What really keeps the Sun shining? How long would it stay warm if fusion stopped? How long would it take before we noticed? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPT (AUTO-GENERATED)

If I snapped my fingers and shut off all fusion reactions in the sun right now, just like that, with every single proton just deciding to sit there and wait around instead of, you know, fuse and release energy, what would happen? Nothing. Nothing would happen. It would just be a normal day. The sun would still shine in the sky. Plants would still grow. You'd still get a sunburn. Absolutely nothing would change. Not today. Not tomorrow. Not for a thousand years. Civilizations would rise and fall. Ages would come and pass. We might develop the technology to reach the stars or we might just sit at home. Either way, the sun would just be the sun. If we killed the sun today, it would take millions of years for anything different to show up to the outside universe. Oh, except the neutrinos. Yeah, we could find out right away because we wouldn't get any solar neutrinos anymore. But who cares about neutrinos? Before we dig into the how, let me explain the what. Let me paint a portrait of our dear old sun.

As stars go, it's large, but not grande. Bigger than smaller, smaller than big. But we don't care about other stars right now. Within the solar system, it's by far the largest single object. It alone holds over 99% of all the mass of the entire system. If you got rid of every planet, every asteroid, every comet, every speck of dust, the solar system... Well, it wouldn't be much of a system anymore, but the point is that the Sun would barely notice. You could cram over 1.3 million Earths inside its volume, and it outweighs 330,000 of our planetary mass. That means a lot of stuff crammed into a comparatively smaller volume. And what does that stuff do? It gravitates. A lot. It's just a pile of stuff all squeezing in as tightly as it can, which sends the pressures of the core skyrocketing to over 260 billion times air pressure at sea level and temperatures in the nice toasty range of 15 or so million Kelvin. So hot and a little bit cramped, which it needs to be to trigger nuclear fusion. But interestingly enough, nuclear fusion is really, really inefficient.

And this is going to explain a lot about why the sun could shut off without us noticing. Check this out. To get fusion to happen, you have to merge two hydrogen atoms, which in these conditions are just bare naked protons, and get them to become a single helium atom. This is hard. Really hard. It's hard because protons have identical charge, which means they repulse each other, and this repulsion gets stronger the closer they are. To get the fusion to happen, the protons have to get... to around a femtometer away from each other, and at that point, the strong nuclear force can take over and make the gluing happen. But at a distance of just one femtometer, the electrostatic repulsion is so strong that just the temperatures and pressures in the core are more than 700 times too weak to do the trick. If it were just a matter of temperatures and pressures, then fusion would never happen. The only way to get around this is to quantum tunnel. Protons aren't exactly point particles, just like all of our quantum forefathers predicted.

They have wave functions. And those wave functions tell us where the protons might be the next time we go looking for them, or where they might be the next time nature calls upon them to take part in a nuclear reaction. Now those wave functions aren't exactly big, but they're bigger than the protons themselves. So as long as you get two protons close together and are patient enough, then randomly they'll find themselves within a femtometer of each other and boom, nuclear fusion. Almost. You see, two protons can form a stable nuclei. It's like being a roommate with yourself. Eventually you get sick of all your own quirks and habits and you start browsing rental options in your city. To make the fusion game work, one of the protons has to convert to a neutron, which is totally entirely possible via the weak nuclear force. Yeah, the weak nuclear force, the worst of the nuclear forces. The force barely ever activates. Two protons meet, their wave functions overlapping, and most of the time, nothing happens.

Once you add it all together. the chances of wave function overlap, the chances of the weak force triggering, you only get one successful fusion event for roughly every 10 to the 27 encounters. Millions and billions and trillions of protons encounter each other and just bounce off like, no, sorry, not today. A typical proton will spend 10 billion years in the core before it successfully fuses. 10 billion! That's more than twice the current age of the sun. Thankfully, to balance this out, the Sun has a lot of protons. The core alone has around 10 to the 56th of them, which last time I checked is a lot, plus more surrounding them in the atmosphere and body of the Sun that have a non-zero chance of making their way down to the core. Every second, any given proton has only a probability of 10 to the minus 19 of fusing, but multiply that by the sheer number of protons just loitering around, and you get steady fusion rates of around 600 million tons of hydrogen consumed every single second. But when you zoom out and look at the efficiencies, in a cosmic sense, the Sun barely works as advertised.

It's actually really awful at fusing. It should be embarrassed, honestly. It's only one step above not working at all. Check this out. The sun emits a ridiculous amount of power, right? Every single second, it's beaming out over 10 to the 26 joules of energy. To put that into perspective, that one second of output could power all of our modern energy needs for over 600,000 years. Only one part in 2.2 billion of that accumulated solar output even strikes the surface of the Earth. And even that tiny fraction is 8,000 times our current energy needs. Yeah, that's a lot of power in total raw numbers. But as far as efficiency, it's awful. Downright awful. Across the entire core, the sun averages an output of around 277 watts per cubic meter. That's nice. Your human body. with all of its metabolism and mitochondria and muscle movement, generates about 100 watts in less than a tenth that same volume. That's right. A human being generates about five times as much power, pound for pound, than the core of the sun.

If you replace the core of the sun with a bunch of warm bodies, it will glow five times brighter. Do not repeat this experiment at home. Heck, a compost heap is more energetically compact than the sun. The sun is so bright and so luminous and so powerful because it's enormous, not because fusion is especially intense. Which means if we shut off fusion, we wouldn't have much to lose, at least at first. When you adjust your stove to heat your pan to cook your eggs, you have to wait a while. It takes time for the heat to transfer to the pan. Once it's hot enough, you cook your eggs. And at least once you're done cooking, you can eat your eggs while waiting for the pan to cool off again. The sun is slightly larger than a stovetop pan, but the same basic physics is at play. At one point, the sun, well, the cloud of stuff that would become the sun, was cold. Now it's hot. It took time to become hot. And last time I checked, the surface of the sun is exposed to the vacuum of space, which is very cold.

And that same surface is constantly emitting radiation, which cools off the sun. All this means that fusion isn't the hot pan itself, it's the flame underneath. It's keeping the sun warm. Without fusion, the sun stays warm until it cools off. Because like all hot things, it takes time to cool off. In fact, the rate of fusion is explicitly calibrated to keep the sun just barely warm enough to avoid catastrophe. And that's through the not-so-magical process known as hydrostatic equilibrium. Check this out. The sun is a giant ball of gas. It's so giant that it has enough gravity to pull itself tighter. And when giant balls of gas get squeezed into smaller volumes, they actually heat up. Back in the 19th century, astronomers were wondering just how the heck the sun stayed warm for so long. So in 1854, a German aristocrat named Hermann von Helmholtz proposed that maybe it stayed warm by being a giant ball of gas that's slowly shrinking. Start with a big ball of gas, let it compress, it heats up, the heat escapes in the form of lovely sunshine, the gas compresses a little more, and the cycle keeps going.

A decade later, another aristocrat, this is definitely the age of if you want to be a scientist, you should probably be independently wealthy, William Thompson, the first Baron Kelvin, Lord Kelvin to us plebs, did some more detailed calculations to estimate the lifetime of the sun and got the wrong answer. He danced around a few options, but generally landed in the ballpark of a few tens of millions of years. Long lifetime for sure, but also way out of step with what the geologists and later biologists were coming up with for the age of the Earth. They were calculating numbers more like hundreds of millions into billions of years. For decades, astronomers would remain the butt of jokes in scientific circles, and I know I'm not interested in hearing discussions of present-day circumstances. Because Lord Kelvin was all high and mighty and public about his calculations, which were, like I said, very wrong. It wasn't until the 1920s and the development of nuclear theory that we were able to propose a working mechanism of solar power that could stretch for billions of years.

So even though Lord Kelvin was wrong, he was wrong in a very useful way. That's the best kind of wrong. Because he didn't know about fusion, it will be his calculations that we will use to calculate what would happen if fusion shut off. So, thanks, I guess. I mean, at the very least, he could have contributed to Patreon. That's patreon.com slash pmsutter. I know Patreon did not exist, nor did computers or the internet or most of modern electricity. But hey, he could have if he really wanted to. He was a smart guy after all. That's patreon.com slash P-M-S-U-T-T-E-R. And that's how you keep this show going. I really do appreciate it. The cool thing about nuclear fusion, which is what powers the sun, is that it self-regulates. If the sun contracts a little, the core compresses. The more protons get to speed date other protons, and the fusion rates increase, which heats up the core and presses outwards, cooling everything off. And if the sun were to expand a little, the fusion reactions would slow down, which takes the pressure off, allowing the sun to shrink back down.

In other words, and you thermodynamics nerds will get a kick out of this, the sun actually heats up as it loses energy, which seems counterintuitive, but nature is under no obligation to be intuitive, especially when it comes to self-gravitating systems. The fusion reactions are just right. keep the Sun from either A exploding in a blaze of glory or B collapsing into a black hole. Which turns out to be not a lot of fusion. Fusion is a trickle, topping off a vast reservoir of heat, efficient just enough to keep the lights on, no more, no less. Which means if fusion shuts off, we still have all that heat already contained in the body of the Sun and we have the Kelvin-Helmholtz mechanism, even though Helmholtz first proposed it, Kelvin Helmholtz rolls off the tongue easier, sorry, Armand, that we can keep the sun powered even longer just by allowing it to shrink. In other words, the sun is hot because it's hot. Fusion just keeps the lights on, and because the sun is gigantic and crowded and complex and messy, changes take a long time to take effect.

Case in point, the photons. Imagine you're standing in the middle of a crowded room. Not just any crowded room, a packed room. Shoulder to shoulder, the room is so crowded that you can't take more than a step in any direction before bumping into someone. And every time you bump into someone, you get spun around to face a random direction. You can't see the walls. You can't see the doors. You just push, bump, spin, push, bump, spin. You can already feel your blood pressure rising. You want to get out now. Well, how long does it take you? The answer depends on how big the room is, of course, but also on something more subtle. You're not walking out of the room. You're random walking out of the room. Every step you take is in a completely random direction. Half the time you're heading deeper into the room without even realizing it. Sometimes you go in circles. Sometimes you make some progress and then you immediately undo it. Needless to say, this is not an efficient way to travel. There's some math behind how long it takes you to reach the exit.

And the math is, well, it's frustrating, especially if you want to leave as quickly as possible. The math says that if you want to travel a certain distance, you can't just take the amount of steps it would normally take to cover that distance because of all the bumping and shuffling and changing of directions and other annoyances. Instead, you need to take the square of the normal number of steps. Like, if the door is four steps away in a normal walk, it's 16 steps away in a random walk. And if it's 10 steps away in an uncrowded room, it's 100 steps. and a packed one. Every photon born in the core of the sun is in exactly this situation, or to be fair, worse. The sun's interior isn't a gas, it's a plasma. Every atom is fully ionized, which means there are bare nuclei and free electrons just floating around everywhere, and photons love to interact with free electrons. A photon born in the core travels approximately one centimeter before it slams into an electron scatters off it in a completely random direction, travels another centimeter, slams into another electron, scatters again, and so on and on and on.

One centimeter. The sun's radius is 70 billion centimeters. That's the straight line, empty room, normal walk distance for a photon in the sun. It requires 70 billion squared steps before it emerges. If you tried to count one step per second of a photon trying to escape the Sun, it would take you longer than the current age of the universe, several times over. Each step only takes in reality a fraction of a nanosecond, which is good, but you've got a lot of them to take, which is bad. Do all the arithmetic and you find that a photon born in the core of the Sun takes around 100,000 years to reach the surface. 100,000 years, folks. If photons could just stream straight out, the trip would take two seconds. Instead, because they have to bounce around like an extremely unhappy pinball, the trip takes 100,000 years. The random walk inflates the travel time by a factor of around a trillion. A photon hitting your face right now was born around the time that anatomically modern humans were just beginning to spread beyond Africa.

Neanderthals were still alive. Agriculture hadn't been invented. Spoken languages we'd recognize didn't exist. Every civilization, every religion, every memory of human history is younger than the trip that photon just took. Sunlight is really old. And by the way, it isn't even the same photon that started the journey. Photons in the solar interior aren't just bouncing around like billiard balls. They're constantly being absorbed by electrons, which then re-emit photons in random directions. with slightly different energies. So a gamma ray that was born in the core with an energy of around a million electron volts gets ground down step by step into longer wavelength lower energy photons. By the time it leaves the surface, it's visible light with an energy of an electron volt, a million times weaker, peaking nicely in the wavelength our eyes are evolved to see conveniently enough. All the energy makes it out of the sun, but the photon itself, not so much. Most of that time that the photon spends inside the sun is in what we call the radiative zone, which is the inner two-thirds, 70-ish percent of the solar interior.

That's where the plasma is dense and hot and the photons are stuck in their pinball nightmare, honestly. Above the radiative zone is the convective zone where the plasma is cool enough and opaque enough that radiation just can't carry the energy fast enough anymore. So the sun gives up on radiation and literally starts boiling with bulk plasma motion taking over. Just hot blobs of gas physically rising to the surface, cooling off and slinging back down. So most of the time a photon is trapped in the sun is in the radiative zone. Once it reaches the convective zone, it's out in only a few months. What this means is that anything that happens in the core of the sun is essentially invisible from the surface for around 100,000 years. Not exactly. That 100,000 is an average number. That's a random walk. If you're in that crowded room and the door is 10 normal steps away, it doesn't take you exactly 100 steps to reach the door. Sometimes you're just lucky and you reach it in 10, 20, maybe 50 steps.

Sometimes you're really unlucky. It takes you a thousand steps. So that 100,000 years for a photon to escape is the average. Sometimes photons can get out sooner, sometimes way longer. But the key idea here is that the light we see from the sun today is reporting on conditions in the core that essentially during the last ice age. If the fusion rate at the core had been drifting slowly for the past 50,000 years, we wouldn't have any idea. The sun's surface, as far as light goes, is tens of thousands of years delayed from what happens in the core. So we put it all together. The sun is gigantic. It's crowded. Changes inside the sun take a long time to propagate. We already knew that fusion is so inefficient that the sun is basically coasting on its stored heat and that the Kelvin-Helmholtz mechanism can keep the lights on for tens of millions of years all by itself. Now add on top of that the fact that the surface itself is reporting from, you know, a hundred millennia in the past. You see where this is going.

Which is exactly nowhere. Let's say fusion turns off magically. Of course, don't ask me how this would actually work. And the sun just sits there. Being the sun. Same mass. Same gravity. Same mix of hydrogen and helium. Still way hot in the center. Less so at the surface. Just no fusion. What happens? Nothing happens. That's the answer. Nothing happens. You go about your day. You go about the next day. You live your entire life completely, totally, 100%. I'm not kidding you. Unchanged. Tides. Equinoxes. Plants eating sunlight. Vacations to the beach. All exactly the same. Temperature. Same. Luminosity. Same. Mass. Same. Spectrum. Same. Size. Same. Your children's lives are unchanged. So are their children's and theirs and theirs and theirs. Written history has been around for, like, what, 5,000 years? For the first 10,000 years after fusion shuts off, twice that entire span of time, nothing really changes. The sun is full of hot stuff, and the hot stuff is really hot, and there's also a lot of it, so it can stay being hot stuff for a good long time.

But after 10,000 years or so... I'm being general here, this is based on a lot of models of solar interiors that do have some wiggle room, so these numbers aren't perfectly precise, but this is an impossible made-up scenario anyway, so don't stress out about the finer details. After 10,000 years or so, things start to drift. Remember that the photons in the core take, on average, 100,000 years to make their way out? That's an average. Some come out a lot faster, they're just the lucky breaks. But now there are no new photons generated in the core. So at around the 10,000 year mark, the Sun starts to seem a little thinner. Just slightly fewer photons coming out than expected. This is the first time that the evidence is clear that something's wrong. It's the first time that the surface finally changes in response to what happened in the core. This is when the world finally wakes up to the unfolding catastrophe. and immediately goes back to business as usual because there's plenty of time before anything interesting happens.

That's because stars, being giant balls of self-gravitating gas, are weird. The core is sputtered out, it's gone cold, or at least cooler because it has a lot of stored internal heat, just the nuclear faucet has been shut off. But without new sources of heat and photons, it can't support itself against gravitational collapse, so it collapses. And as it collapses, like a house built on sand, so does the rest of the house. The rest of the house being the entire sun, of course. Over the course of 100,000 years or so, the last core-generated photon leaves the sun. There are still more photons. It's still hot. But they're not fusion-generated photons anymore. They're just regular old heat photons. The sun starts to drift out of hydrostatic equilibrium. Because if gravity and pressure are two kids on a teeter-totter, well, one kid just got up and left. Which means it's time for our two favorite aristocrats, Kelvin and Helmholtz. The sun is shrinking. Nothing is holding it up. You expect it to cool off.

We literally shut off fusion. But because it's shrinking, it actually heats up. In fact, depending on exactly how this plays out, it not only gets smaller and hotter, it might actually get brighter. That's right, for a few million years, a no-fusion sun is actually smaller, hotter, and brighter than a yes-fusion sun. I told you stars were weird. This goes on for a few tens of millions of years. We can go ahead and call this the Kelvin-Helmholtz coasting phase because that sounds gentle. The sun is a giant warm thing that can keep generating new heat through gravitational contraction. And then it dies. But slowly. Over those tens of millions of years, it does begin to cool down. Its luminosity dwindles. Won't be an overnight apocalypse. It will be a gentle shifting of the Earth's climate. 30 million years, though, is the timescales we're talking about before our climate noticeably shifts. 30 million years is a long time. 30 million years ago, whales just started whaling. And grass was just starting to grass.

That's evolutionary timescale. weather patterns shift, glacial patterns change, life on Earth has suffered worse catastrophes over much shorter timescales. A giant impact, anyone? Life could have a chance at adapting to the slowly cooling sun. And no, it won't be pretty. Definitely won't be warm. But at least it will be slow enough that life could adapt. After 100 million years, the sun becomes... Well, we don't have a name for it because this is a made-up scenario. So it becomes something, but it's cold. And then it becomes too cold for the earth. We freeze up. And that's it. 100 million years. On one hand, that seems ridiculously far away. On the other hand, it seems a little frighteningly early. But if that seems a little too frighteningly early, remember that if fusion stays on, the sun gets steadily brighter and warmer as it goes. And in 300-ish million years, the sun will be too hot to For our planet, our oceans will start to boil, continents will lock up, and we'll turn into Venus.

So no matter what, we're in the last few hundred million years of life on this Earth. I guess that didn't help much. Oh, right, the neutrinos. Oh, those sneaky little devils always give the game away right off the bat. See, nuclear fusion produces neutrinos. It's that weak force action that changes protons into neutrons. And they just stream right out while the photons get all tangled up. which means just eight minutes after the core shuts down, neutrino observatories all around the Earth all of a sudden stop registering anything coming from the sun. And they always spoil the surprise, don't they? Thanks to Steve T, at Ion at Breneia, Brian D, Todd D, Duncan S, and Smokey for the questions that led to today's episode. Thank you for all your amazing questions. It really is what powers the show. That's AskASpaceman at gmail.com. or the website justaskthespaceman.com. There's a little form there that you can use to submit a question. Please keep dropping reviews and ratings for the podcast on your favorite podcasting platform that helps to show visibility.

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