What’s the balancing act happening inside stars? Why is iron the fusion limit? How long does it take for a star to blow? I discuss these questions and more in today’s Ask a Spaceman!
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Hosted by Paul M. Sutter, astrophysicist at The Ohio State University, Chief Scientist at COSI Science Center, and the one and only Agent to the Stars (http://www.pmsutter.com).
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
You know what time it is. It's time for Ask a Spaceman. I'm your host, Paul Sutter. You've got questions and I've got answers. You know how this show works, but let's blow it up one more time. You go online to Twitter, Facebook, use the hashtag Ask a Spaceman. I'll pick those questions out and I'll blast an answer right back to you or six months later or two years later, but eventually an answer will get back to you. You can also follow me directly on Twitter and Facebook. My name is at Paul Matt Sutter. You can also go to the website at AskASpaceman.com. Leave some comments on the episodes. You can have a conversation there. It's all good. You can also go to YouTube.com slash Paul M. Sutter. We have one simple goal with this show. Complete knowledge of time and space. And on the road to complete knowledge of time and space, we have today's questions from at Ianut Branea. asking, why do supernova happen? I don't understand what triggers the explosion. And also, why is iron the limit when fusion stops inside a star? And also, at thomaspadden1, also on Twitter, supernova, coolest or hottest thing ever? Great questions.
And today, I'm going to give you a simple recipe that you can follow at home to make your own supernova. You need a load of hydrogen. Around 10 to the 31 kilograms or two times 10 to the 31 pounds. Don't worry if you're a little bit off. This measurement doesn't have to be exact. You need a dash of carbon, nitrogen, and oxygen. Set your oven to 10 million Kelvin and set your timer to around 10 million years. And all you need to do is wait and eventually you'll get... The big boom. A supernova explosion. It is just that easy. It's crazy to think how simultaneously simple and complex the natural world can be. It's a very simple ingredients. It's a very simple process. And yet it leads to this incredibly rich, dynamic situation and one of the most powerful explosions in the universe. That simple process. that takes all these ingredients the hydrogen the carbon the nitrogen the oxygen and turns it into a supernova is gravity it's all driven by gravity good old-fashioned newton and einstein down at the bottom of your feet gravity gravity plus time gives you supernova chew on this The same force.
The same force that holds your feet to the ground is the exact same force that provides the energy required for a star to go supernova. That is one of the most beautiful things. One of the things I really love about physics is the universality of physics, how the same physics here on Earth apply across the universe. And the understanding that we develop to figure out why our feet stick to the ground help us understand some of these crazy exotic phenomena happening on the other side of the universe. And I need to say, so much of astrophysics is driven by simple gravitational collapse. You have a cloud of stuff. It's unstable. It starts collapsing in on itself. You get galaxies. You get galaxies. Stars, you get supernova. This is an example of the power of gravity. Sure, gravity may be the weakest of the four forces, but it can play some tricks that the others can't. Just give it, you know, a few million years. In the case of stars, it's a balancing act. You have a giant blob of hydrogen, and gravity is pulling it together.
Gravity is what pulls it into a spherical shape. Gravity would love for the star to collapse into an infinitely dense point. That's all gravity desires, is to take stuff and squish it down even tighter. But it gets stopped. It gets prevented from squishing that star down into an infinitely dense point because... The deeper you go into the core, as all this material is compressing in on itself, the deeper you go, the higher the pressures get, just like diving into the ocean. The gravity of the Earth would love to pull the ocean down into the core. And so as you go lower and lower in the ocean, the pressures get higher and higher. Higher pressures when it comes to gases like the sun or a star, higher pressures mean higher temperatures. eventually. The pressures are so high that protons, who normally hate each other and want nothing to do with each other, they are like charges, so they repel. They are forced to live together inside the same nucleus, and that's a fusion party. The pressures, the gravity delivers a enough overwhelming pressure.
In fact, this is the definition of a star. The dividing line between a planet or a brown dwarf and a star is, is there enough gravitational pressure at the core to ignite a fusion reaction? There's two main chain reactions that happen to convert hydrogen into helium. And since I'm talking about supernova, I'm going to talk about more massive stars. Stars at least around eight or so times more massive than the sun. And in this reaction, you start off with four hydrogen atoms. And you sprinkle in some carbon, nitrogen, and oxygen. And these act like a catalyst to... shall we say, enhance the reaction. They make the nuclear chain reaction go faster than it normally would and be a little bit more energetic than it normally would. At the end of the day, you start off with four bits of hydrogen and some sprinkling of carbon, nitrogen, and oxygen. They return. You still keep the carbon, nitrogen, and oxygen. They get recycled to be used in another reaction. But instead of four hydrogen atoms, you get a helium atom.
And the helium atom is slightly less massive than the original four hydrogen atoms. And we live in a universe where mass is equal to energy. And so this process gives off energy. You lose mass, boom, you get some energy out of it. This process, this nuclear reaction has a few other byproducts. It spits out some positrons, which are the evil twin of the electron. I need to get into antiparticles someday. Just ask the question. Spits off a couple neutrinos, and neutrinos, a very ghostly particle, usually don't play a role, but they'll become important later. And of course, photons, light, radiation. Ladies and gentlemen, this is a nuclear bomb. A fusion bomb, to be precise, a hydrogen bomb. And what do bombs do? They explode. So gravity wants to pull the star ever smaller, but it's resisted by this perpetual nuclear explosion going on in the core. These stay in balance. If gravity were to win a little bit, let's say you were to temporarily stop The nuclear reaction is happening in a star.
Then gravity would start to wind, pull it even tighter. But the core of the star would get scrunched up even tighter with even higher pressures, with even higher temperatures, which would increase the reaction rate, which would make the star want to blow up a little bit more. So it reinflates. And if the fusion were to win a little bit, say you're able to temporarily ramp it up where you can get a lot of fusion going, well, that would tend to inflate the star, expand the star, which would cool it off in the core, which would lower the reaction rate, which would mean gravity would temporarily have the upper hand. And so we have a balance between the crushing gravity of the star's own weight and the energetic explosion happening because of these nuclear chain reactions. This balance is called hydrostatic equilibrium. Equilibrium because it's in balance. Hydrostatic because, well, it's not hydro. It's not water. It's gas. Static is not moving. It's just a historical term, all right? We're going to live with it.
In this case, in the case of stars, this happens all over the universe, by the way. Anytime another force resists gravitational collapse, you get what's called hydrostatic equilibrium. Why doesn't our atmosphere collapse? Hydrostatic equilibrium. How do giant clusters of galaxies maintain their shape for billions of years? Hydrostatic equilibrium. How does a star keep from collapsing into an infinitely dense point? Hydrostatic equilibrium provided by nuclear fusion. Anytime another force resists gravitational collapse, and this is usually a byproduct of the gravitational collapse itself, gravity is kind of its own worst enemy. Gets a little bit too eager, wants to collapse things into an infinitely dense point, but that leads to a condition that resists the collapse itself. So... Sorry, gravity. I mean, maybe if you're a little bit nicer, you'd get what you wanted, but you had to go all in single-mindedly and you end up with all this fusion. So stars stay in balance for a long time. It's all hunky-dory for millions or even billions of years until you run out of hydrogen in the core.
Now, you can have tons of hydrogen left over in the star everywhere else, but if it's not in the core, no fusion for you. Just like Patreon. Hey, there can be all these monies floating around the world, but unless they're put into patreon.com slash pmsutter, they're not doing anything useful or important. or vital for the perpetuation of our civilization. You know, they have to be in a very specific place. They have to be submitted to patreon.com slash pmsutter to help this show keep going. Very apt analogy. One of my best analogies ever. So you can have tons of hydrogen in the star, but if it's not in the core, it's not going to get any work done. You're not going to have any fusion. But what happens in the core? As this fusion party is happening, you're creating helium. Where does the helium go? It sinks down into the core, deeper into the core. And you can fuse helium, but in the normal everyday life of a star, it's not hot enough. The pressures aren't high enough. So it gets shoved into a little ball.
And you end up eventually, and eventually depends on the size of the star, how quickly it fuses, blah, blah, blah. The fusion, the actual hydrogen fusion, the furnace gets moved into a shell around the helium core. And how intense is nature? What's happening, it's starting to happen inside of our own sun right now. It'll take a few more billion years for it to really play out. But it's building up a core of inert, dead helium. And surrounding that, beginning to surround that, is a shell of hydrogen fusion. And it's surrounded by the ferocity of the rest of the star. Eventually, eventually, eventually... The burning of hydrogen in the shell turns off. And yes, I know it's not technically burning. It's not combustion. It's not fire. But it's called burning because it's fun to call it hydrogen burning. So it's fun. So shut up. Don't worry about that. Just call it hydrogen burning. It's the same as hydrogen fusion. It's just a name. Another historical term. Eventually, that shell runs out of gas.
It runs out of hydrogen. So gravity wins! Yay! Finally, its resistance is futile. The fusion has finally shut off once and for all, and gravity can get back to work of collapsing that star into a single, infinitely dense point. But now you have a bunch of helium in the core, and initially the helium isn't doing anything at all, just sitting there being hot. But as gravitational collapse continues and continues and continues, once the core reaches about 100 million degrees, the pressures are high enough, the temperatures are high enough, helium can start to fuse. Helium fuses? turns into carbon, oxygen, a little bit of nitrogen sometimes, and releases energy. Hydrostatic equilibrium rules the day again. But helium burning is hotter and furiouser. Why? Well, helium fusion isn't quite as efficient as hydrogen fusion. It doesn't release as much energy. So it has to really struggle to overcome gravity. This is operating at 100 billion degrees with an incredibly compressed core. To maintain that balance, the helium has to burn a lot faster than the hydrogen did.
So this phase, the helium burning phase of a star is much shorter. And the cycle is repeating again inside of massive stars. It's building up a core of unburnt, of inert carbon and oxygen. With the helium fusion happening in a shell around it, and sometimes hydrogen fusion can reignite in a shell around that, but eventually you run out of helium in the shell and gravity wins. Then carbon and oxygen can start fusing. That happens at the billion Kelvin mark. So 10 times hotter than where helium fuses, you get carbon and oxygen fusion. With a shell of helium fusion around that, a shell of hydrogen fusion, it's crazy. But then that stops because you run out of carbon and oxygen. Well, what do carbon and oxygen fuse into? They fuse into silicon. And silicon fusion ignites at around 3 billion Kelvin. And at this stage, you end up with the hottest, nastiest, seven-layer bean dip you've ever shoved your tortilla chip into. You have on the surface a skin of hydrogen. that's not doing anything. Below that, you have a skin of hydrogen that's fusing.
Below that, you have a skin of helium that's fusing. Below that, carbon that's fusing, neon that's fusing, oxygen that's fusing, silicon that's fusing. And at the very, very core, a ball of iron. That's what silicon fuses into, nickel and then iron. And as we move up the fusion chain to more and more massive elements, the energy production gets less efficient. You get less bang for your buck. fusing carbon or silicon than you do fusing helium or hydrogen. And it's at iron where the nightmare begins because it costs energy to fuse iron. When you fuse lighter elements, you gain energy. When you fuse iron and heavier elements, you lose energy. So gravity, the incredible pressure of this massive star weighing down on this core of iron, squeezes two iron bits together, but it loses air. There's nothing to fight it. There's even less resistance to gravity. So gravity pushes the attack, drives up the temperatures even more. And the fusion rate goes up even more and almost instantaneously. The star is thrown out of balance.
Some iron gets fused in this process, but most of the iron remains because this happens very quickly. That first hydrogen burning stage in a star lasts, you know, a few million years. And for small stars can last tens of billions of years. But for these big massive stars, at least eight times more massive than the sun, a couple dozen times more massive than our sun. This is over in a few million years. The helium lasts a million years. Tops. Carbon fusion goes on for 600 years. Neon burning lasts a single year. Oxygen lasts six months. Silicon fusing into iron, that takes a day. A single day. And you've run out of silicon. Then you make that ball of iron in the core. It tries to fuse, but there's nothing to stop the overwhelming gravitational collapse. Spoiler alert, gravity is going to win. But in this battle between collapse and explosion, there's going to be one big epic showdown. You have a core of iron. This core of iron is only a few miles across or kilometers across, surrounded by enough gas to fill a dozen suns, and there is nothing holding that gas back.
The iron core itself is made of protons and neutrons. And there's a bunch of electrons just hanging out, not bothering anybody. But the pressures are intense enough to shove, to literally shove those electrons into the protons. This process turns the protons into neutrons and also spits out some neutrinos. The iron, you start off with a ball of iron, it gets converted into a giant ball of neutrons, right? raw neutrons, like a gigantic citywide atomic nucleus. And the neutrons can hold themselves up from degeneracy pressure. Just like electrons, you can only fit so many electrons in a box before the box starts spilling over. You can only fit so many neutrons in a box before the neutrons start spilling over. Once this ball of neutrons is formed, this neutron core, You have the entire weight of the star, dozens of suns worth of material, that the intense gravitational pressure converted that iron core into a ball of neutrons, but it can't press it any further. It is supported. It does resist gravity.
So the entire material slams into that neutron ball and ricochets. And boom. That process, going from formation of the Iron Core to triggering the launch of a supernova, about 15 minutes. 15 minutes. This is a process that was set up millions of years in the past. And the final moments happen before this show is over. That's the initial explosion. That's the triggering of a supernova. But here's where things get tricky. We don't fully understand supernova. Welcome to, you know, science. But we've been running simulations of supernova. What's going on inside these explosions? It's complicated stuff. You know, you've got all these fusion chain reactions going on, the multiple layers, the multiple elements. You've got radiation. It's spinning. You bet there's magnetic fields. And I swear there should be a special tone every time I say magnetic field in a show. I should get a different voice like magnetic fields. Like it's the secret word of this show. There are magnetic fields happening. There are neutrinos.
Which usually no one cares about neutrinos. But what we found when we started making our initial simulations of supernovas, you know, 10, 20, 30 years ago, the shock front, the rebounding, you get this rebound off the core and then you get this shock front, this wave of explosive energy that starts propagating away to start filling out the rest of the star and make it go boom. It kind of stalls, peters out, slows down. It was thought that neutrinos might come to the rescue. Even though they don't usually interact with matter, there's enough neutrinos produced And this kind of event qualifies as enough that enough of them blow. It's like a subtle wind, a gentle wind. So there's this stalled shock front in the middle of the star. It wants to explode, but it doesn't quite have enough oomph to do it. But then there's this flood of neutrinos and kind of reinvigorates it. We thought neutrinos would do the trick, but complex simulation so that... That again, the shockwave that wants to blow up the star, the final revenge of nuclear fusion against gravity, but that peters out too.
What happens after that? We don't know. Because it's in this stage that things get really complicated really fast. In our simulations of supernova, we have a lot of trouble getting supernova to, you know, supernova. We're not exactly sure. We're pretty confident on the triggering mechanism, that this is the physics, that the iron core forms, it turns into a neutron ball, and there's a bounce and a rebound. And we know this because we've seen the neutrinos themselves. This was a major revelation back in 1987. There was a supernova that we saw. With our eyeballs, our telescopes. And we also detected this supernova using our neutrino detectors. And we actually saw the neutrinos first because the neutrinos were able to burst out of there, out of the scene, and stream straight to our eyeballs while the explosion hadn't quite happened yet. So the neutrinos actually popped out first. There was a flash of neutrinos. There's almost a supernova neutrinos first before there was a supernova of light.
And yes, I'd love to do an episode on neutrinos. Just feel free to ask. We saw the flash of neutrinos in our neutrino detector, and then we saw the flash of the explosion in our telescopes. So we knew, we know this general picture is right, but the details, what happens to actually... motivate that explosion to actually get it going, to get it through, get its second wind or its third wind to actually produce the brilliant display that we see. We think it might have something to do with unstable modes where these explosions aren't perfectly spherically symmetrical. It's not like a ball. or a shell, maybe this shock front happening inside of a star slows down, but then it starts to get unstable and starts to slosh back and forth. And then eventually one of these sloshing modes, like a resonating bridge or something just goes nuts. And that's what goes to the explosion. We're not exactly sure. And sometimes we see hints of failed supernova. Stars that get turned into neutron stars, the leftover raw cores, without having the big explosion.
We call these core collapse or type two supernova. It's the end of the line for massive stars in our universe. Just like our sun will end up as a planetary nebula and eventually a white dwarf, massive stars almost all the time end up as supernova. The core, what happens to that? It either continues on as a core in which we call it a neutron star. Sometimes it gets ripped apart and there's nothing left. It gets vaporized. And sometimes gravity wins. And in that final moment, that unrush of collapse, there is enough pressure to even overwhelm the degeneracy of neutrons. And it does tip the scale and it does get to collapse all that matter into an infinitely dense point, which is what we call a black hole. This is how black holes are formed in our universe, are through the deaths of massive stars. In these final moments, in that 10 or 15 minutes of catastrophe at the end of a life of a massive star, the amount of energy released is around, in a typical supernova like this, 10 to the 44 joules.
And if 10 to the 44 seems too abstract, that's about the same amount of energy released by the sun over its entire 10 billion year lifetime. The formation of the iron core, the fusion of the iron core takes about a day. The intense pressures converting that into a ball of neutrons takes about 15 minutes and the explosion itself takes about a couple seconds. The source of all that energy, 10 billion years of our sun's output released in those moments is gravity. Gravity drove this entire crazy train. Yeah, there was nuclear fusion and neutrinos and blah, blah, blah. But those processes were set in motion by simple gravity. The processes were driven by gravitational class, by the conversion of gravitational energy into blowing up energy. So are supernova the coolest or hottest things? I would say neither and kind of both. Supernovae obviously are not the coldest thing, taking it literally, the coldest thing in the universe, but they're not the most energetic events. The active galactic nuclei swamp them out in terms of raw energy output.
They are the most intense explosions, but they're also kind of both. You know what? It's not often that a star dies in such spectacular fashion. And when they do, 10 to 44 joules released as photons and neutrinos, you can literally see these stars blow up from the other side of the universe. Which is pretty cool. There's not a lot of things you can see from the other side of the universe. They will outshine galaxies for a couple weeks. Kind of bright. Thank you so much for listening, and especially thanks to all my Patreon contributors and the top ones, Helgi B., Justin G., Justin R., Kevin O., Michael Z., and Chrissy. For this month, who knows what the list will be next month. Go to patreon.com slash pmsutter for more info. Thank you so much for iTunes reviews. Thanks so much for asking me follow-up questions on Twitter and Facebook and on the website. I love the discussions. You can find out more about following me directly on Twitter and Facebook. My name is at Paul Matsar. Or go to YouTube.com slash Paul Matsar.
And, oh, how could I not thank the people who asked the questions at Ian Uprenea and at ThomasPatton1 on Twitter for asking these wonderful questions that led to today's episodes. And I will see you next time for more Complete Knowledge of Time and Space.