will not fuse any helium. It's
• 28:35
  Speaker 1: just not massive enough to create the temperature and pressure
• 28:38
  Speaker 1: at its core necessary to do that. Interesting. I feel
• 28:41
  Speaker 1: like that's something that testifysist don't talk about often, you know,
• 28:43
  Speaker 1: when they say we're made out of star stuff. Really,
• 28:45
  Speaker 1: they mean we were made out of some stars stuff.
• 28:48
  Speaker 1: Not all stars make stuff like us, Right, you need
• 28:51
  Speaker 1: special stars. Yeah, only the bigger, more massive stars are
• 28:54
  Speaker 1: capable of fusing heavier and heavier elements. Okay, but even
• 28:58
  Speaker 1: those big stars can combine carbon and hydrogen and make
• 29:01
  Speaker 1: these heavier elements. But even the biggest and hottest stars
• 29:04
  Speaker 1: have to stop at ore. And so the question is
• 29:07
  Speaker 1: why is that what's so special about? All Right? So,
• 29:09
  Speaker 1: as you move up the periodic table, you're getting more
• 29:11
  Speaker 1: protons and more neutrons in there. Things are getting tighter
• 29:15
  Speaker 1: and tighter. The binding energy actually increases because now you
• 29:18
  Speaker 1: have more of these things feeling the strong force and
• 29:21
  Speaker 1: pulling on each other. So as you go up the
• 29:23
  Speaker 1: periodic table, you're releasing more energy because the binding is
• 29:26
  Speaker 1: getting stronger. Remember, binding getting stronger means you need more
• 29:30
  Speaker 1: energy to break it up. So it takes a more
• 29:32
  Speaker 1: powerful laser to break up carbon than it does to
• 29:35
  Speaker 1: break up helium. Takes an even more powerful laser to
• 29:38
  Speaker 1: break up heavier elements than carbon, because as you keep
• 29:41
  Speaker 1: adding nucleons, they like to stick together even more. They're
• 29:44
  Speaker 1: like all working together to make this stuff even stickier,
• 29:47
  Speaker 1: And that's really the key that the binding energy per
• 29:51
  Speaker 1: nucleon is going up. As you do fusion all the
• 29:55
  Speaker 1: way up to iron, the potential well is getting deeper.
• 29:58
  Speaker 1: The atoms are getting stick year. You stick two atoms together,
• 30:02
  Speaker 1: but the combined atom has more than twice the original
• 30:06
  Speaker 1: binding energy, so it's more tightly bound per nucleon, which
• 30:10
  Speaker 1: is why it releases more energy to make that combined atom.
• 30:15
  Speaker 1: But wouldn't then release more energy? Is that kind of
• 30:17
  Speaker 1: like a runaway reaction in a way, Like you know,
• 30:20
  Speaker 1: Sun starts to make heavier elements and when you fuse
• 30:22
  Speaker 1: those together they release even more energy. It sounds like
• 30:25
  Speaker 1: what you're saying, Merging two carbons together releases a whole
• 30:28
  Speaker 1: bunch of energy, Yes, exactly. And so as you keep
• 30:30
  Speaker 1: going up to the periodic table, things get tighter and
• 30:33
  Speaker 1: tighter and you keep releasing energy. You can keep doing
• 30:35
  Speaker 1: fusion and it keeps releasing energy until you get to iron.
• 30:38
  Speaker 1: Iron is the tipping point when the electrostatic force takes
• 30:41
  Speaker 1: over again. And what happens is that the nucleus is
• 30:44
  Speaker 1: now so big that the strong force between protons on
• 30:47
  Speaker 1: like different sides of the nucleus can't really do its
• 30:50
  Speaker 1: thing anymore because the protons are so far apart. But
• 30:53
  Speaker 1: the electrostatic force, which is a longer range, much much
• 30:56
  Speaker 1: longer than the size of the nucleus, can so the
• 30:59
  Speaker 1: atom becomes a little bit less tightly bound. Instead of
• 31:02
  Speaker 1: adding another proton, which sticks everything together more, you're adding
• 31:05
  Speaker 1: another proton, which sticks everything together a little bit less.
• 31:08
  Speaker 1: So you're reducing the binding energy of the nucleus. You're
• 31:11
  Speaker 1: making it easier to break it up than it was before. Alternately,
• 31:15
  Speaker 1: you're saying it gets harder to fuse things to a
• 31:18
  Speaker 1: really big atom like art, Like, there are so many
• 31:20
  Speaker 1: protons inside of the nucleus of an iron atom that
• 31:24
  Speaker 1: is just super duper positive. There's a lot of positive
• 31:26
  Speaker 1: charge there in one spot, and so like, adding one
• 31:29
  Speaker 1: more proton just gets harder and harder a because there's
• 31:31
  Speaker 1: so much positivity they're repelling you. But also like even
• 31:35
  Speaker 1: the strongforce that's holding all those protons together gets kind
• 31:39
  Speaker 1: of more diluted. That's exactly right. But the thing that
• 31:41
  Speaker 1: controls whether or not this happens very often in stars
• 31:44
  Speaker 1: is really the energy flow, because what it means is
• 31:46
  Speaker 1: that to fuse iron together, for example, costs energy rather
• 31:50
  Speaker 1: than releasing energy, because fusing iron together I means sticking
• 31:54
  Speaker 1: it together into a bigger nucleus which is not as
• 31:57
  Speaker 1: tightly bound. Right, And so remember if something it's really
• 32:00
  Speaker 1: tightly bound, it costs more energy to break it up,
• 32:03
  Speaker 1: which means it releases energy to make it. If something
• 32:06
  Speaker 1: is less tightly bound, then it doesn't take as much
• 32:09
  Speaker 1: energy to break it up, so it costs energy to
• 32:12
  Speaker 1: make it. And so what happens after iron is now
• 32:14
  Speaker 1: you're making things that are less and less tightly bound,
• 32:17
  Speaker 1: and so it actually absorbs energy. It costs energy to
• 32:20
  Speaker 1: do it. You want to fuse iron together, you can
• 32:23
  Speaker 1: do it, but you take energy away from the star.
• 32:26
  Speaker 1: So in effect, you're like putting out the fire of
• 32:28
  Speaker 1: the star instead of fueling it for the next reaction,
• 32:31
  Speaker 1: you're cooling it down. Yeah, I feel like you're making
• 32:33
  Speaker 1: folks here do some superhero style mental gymnastics here. With
• 32:37
  Speaker 1: so many inversions, I think maybe it's simpiliary to put it.
• 32:41
  Speaker 1: Is that when you're fusing something simple like hydrogen, it
• 32:43
  Speaker 1: takes a little bit of energy to get the hydrogen's
• 32:45
  Speaker 1: atoms together, but once they fused, they release much more
• 32:48
  Speaker 1: energy than the energy took to get them together. But
• 32:51
  Speaker 1: as you get into these heavier and heavier and bigger atoms,
• 32:53
  Speaker 1: the energy it takes to like fuse something to them
• 32:56
  Speaker 1: is more than the energy that gets released when it
• 33:00
  Speaker 1: actually happens. Yeah, fusing iron together is an energy loser, right,
• 33:04
  Speaker 1: it costs energy to do that. The reason and we
• 33:07
  Speaker 1: say that a loot on the podcast, that you can't
• 33:09
  Speaker 1: make things heavy than iron because it cool stars. The
• 33:12
  Speaker 1: reason for the fundamental reason for why fusing iron together
• 33:16
  Speaker 1: cools star instead of heating a star the way fusing
• 33:19
  Speaker 1: hydrogen together does calm down to this nuclear binding energy.
• 33:23
  Speaker 1: How the nucleus is put together. When you put heavier
• 33:26
  Speaker 1: and heavier nuclei together, they are not as tightly bound.
• 33:29
  Speaker 1: They're easier to break up. So if you transition from
• 33:32
  Speaker 1: a nucleus which is more tightly bound to less tightly bound,
• 33:36
  Speaker 1: then it costs energy to do that. You have to
• 33:38
  Speaker 1: absorb energy to go from more tightly bound to less
• 33:41
  Speaker 1: tightly bound. You're like moving up energy levels, So it
• 33:44
  Speaker 1: costs energy. If you don't like thinking about the binding energy,
• 33:47
  Speaker 1: here's another way to think about it. Fission releases energy
• 33:51
  Speaker 1: because a heavy nucleus like uranium two thirty five is
• 33:54
  Speaker 1: like a cocked mouse trap. It took energy to squeeze
• 33:58
  Speaker 1: all those protons and neutral is hard enough together to
• 34:01
  Speaker 1: make them barely stick together using the nuclear force that
• 34:05
  Speaker 1: fights against the natural tendency of all those protons to
• 34:08
  Speaker 1: fly violently apart due to their electrostatic repulsion. So when
• 34:12
  Speaker 1: that heavy nucleus, the uranium two thirty five, is struck
• 34:16
  Speaker 1: by an incoming neutron, for example, it's like a mouse
• 34:19
  Speaker 1: touching the trigger pedal of the trap. Bang goes the
• 34:22
  Speaker 1: nucleus as it breaks apart. In the case of fusion,
• 34:25
  Speaker 1: the mechanism really is different. The nuclear force between the
• 34:29
  Speaker 1: nucleons is very powerfully attractive, but only kicks in when
• 34:32
  Speaker 1: the particles are so close to each other that they
• 34:35
  Speaker 1: are almost touching. That attraction is not quite enough to
• 34:38
  Speaker 1: stick the two protons together against their electrostatic repulsion. But
• 34:42
  Speaker 1: if you add two neutrons to the recipe, you get
• 34:45
  Speaker 1: enough mutually attractive nuclear force stickiness to overcome the electrostatics,
• 34:50
  Speaker 1: and the particles fall into each other's potential well like
• 34:54
  Speaker 1: a ball getting trapped in a cup, giving off energy
• 34:57
  Speaker 1: as they fall in, and that gives a powerful bang. Well,
• 35:00
  Speaker 1: maybe a question I might have is in the sun,
• 35:02
  Speaker 1: there's a lot going on, right Even in a big,
• 35:05
  Speaker 1: powerful sun that can make things like iron, there's still
• 35:08
  Speaker 1: hydrogen being fused and the lower elements being fused that
• 35:10
  Speaker 1: create a whole bunch of energy. You know, why can't
• 35:13
  Speaker 1: the Sun sustain an iron fusing reaction with the energy
• 35:18
  Speaker 1: from these other reactions? You know, like maybe making iron
• 35:21
  Speaker 1: cost energy, but it's also you know, getting a lot
• 35:24
  Speaker 1: of surplus energy from some of these other reactions it has,
• 35:27
  Speaker 1: And so why can't the Sun just keep making heavy
• 35:29
  Speaker 1: and heavier elements with its surplus energy. Yeah, that's a
• 35:32
  Speaker 1: good question. And remember that our sun can't even get
• 35:34
  Speaker 1: close to that situation because it can't even make iron,
• 35:37
  Speaker 1: so it's not really in the situation of trying to
• 35:39
  Speaker 1: fuse iron together with heavier stuff. But imagine some more
• 35:42
  Speaker 1: massive star that's hotter at its core and is capable
• 35:45
  Speaker 1: of fusing all the way up to iron, and you
• 35:47
  Speaker 1: can ask, like, why can't you make heavier stuff and
• 35:49
  Speaker 1: use up a little bit of that energy that you're
• 35:51
  Speaker 1: producing with all the other fusion processes you can? And
• 35:54
  Speaker 1: that does happen a little bit, but it tends to
• 35:56
  Speaker 1: kill the star, right, it tends to cool the star down,
• 35:59
  Speaker 1: and that's the end of the star. That's the death
• 36:01
  Speaker 1: of a star. Like if you put water onto a fire,
• 36:04
  Speaker 1: what happens, Well, you do heat the water up, certainly,
• 36:06
  Speaker 1: but it also cools the fire down, and so then
• 36:08
  Speaker 1: the fire goes out. So in the same way, once
• 36:10
  Speaker 1: you get to the point where you were cooling the star,
• 36:12
  Speaker 1: then the star is dying. So you do make a
• 36:14
  Speaker 1: little bit of stuff heavier than iron. It's not like
• 36:17
  Speaker 1: there's a total wall there, but you just can't make
• 36:19
  Speaker 1: very much of it. I think I still have questions
• 36:22
  Speaker 1: about that, And so let's talk about that and what
• 36:24
  Speaker 1: happens after you make iron, and why you can't even
• 36:27
  Speaker 1: use surplus energy to make it as far as let's
• 36:29
  Speaker 1: take another quick break. All right, we're having a stellar
• 36:42
  Speaker 1: conversation that's not ironic at all about how stars make iron.
• 36:46
  Speaker 1: And it seems like stars can fuse all of the
• 36:49
  Speaker 1: lighter elements, starting from hydrogen right up until carbon oxygen,
• 36:54
  Speaker 1: and but then it gets to iron and it can't
• 36:56
  Speaker 1: do it anymore sustainably. I think maybe that's the footnote
• 37:00
  Speaker 1: you would have to add here, is that stars can
• 37:02
  Speaker 1: make heavier elements and iron, it just can't make them
• 37:04
  Speaker 1: sustainably because it costs energy to make anything heavier than iron. Yeah,
• 37:10
  Speaker 1: the same way, like a fire can't make steam sustainably
• 37:13
  Speaker 1: if you just pour water onto the fire. What if
• 37:15
  Speaker 1: I just had a bigger fire, Like, can you imagine
• 37:19
  Speaker 1: a sun or a star that somehow can make heavier
• 37:23
  Speaker 1: elements and iron sustainably, Like it has so much hyngen
• 37:26
  Speaker 1: in it, perhaps that it can keep making heavier elements
• 37:29
  Speaker 1: for a while. I mean, I think that's basically what
• 37:31
  Speaker 1: happens inside stars. But it spells the end of a star. Right.
• 37:34
  Speaker 1: You have a huge corporation, for example, and you have
• 37:36
  Speaker 1: money losing divisions that are just growing bigger and bigger
• 37:39
  Speaker 1: and bigger and sapping the profits from the money making divisions,
• 37:42
  Speaker 1: then your business is not going to last very long
• 37:44
  Speaker 1: before you go bankrupt. So these stars basically start to
• 37:47
  Speaker 1: go bankrupt as soon as they turn over past iron
• 37:50
  Speaker 1: and nickel because they start to use up their own
• 37:52
  Speaker 1: heat instead of producing more heat. So maybe the answer
• 37:55
  Speaker 1: here is that stars can fuse iron. Right, I feel
• 37:58
  Speaker 1: like maybe we like or Innerstanuel. Because stars can make iron,
• 38:03
  Speaker 1: it can probably go all the way up to heavier
• 38:05
  Speaker 1: and every amassie. It just can't do it sustainably, right,
• 38:09
  Speaker 1: But it can, and they do make heavier elements in iron.
• 38:12
  Speaker 1: But you're saying, it's kind of marks the point in
• 38:15
  Speaker 1: the Sun's balance sheet where it starts to lose energy.
• 38:18
  Speaker 1: But then, how much longer after that does the star
• 38:20
  Speaker 1: have before it dies or collapses? Not very long. One
• 38:24
  Speaker 1: of the really cool things about stellar evolution is how
• 38:26
  Speaker 1: the first stages can take a very very long time
• 38:29
  Speaker 1: hydrogen burning, and the next stages become much faster. So
• 38:32
  Speaker 1: you can burn hydrogen for millions or billions of years
• 38:35
  Speaker 1: and then burn helium for like days or minutes, and
• 38:39
  Speaker 1: every step after that gets faster and faster. The wise
• 38:42
  Speaker 1: that because what happens is that the temperature is increasing,
• 38:44
  Speaker 1: and as the temperature increases, fusion happens faster, which then
• 38:48
  Speaker 1: increases the temperature, which makes fusion happen faster. So it's
• 38:51
  Speaker 1: a runaway effect. But then when you get to iron,
• 38:53
  Speaker 1: when that help cool it down and stabilize it. When
• 38:56
  Speaker 1: you get to iron, that does help cool it down.
• 38:58
  Speaker 1: But now you have a heart of a star which
• 39:00
  Speaker 1: doesn't have what it needs in order to fuse. Right,
• 39:03
  Speaker 1: this cold blob of iron at the heart of your star.
• 39:06
  Speaker 1: You know, these shells of lighter materials going all the
• 39:09
  Speaker 1: way out, like the hydrogen has been pushed all the
• 39:12
  Speaker 1: way out to the outside of the star, and there
• 39:14
  Speaker 1: was only hydrogen burning happening on the edges. And they
• 39:16
  Speaker 1: have a layer of helium which is burning, and then
• 39:18
  Speaker 1: you have a layer of carbon which is burning, and
• 39:20
  Speaker 1: layers of oxygen and neon, et cetera all the way
• 39:23
  Speaker 1: down to iron at the heart. So now the core
• 39:25
  Speaker 1: of the star starts to cool down, and that's what
• 39:27
  Speaker 1: triggers this collapse. By the way, iron Heart is the
• 39:31
  Speaker 1: name of a superhero as well. Tony starts protege who
• 39:34
  Speaker 1: built her own iron suit. So then then that's where
• 39:36
  Speaker 1: stars basically collapse and becomes super nervous. Right for some stars,
• 39:41
  Speaker 1: I should say, the reason that stars aren't collapsing in
• 39:43
  Speaker 1: the first place is this heat produced from fusion. Fusion
• 39:47
  Speaker 1: is what's pushing back against gravity to keep a star
• 39:50
  Speaker 1: in balance. That's why it keeps going for billions of
• 39:53
  Speaker 1: years the way that it can. And so once the
• 39:55
  Speaker 1: star starts to cool and fusion starts to slow down,
• 39:57
  Speaker 1: then that spells the end of it, and it starts
• 39:59
  Speaker 1: to collapse, and then you can get a supernova. In
• 40:01
  Speaker 1: some cases, you can just get a gravitational collapse, which
• 40:04
  Speaker 1: leads to like a white dwarf or a neutron star
• 40:07
  Speaker 1: or a black hole. All sorts of fun outcomes, right,
• 40:10
  Speaker 1: because what happens is that the star makes heavier and
• 40:12
  Speaker 1: heavier elements. It gets up to iron iron causes it
• 40:15
  Speaker 1: to cool down. Then you've got all this cold iron
• 40:17
  Speaker 1: in the middle of the star. And then basically that's
• 40:20
  Speaker 1: when gravity kind of wins, right, takes all that iron
• 40:22
  Speaker 1: and squishes it down to like super duper dense materials
• 40:26
  Speaker 1: and which can either stay there or cost the whole
• 40:29
  Speaker 1: star to collapse and explode. Yeah, I think gravity only
• 40:32
  Speaker 1: really wins if you get to a black hole. Even
• 40:34
  Speaker 1: if you get to something really dense like a neutron star,
• 40:37
  Speaker 1: gravity is still being resisted. There's still some force there
• 40:40
  Speaker 1: that's pushing back to prevent the collapse into a black hole.
• 40:44
  Speaker 1: And so like a white dwarf, for example, is just
• 40:46
  Speaker 1: like a big hot lump of that metal that was
• 40:48
  Speaker 1: made inside the star and it's resisting gravity trying to
• 40:52
  Speaker 1: compress it into a black hole. But Yeah, there are
• 40:54
  Speaker 1: various stages of retreats sort of against gravity. As gravity
• 40:58
  Speaker 1: gets stronger and stronger, collapses a huge burning star into
• 41:01
  Speaker 1: a white dwarf, into a neutron star, or maybe even
• 41:04
  Speaker 1: into a black hole. Yeah, but some stars collapse, and
• 41:08
  Speaker 1: that collapsation causes kind of like a rebound, right, because
• 41:12
  Speaker 1: there's all the star basic collapses in on itself with
• 41:15
  Speaker 1: like rebounds, and that's one of the kinds of supernova
• 41:18
  Speaker 1: that exists out there, right, That's what causes some supernova. Yeah.
• 41:22
  Speaker 1: If you have a massive enough star to start from
• 41:24
  Speaker 1: something like eight times the mass of our Sun, then
• 41:27
  Speaker 1: this last stages, it puffs out to be a red
• 41:29
  Speaker 1: supergiant and then it collapses until we call it type
• 41:32
  Speaker 1: two supernova. It's a gravitational collapse, and then you can
• 41:35
  Speaker 1: get like a black hole or a neutron star at
• 41:37
  Speaker 1: its heart. If you have a lower mass star, like
• 41:40
  Speaker 1: less than eight times the mass of the Sun, then
• 41:42
  Speaker 1: you're more likely to get like a white dwarf as
• 41:44
  Speaker 1: an outcome. And if that white dwarf then later gets
• 41:47
  Speaker 1: some more mass added to it, then it can collapse
• 41:50
  Speaker 1: into a type one a supernova. Right, But then I think,
• 41:53
  Speaker 1: what's interesting is that in these events, like when a
• 41:56
  Speaker 1: star goes supernova, that's then when the heavier elements get made, right,
• 42:01
  Speaker 1: That's when there's so much energy being released that the
• 42:03
  Speaker 1: shock wave compresses things and there's enough energy there to
• 42:07
  Speaker 1: actually fuse these heavier elements. Yes, so, as we said earlier,
• 42:10
  Speaker 1: there's a little bit of fusion of these heavier elements,
• 42:12
  Speaker 1: things heavier than iron inside the star that tends to
• 42:15
  Speaker 1: cool the star. But most of the heavy elements in
• 42:18
  Speaker 1: the universe, the gold and the platinum and the uranium,
• 42:21
  Speaker 1: are not made inside those stars. They're made either at
• 42:24
  Speaker 1: the end of the star, like during the supernova, when
• 42:26
  Speaker 1: so much energy is released that you can use some
• 42:29
  Speaker 1: of it up to fuse these even heavier things, and
• 42:32
  Speaker 1: also much later on when neutron stars, which are the
• 42:36
  Speaker 1: remnants of some of these supernovas, when those collide, and
• 42:39
  Speaker 1: then you can get enormous creation, like entire earth sized
• 42:43
  Speaker 1: chunks of gold or platinum can be made in those
• 42:46
  Speaker 1: neutron star collisions. Right. So, in a way, stars doop
• 42:49
  Speaker 1: use iron and make heavier elements, right, I mean, they
• 42:52
  Speaker 1: don't just make it at the end of their lives.
• 42:54
  Speaker 1: But also in these new ways they do make the
• 42:57
  Speaker 1: heavier elements, that's true. I guess they can get some
• 43:00
  Speaker 1: for that as well. I mean they get all the credit, right, Like,
• 43:03
  Speaker 1: are these heavier elements made any other way. It's almost
• 43:05
  Speaker 1: entirely supernova and neutron star mergers. And I guess you
• 43:09
  Speaker 1: could say the neutron star comes from the original star,
• 43:12
  Speaker 1: and so when you merge it together to make gold
• 43:14
  Speaker 1: or platinum or plutonium or whatever, that gets credited on
• 43:17
  Speaker 1: the account of the original star. It's a different sort
• 43:20
  Speaker 1: of process though, right. It's not fusion happening at the
• 43:22
  Speaker 1: heart of the star the way you make carbon and silicon,
• 43:25
  Speaker 1: though it is a product of the star which is
• 43:27
  Speaker 1: later fusing together to make these heavy elements. Not the
• 43:31
  Speaker 1: same process, but still fusing the elements together to make
• 43:34
  Speaker 1: heavier elements. Yeah, it certainly is fusion, and it costs
• 43:37
  Speaker 1: energy in that case, right, instead of creating energy. And
• 43:40
  Speaker 1: that's kind of why these heavier elements are also so rarer, right,
• 43:43
  Speaker 1: because they're only made at the end of the life
• 43:46
  Speaker 1: of a star and only briefly. And so that's why,
• 43:49
  Speaker 1: for example, here on Earth, there's a ton of iron,
• 43:51
  Speaker 1: but not a lot of gold or titanium. Yeah, that's
• 43:54
  Speaker 1: why the core of the Earth is mostly iron and
• 43:57
  Speaker 1: nickel and these kinds of things, and not gold and
• 44:00
  Speaker 1: platinum and not uranium. That's why these really heavy elements
• 44:03
  Speaker 1: are traced in the universe, compared to iron and nickel,
• 44:06
  Speaker 1: which are like the endpoints of stellar nucleosynthesis. Stars are
• 44:10
  Speaker 1: like factories for turning hydrogen into heavier stuff, but they
• 44:14
  Speaker 1: can only do that sustainably up to about iron, which
• 44:17
  Speaker 1: really controls what life on Earth is made out of,
• 44:20
  Speaker 1: what Earth itself is made out of. Yeah, including us
• 44:23
  Speaker 1: and Danny. Are you saying that gold and titanium are
• 44:26
  Speaker 1: basically made out of iron as well, because you have
• 44:28
  Speaker 1: to merge iron to make gold and titanium. Yeah, I
• 44:30
  Speaker 1: suppose so. In that same sense, everything is made out
• 44:32
  Speaker 1: of hydrogen, right, You and I were both just hydrogen.
• 44:35
  Speaker 1: I'm just saying iron Man is named appropriately because even
• 44:38
  Speaker 1: if it's made out of golden titanium, it does have ironum.
• 44:41
  Speaker 1: Although technically you're saying you should be maybe called hydrogen man,
• 44:44
  Speaker 1: should hyrogen man exactly. Something that's really interesting for me
• 44:47
  Speaker 1: to think about, sort of philosophically, is to think about
• 44:50
  Speaker 1: the difference between for example, oxygen and helium right in
• 44:54
  Speaker 1: the end, they're really the same components. Like a single
• 44:57
  Speaker 1: atom of oxygen has twenty four up corks, twenty four
• 45:00
  Speaker 1: down corks, and eight electrons. If you take four atoms
• 45:04
  Speaker 1: of helium, it has the same thing. It's twenty four ups,
• 45:06
  Speaker 1: twenty four downs, and eight electrons. It's just arranged differently.
• 45:10
  Speaker 1: And so all we're doing in the hearts of stars
• 45:13
  Speaker 1: and in supernova and in neutron star collisions is just
• 45:16
  Speaker 1: moving the pieces around to make different arrangements. And that's
• 45:19
  Speaker 1: what makes oxygen different from helium, and different from iron,
• 45:23
  Speaker 1: and different from titanium. And so it's fascinating to me
• 45:26
  Speaker 1: to see this happened. That the crucial thing is the
• 45:28
  Speaker 1: arrangement of those bits and the energy needed to put
• 45:31
  Speaker 1: the bits together into these special arrangements to make up
• 45:34
  Speaker 1: me and you and Iron Man. Yeah, it's fascinating. So
• 45:37
  Speaker 1: you're saying, instead of saying we're all made out of
• 45:39
  Speaker 1: star stuff, we should be saying we're all made out
• 45:41
  Speaker 1: of quirks, and all superheroes should just be called Corkman exactly,
• 45:46
  Speaker 1: or also Corkman, Captain America, Captain cork Hey. I'm a reductionist, right,
• 45:50
  Speaker 1: I'd like to reduce everything to the simplest possible terms.
• 45:53
  Speaker 1: There you go, just call everything cork. All right. Well,
• 45:56
  Speaker 1: it's interesting to think about the processes that lead to
• 46:00
  Speaker 1: all of the things that we see around us and
• 46:02
  Speaker 1: the inside of our phones, inside of our bodies, and
• 46:05
  Speaker 1: kind of reflect on why we're here. We're here because
• 46:08
  Speaker 1: of these processes that happen inside of previous generations of stars,
• 46:12
  Speaker 1: and how the physics of the universe limits that and
• 46:15
  Speaker 1: kind of determines the things that we are made out of,
• 46:18
  Speaker 1: and how those bits are arranged together. And even though
• 46:21
  Speaker 1: the universe has been working for billions of years to
• 46:24
  Speaker 1: convert hydrogen into more interesting stuff, it's still got a
• 46:27
  Speaker 1: long way ago. Universe is about nine two percent hydrogen
• 46:31
  Speaker 1: still after billions of years, and most of the rest
• 46:34
  Speaker 1: of that is just helium. The bits that make up
• 46:37
  Speaker 1: me and you and everything else, The interesting bits, the
• 46:39
  Speaker 1: heavy bits, are a tiny fraction of one percent of
• 46:42
  Speaker 1: the universe. Yeah, so even when the Avengers assembled, they're
• 46:45
  Speaker 1: still made out of court. Alright, Well, we hope you
• 46:49
  Speaker 1: enjoyed that. Thanks for joining us, See you next time.
• 46:59
  Speaker 1: Thanks you're listening, and remember that Daniel and Jorge Explain
• 47:02
  Speaker 1: the Universe is a production of I heart Radio. For
• 47:05
  Speaker 1: more podcast from my heart Radio, visit the i heart
• 47:08
  Speaker 1: Radio app, Apple Podcasts, or wherever you listen to your
• 47:12
  Speaker 1: favorite shows. H