The Sun. It’s a big ball of fire, right? Apparently not. In fact, what’s going on inside of the Sun took us some time and knowledge of physics to finally figure out: stellar fusion. Let’s talk about the different kinds of fusion, and how we’re trying to adapt it to generate power here on Earth.
- Brown Dwarfs:
- Mass > 13 M_Jupiter, Deuterium burning
- Mass > 75-80 M_Jupiter, Lithium burning, 7L + p → 4He
- Stellar Fusion/Stellar Nucleosynthesis
- Hydrogen isotopes Deuterium and Tritium
- Deuterium Tritium Fusion
- Proton-Proton Cycle
- Silicon-burning process
Fraser: Astronomy Cast episode 513, Stellar Fusion. Welcome to Astronomy Cast, your weekly facts based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, Dr. Pamela Gay, a senior scientist for the Planetary Science Institute, and the director of CosmoQuest. Hey Pamela, how are you doing?
Pamela: I’m doing well, how are you doing Fraser?
Fraser: Good. I want to take a moment to promote something that we are going to be at.
Pamela: Yes, in Joshua Tree, or at least what’s left of it.
Fraser: What’s left, apparently people are cutting down trees in Joshua Tree –
Pamela: Yeah, it’s horrible. It’s horrible.
Fraser: – so that they can – oh. This is just terrible. But yes. So Pamela, me, Paul Sutter, Skylias, John Michael Godier are going to be in Joshua Tree from June 26-30th for the All-Stars Party. And this is where we’re gonna be staying at this really fantastic resort four nights. We are gonna be setting up a whole bunch of really fancy telescopes, and it is going to be your personal star party with just us teaching the sky, taking requests, finding objects, doing astrophotography. If you just wanna be at the heart of super space nerdery, this is it. So if you wanna find out more information, go to Astrotours.co/allstars and you can find out more about what it is, what it costs, and what we’re planning to do.
It’s gonna be a really fun experience if this is the kind of thing you’ve ever wanted to do. And it’s gonna be fairly small, so it’s gonna be a fairly intimate time with cool telescopes.
Pamela: I will have hammocks.
Fraser: Is that your plan? You’re gonna hammock it up?
Pamela: I’m gonna hammock it up, and that is the best way to do binocular sky, which is my favorite sky, and also the easiest to pack.
Fraser: I think I am just going to be running around, moving telescopes in new directions, and showing people what’s up.
Pamela: That works.
Fraser: It’s gonna be good. All right. And now for the actual episode part, which I have lost. Hold on. Hold on, wait for it.
Pamela: Oh, no!
Fraser: There it is. Okay. The sun. It’s a big ball of fire, right? Apparently not. In fact, what’s going on inside of the sun took us some time and knowledge of physics to finally figure out. Stellar fusion. Let’s talk about the different kinds of fusion, and how we’re trying to adapt it to generate power here on Earth. All right, Pamela. So, the sun is not a ball of fire?
Pamela: It’s more like a ball of plasma. It’s a thing.
Fraser: Plasma isn’t fire.
Pamela: And I just wanna say, this episode is because I had a moment of oh. I suddenly realized, I totally don’t understand something I thought I understood last week. During the Q and A that followed our live recording, we got to talking about brown dwarfs, what makes brown dwarfs. Maybe stars, maybe planets, definitely confusing. And I brought up tritium burning, how they burn tritium. And someone’s like yeah, but tritium only has a half-life of 10 point something years, and I became super confused. And I’m like okay; I clearly need to do some research on this. And so this episode comes out of you guys totally stumping me for which I am grateful, because now I know new things.
Fraser: That’s awesome. That is the trick. I find that since I’ve been doing YouTube channel, and I’ve been answering questions, and I just get this – I get a lot of the same questions again and again and again. And every time that I don’t know the answer to a question, I have to go and research the question, and now I get the answer. And if I make a mistake, then people slap my wrists on the YouTube channel. And so I just get better and better and better. I just get turned into this – I feel like I’m being honed in the fires of Mt. Doom to answer space questions.
And this is this exact process, where you’re just like I wanna know more about this thing that I kind-of know a little bit about, but not enough to be able to explain it. So let’s get into it. So where do you wanna start? I think the first thing is just maybe some of the history. How did we figure out what’s going on inside the sun?
Pamela: Well, it came down to realizing what it wasn’t. In the 1800s, we had a whole lot of different scientists who are like well, if the sun is made out of this highly combustible thing that burns very effectively and gives off energy, then it could only burn for this many thousands of years. Well, if it’s made with this other thing, it can only burn for this many thousands of years. And no matter what they came up with, there seemed to be no legitimate way to have a sun that was giving off the amount of energy we have for as long as seemed sensible.
And, making it more confusing, this was about the same time that we started to understand that the fossil record was indicative of more than a few thousand years of Earth history. That dinosaurs might not have coexisted with humans, and we weren’t really up on plate tectonics yet, but geologists were starting to get more and more indications that a 6000 or an 8000 year old Earth was not what we were dealing with.
Fraser: Right, you can’t add up all the begats and get a proper age of the universe.
Pamela: It does not work. Now, this is the same time that we started to have quantum mechanics. And so we had folks like Beta that were looking at well, what happens when you combine different atoms in different ways? And the answer was well, in the right conditions, you have energy that is released. And then it becomes a question of what are the right conditions? And so they started out figuring out okay, so if we put these two things together, what happens?
And for a long time, they kept running into these issues of – well expletive, this reaction doesn’t seem like it’s probable to happen because, well, it took a while to figure out quantum tunneling, it took a while to figure out all the little nuances of quantum mechanics that allows the improbable to actually happen in stars.
Fraser: And so, for example, if you take two atoms of hydrogen, put them together, or four atoms of hydrogen, they don’t just turn into helium. You need to put an enormous amount of energy into this process to make them – pressure, to make them do this thing. If you can, then you do get energy out of it, but – but it didn’t seem possible.
Pamela: Yeah, and it’s even harder to try and figure out how the heck do you get carbon and nitrogen and oxygen going in a cycle? Because those helium atoms really don’t seem to want to form carbon. And so it was figuring out all the tricks of the trickster-y quantum mechanics that were necessary to make stars actually capable of having sustained, and the keyword here is sustained, nuclear fusion in their cores.
Fraser: So when did they really start to feel like they had a handle on that stars are doing fusion inside the core? They are mashing one kind of atom together to make another kind of atom, and releasing energy in the process.
Pamela: This started to come about in bits and pieces with Eddington and Beta, and all of them working in the early 1900s. It took a while to fully work out all the pieces, and in fact I think we’re still working out some of pieces. For instance, how the heck does a brown dwarf work? But it was really piece by piece figured out in the early 1900s, and one of the things that’s fascinating to me, personally, is here we had Eddington was one of the major figures working in England, figuring out what it was that powered stars, how all of this nuclear fusion took place. And then Chandrasekhar sailed across the sea to seek an education with him.
And in the process of that journey, Chandrasekhar figured out okay, and after it’s done burning, we can end up with white dwarfs that have a limit in their mass and above that limit, they become a neutron star. And Eddington was like that’s too crazy. No. We’re not – just like, how can one human being in one lifetime go from we have no idea how start work to oh, fusion! To no, that’s too crazy. We can’t have electron degenerate pressure.
Fraser: That’s funny. So now what is the modern idea of what’s going on inside a star?
Pamela: Well, it all starts out with a collapsing blob of gas and dust. Something that can be light years across will collapse down due to whatever triggered reason, supernova went off, shocked by hitting another cloud of material. And all of that diffuse material that may be thinner than the Earth’s own atmosphere at the beginning will collapse and collapse and collapse, until deep in the center of this proplyd, this cocoon, this collapsing solar nebula, you start to reach the point where at the very center, the pressure is high enough and the temperature is high enough that you have atoms that are trying really hard to move around really fast.
They’re colliding with large amount of energy. Because they’re packed together, a lot of collisions are happening. This is why you need both the high pressure and the high temperature. The temperature says things are moving rapidly, the pressure says you’re gonna get a whole lot of collisions taking place. And at a certain point, deuterium fusion turns on. And this isn’t, at it’s beginnings, the most effective way of generating energy. You end up with two atoms of hydrogen the each have a neutron attached to them. So deuterium, two particles, proton and neutron, bound up. That’s deuterium. And when they collide, they create tritium, and they give off a neutron, and they give off a wee amount of energy.
Fraser: So sorry, just – what is deuterium for people who don’t know?
Pamela: So deuterium is that heavy hydrogen that is one proton and one neutron. So hydrogen, we like to come up with lots of special words for. Every day run-of-the-mill hydrogen is just a proton and electron flying around having fun. Or not. When you add –
Fraser: Right. Going about their solemn business of creating the universe.
Pamela: Exactly. You add a neutron into that, and now you have deuterium. You add two neutrons into that, and now you have tritium. You have three particles. So hydrogen –
Fraser: And sorry, once again, sorry. And deuterium and tritium are very rare compared to the amount of just regular old hydrogen.
Pamela: Regular hydrogen. And at the end of the day, ionized hydrogen is a proton. So essentially we have three particles, a proton, an electron, and a neutron that we combine in various ways to get various words that all mean hydrogen in different states.
Fraser: Right. And so I’m just kind-of imagining now, the core of this star, where you’ve got this intense pressure. Most of the stuff that’s in there is just a bunch of protons bumping into each other. Occasionally, there’s gonna be these proton-neutron pairs, and occasionally there’s gonna be these proton-neutron-neutron pairs, and they’re all whacking into each other, there in the core of the star.
Pamela: And the tritium don’t occur naturally. They have a very short half-life, a little over 10 years. So you create a tritium, and it’s gonna go away. So this is where you have to start off with this deuterium burning. The nucleus, it’s a proton and a neutron, plus another nucleus that’s a proton and a neutron. You collide them together, it gives off a wee amount of energy, but in the process creates that tritium. And it’s that tritium that opens the door for energy generation that really matters. Once you have tritium, tritium burning, which is where you take one deuterium, so proton and a neutron, you take one tritium which is a proton and two neutrons, you combine them together. This gives you helium, so you finally have a new atom. This gives you helium, this gives you a neutron, and this gives you 17.6 megaelectron volts of energy. So this is a highly energetic reaction. Now you can do this reaction in slightly different ways.
So you start burning deuterium and helium at this point, because now you have helium. And you can start releasing even more energy. And this is all referred to the deuterium cycle, but it’s that deuterium-tritium combination that’s most important. It is, once the tritium’s there, it occurs 40 times more frequently than just a straight old deuterium-deuterium reaction. So you’re gonna have more frequent reactions, and they give off more energy. So that’s where you really start generating – well, generating that light that makes a star a star.
Fraser: And that increased energy increases the temperature that’s down in the core of the star, allowing the less likely protons to bounce into each other to give them a better environment. A better chance of being able to do their part.
Pamela: And there, you still also need the added mass. And this is where we start looking at what are the boundary conditions between a brown dwarf and a red dwarf? So, if you have an object that is just a dozen-ish times the mass of Jupiter, there is, theoretically, the possibility that – so what we were talking about last week was that boundary between the brown dwarfs and the red dwarfs. And that boundary is a function of what is able to burn. We have theoretical models that say at about the 12 Jupiter mass level, 13 Jupiter mass level, you’re going to start to be able to get some of that deuterium burning. Not a lot, because there’s not gonna be a lot of deuterium.
But while there’s deuterium, it’s gonna burn, it’s gonna burn some of it into tritium, you’re gonna have that deuterium-tritium burning taking place. It’s gonna give off energy. But at that level, you will never hit the energy, you’ll never hit that temperature/pressure combination necessary for the regular, every-day hydrogen, by which I mean protons, to start having fusion, and having a prolonged nuclear reaction.
Fraser: And so with the brown dwarf stars, they’ve still got the same elements in them that a regular star might have. They’re gonna have mostly hydrogen and helium left over from the Big Bang, they’re gonna have some flavor, some amount of deuterium. That deuterium is gonna be able to fuse and produce tritium, and it’s gonna have those kinds of reactions, but it’s not gonna be able to get hot enough and high enough pressure enough to be able to turn the regular hydrogen into energy.
Fraser: And so it eventually just runs out of fuel in its core whenever it runs out of deuterium.
Pamela: Yeah. And sometimes with some of the higher mass – we’re not exactly sure how much higher mass brown dwarfs, you can also get lithium burning. Lithium-7, when you hit it hard enough with a proton, which I mean an ionized hydrogen atom, you can end up with helium as an outcome. But lithium, again, is another one of those super scarce elements. So once the star has burned up all of its lithium, again, no more burning for you. So somewhere between 75 and 80 Jupiter masses, you can get more fusion, but you can’t get sustained fusion.
Fraser: Right. Right. Okay, so then – I guess we’ve sort-of – you’ve answered your question from last week, and how you define that difference. So let’s go back to the regular form of fusion as we understand it. You know, the ones with the protons. Once you’ve got that temperature, that pressure in the core, what is going on in the core with the atoms? What is the cycle that goes through for that?
Pamela: Well, this is what we call the proton-proton cycle. And I, for one, feel it is deeply wrong and annoying that they go from calling it tritium and deuterium to when they get to actual hydrogen, suddenly they call it protons. Anyways, with the proton-proton cycle, what we have is two hydrogen atoms collide, form a deuterium, then that deuterium and another hydrogen collide, they create helium-3 and they give off energy, and a neutrino, and people get upset because we don’t see the neutrinos in the correct numbers, but it turns out neutrinos have identity issues and they like to change a lot.
Fraser: Yeah, they just change flavors.
Pamela: Yeah. So that’s a different episode. But it’s solved. When I was a student, we hadn’t solved that yet.
Fraser: I think it had gotten solved during AstronomyCast.
Pamela: Yes. So we got to discuss that early on. So we have this process where regular every day hydrogens, by which I mean protons, come together, build up the deuterium. The deuterium and the hydrogen build up to helium and neutrinos, and give off energy. Ad this starts to get us then to the heliums combining to form helium-4, which is far too stable, but also more hydrogen.
Fraser: Sorry, is that helium-4 cycle happening in the sun right now?
Fraser: Okay. And so there’s a limit, right? There’s as much – as we mentioned, a brown dwarf only has enough temperature and pressure to allow deuterium to form. What is the limit for what can happen in our sun in its current state?
Pamela: So with our sun, what’s going to happen, it’s what we call a main sequence star, is it’s gonna work on building that hydrogen up into helium over time. And as it builds up and ends up with more and more of that helium-4, eventually the helium-4 is just gonna be like and we’re done now. And for a little while, light generation is gonna stop, the star’s gonna collapse a bit, and then it’s gonna change state as it evolves.
Fraser: And this is when it’s starting to die?
Pamela: Well, in this case – yeah. That’s the correct phrase. As it’s starting to die. So stars like our sun will spend the majority of their evolution with this proton-proton cycle going on in the core. Once they’re done with that, once they have that helium rich core, they still have some life left on them but really, they’re starting to angle their way across the color magnitude diagram towards that great right-hand corner where they become planetary nebula. And at that point, they have added shells of material. So they’ve begun, at one point or another, burning a shell of hydrogen. They have done some helium burning.
All of these different phases are working towards that point where depending on the mass, you can end up with either a helium white dwarf, a carbon white dwarf, and an atmosphere that drifts away to just form something really pretty for somebody else far across the galaxy to look at.
Fraser: And in each one of these steps, you have this situation where the waste is building up in the core of the star, in this case say it’s helium, the sun can’t turn that waste helium into – it can’t turn into anything else. And so it just gives up, and goes well, I’m out. And so then the star starts to cool down, collapse down because it no longer has that light pressure that’s pushing outward. And then things get more and more concentrated. Higher pressures, higher temperatures, it goes oh, wait a minute. I got this.
Pamela: Now I can do something else.
Fraser: Now I can do this. Now I can do this next thing. And then it – not reignites necessarily, but heats back up, expands, gets much bigger, and then uses up this waste, and then goes okay, that’s it, I promise. This time’s the last time. I’m done. Now I’m gonna shrink – oh wait a minute, I can do this! And it just expands again, and does it again until it runs out. Until it’s out of tricks.
Pamela: And it all depends on the size of the star. Even on the main sequence, not all stars are the same. So while our star is happily doing this proton-proton chain, there are other stars that are higher mass than us, that are sitting on the main sequence, doing what we call the CNO cycle. The carbon-nitrogen-oxygen cycle. And in this particular situation, they’re combining hydrogen atoms, they’re combining positrons, they’re getting to helium rather rapidly. And as they go through the cycle, they’re building themselves up to heavier elements, but it’s still a main sequence star.
Fraser: So sorry, so how big a star do you need? How much more massive than our sun do you need to be able to have one that uses this different cycle, this C – it’s the CNO cycle, right?
Pamela: It’s the CNO cycle, and you only need to be about one and a half, not quite one and a half times the mass of the sun. So it’s just a little bit bigger, not a lot bigger. But when you look at the fact that the majority of the stars in the universe are smaller than our sun, it’s really something that not a lot of stars do. So when we teach our students oh yeah, it’s all just burning hydrogen, it’s an oversimplification, but it works for the majority of the stars.
Fraser: And so will you get both going on at the same time? Like a much more massive star than our sun, it’s gonna have the proton-proton fusion going on –
Fraser: – as well as the CNO cycle, as well as deuterium burning, whatever is possible will be happening inside that core?
Pamela: In different ratios.
Fraser: In different ratios. And will you get – and this question just came up on the comments and I just wanted to throw this in as well, will you get different kinds of fusion in different layers at the same time of the star, depending on how massive it is?
Pamela: Yes. Once it moves off the main sequence. So, during the main sequence, in those slightly larger stars, you’re going to have those hydrogens coming together, getting you to the helium which allows you to get to the carbon and start the CNO cycle on. Now as start evolve, you will initially end up with the core gets filled up. And when that occurs, you can have a shell of hydrogen that begins burning around it. As that core of hydrogen burns, the helium core beneath it – sorry, as that shell of hydrogen burns, the core of helium beneath it will get more massive, will get heated up, will eventually light up and begin helium fusion.
Now eventually, if the star is big enough, when you have burned up that entire helium core, you’re gonna keep expanding out. You’re gonna keep getting more different kinds of materials. From that, you end up with a carbon fusing core that will fill up. From there, you end up with neon. Eventually, you’ll end up – well, if you have a star that is eight times the mass of our sun, you can end up burning all the way through so that your layers are the outermost layers, a shell burning hydrogen, that sits atop a shell burning helium, that sits atop a shell burning carbon, that sits atop a shell burning neon, that sits atop a shell burning oxygen, that sits atop a shell burning silicon, which is getting fused into iron.
And when all of that silicon fusion creates an iron core – well, the thing with iron is, everything up until you get to iron releases energy during the fusion process. But if you wanna fuse a couple of iron atoms, they’re going to demand you give them energy. And that giving the energy is kind-of the opposite of how stars work on the insides. So that’s when supernovae occur.
Fraser: Right, and this is when it all – and that process, those heaviest – the silicon going into iron, these all happen incredible quickly.
Pamela: Yeah, just millions of years. It’s one of these things where because you have so massive a system, and you have stuff that is burning at higher temperatures, you have extraordinarily rapid fusion going on. It is burning through a significantly larger amount of fuel very, very quickly and it – they die and go boom. And for this, we are grateful, because this is where all of our heavy elements that got freely released to create stars like our sun that are capable of having planets. This is how all of that happens.
Fraser: Yeah, so we kind-of need it. And so, and then what? What happens to those – for the smaller stars where they can’t support the fusion anymore, up to the ones that go supernova, like in that .13 to 7.5 times the mass of the sun, what do you get?
Pamela: There’s basically four fates across the entire spectrum to broaden it all the way out. There’s brown dwarfs, they’re gonna burn their deuterium. The bigger ones are gonna burn their lithium and then they’re gonna sit there going, I was, maybe, a star. Hi, I look like Jupiter now. So they’re just gonna be some pathetic. The red dwarf stars, the extraordinarily low mass objects that are in the hundreds of Jupiter size but tiny. They are actually boing to be capable of fully burning their entire atmosphere in this proton-proton process.
They are convecting a little tiny lava – well, like a giant lava lamp star where material in the center is broiling up to the surface, roiling up to the surface, and they’ll burn that proton-proton cycle for trillions of years. As you get to stars like our sun, after about five, six billion years, they’re like and we’re done with this proton-proton cycle. We’re out. We’re gonna start the rest of our evolution. And they end as white dwarfs. They end with those planetary nebulas. And this is what you get up until you start getting around the eight solar mass mark. At the eight solar mass mark, this is where things start going boom, because they have formed an iron core.
Now, exactly how much they go boom, how they go boom, is gonna depend on mass loss rates. And here’s where things start to get messy, because the mass that a star starts with is not the mass that a star ends with. You could conceivably have something that starts out as four solar masses and ends up quite happily being a white dwarf, because it’s lost enough mass. Then you could have something that started out as a few hundred solar masses, and ends up as like 10 solar mass black hole. Solar wind and mass loss is a real issue. We still don’t fully understand it. We don’t know how much mass is gonna be lost. All we know is if you die at a certain size, you can go boom.
Fraser: Right. Sometimes you end up as a neutron star, sometimes you end up as a black hole, sometimes you end up as nothing.
Fraser: It just goes kaboom and it’s all gone. Well, that was awesome, Pamela. I’m really glad we got the question last week, and that took you the time to go into great detail. And now we will all remember forever.
Pamela: That tritium decays, and is made my merging deuterium.
Fraser: Awesome. There you go. See you next week.
Pamela: See you. Bye bye.
Female Speaker: Thank you for listening to AstronomyCast, a non-profit resource provided by the Planetary Science Institute, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at AstronomyCast. You can email us at email@example.com, tweet us at AstronomyCast, like us on Facebook, and watch us on YouTube. We record our show live on YouTube every Friday at 3:00 pm eastern, 12:00 pm pacific, or 19:00 UTC. Our intro music was provided by David Joseph Wesley. The outro music is by Travis Surrel, and the show was edited by Susie Murph.
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