Our Sun isn’t just a terrifying ball of white hot plasma, it’s actually a lot more complex. It’s got layers. And today, we’re going to peel back those layers and learn about the Sun – from the inside out.
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Female Announcer: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online. The world’s longest-running online astronomy degree program. Visit astronomy.swin.edu.au for more information.
Fraser Cain: Astronomy Cast Episode 320, “Layers of the Sun.” Welcome to Astronomy Cast, your weekly facts-based journey through the cosmos. We help you understand not only what we know, but how we know what we know. My name is Fraser Cain. I’m the publisher of Universe Today and with me is Dr. Pamela Gay, a professor at Southern Illinois University – Edwardsville and the Director of CosmoQuest. Hey, Pamela, how are you doing?
Dr. Pamela Gay: I’m doing well. How are you doing, Fraser?
Fraser Cain: Doing really well. So, this is a very special episode of Astronomy Cast. I feel like every episode is a very special episode, but this is a very special episode of Astronomy Cast that we’re actually recording live as a Google+ Hangout from the YouTube studio in Los Angeles.
Dr. Pamela Gay: And my attic.
Fraser Cain: Yeah, the YouTube creator space in Los Angeles, and Pamela’s attic in southern Illinois. So, you can all sort of watch along as we use it. So, we’re actually in a green room at the creator space, and hopefully you’ll be able to see a little bit of behind-the-scenes when we post some more videos later on. So, what’s happening there?
Dr. Pamela Gay: Well, I think the most exciting thing is, it’s harvest season, which means the leaves the leaves are yellow and the pollen levels are high, but the apple cider is in large amounts.
Fraser Cain: So, a couple of quick announcements. One announcement is that, for those who watched, we did a special episode of the Weekly Space Hangout just on Friday, to talk about comet ison, and this is actually a collaboration with the Discovery Channel, Discovery Channel Canada, and the Discovery Science Channel out of the United States, and they’re gonna be doing, they’re gonna be broadcasting a special about ice on December 4, 2013, so if you get a chance to watch it, I’m in it, Pamela’s in it, and sort of one of our hangouts is in it, and it should be a lot of fun. So, keep an eye out for that.
Dr. Pamela Gay: Nicole Gugliucci joined us.
Fraser Cain: Nicole Gugliucci and David Dickinson. Yeah, it was a really good time. So, the second thing that I want to promote is that we’re doing a photo contest on comet ison, and so this is a promotion that’s being done with Universe Today and Space Weather and O.P.T. Telescopes. I know you’re good friends with the folks at O.P.T Telescopes, Pamela.
Dr. Pamela Gay: Yeah, they’re great.
Fraser Cain: Great people. So, the way this is gonna work is that we want you to capture your best images of comet ison, and then enter daily for prizes, and O.P.T. is giving out more than $10 thousand worth of equipment and telescopes and amazing gear from lots of different prize donors: Solostron, Explore Scientific, Takahashi, Vixen. It’s pretty great. So, what you’ll wanna do, we’ve got a poster on Universe Today and we’ll put it into the show notes or you wanna go to Space Weather, or you wanna go to O.P.T. Telescopes and look for the comet ison photo contest, and you can enter this. The contest began on November 1st and it ends on December 31, 2013, and the winners will be announced on January 7th.
So, lots of time. So, take some great pictures of comet ison, post them, and get a chance to win some amazing prizes. So, thanks to O.P.T., thanks to Space Weather, and from us at Universe Today.
Female Announcer This episode of Astronomy Cast is brought to you by 8th Light, Inc. Eighth Light is an agile software development company. They craft beautiful applications that are durable and reliable. Eighth Light provides discipline software leadership on demand and shares its expertise to make your project better. For more information, visit them online at www.8thlight.com. Just remember, that’s www dot the digit 8 T-H L-I-G-H-T dot com. Drop them a note. Eighth Light, software is their craft.
Fraser Cain: So, our sun isn’t just a terrifying ball of white-hot plasma. It’s actually a lot more complex. It’s got layers. Today, we’re gonna peel back those layers and learn more about the sun from the inside out. Now, Pamela, we’ve talked about the sun in the past. It’s a, as I’ve said, it’s a terrifying ball of plasma just located a mere 149 million kilometers away from us, so, what should we know about the sun?
Dr. Pamela Gay: It’s a terrifying, giant ball of plasma.
Fraser Cain: Yeah, that really is all we need to know. But for those – it’s actually pretty funny. So, like I said, Jason Harmer, the guy who I do these videos with, and whenever we do episodes on the sun, he turns to me and goes, “Does it ever freak you out, the fact that we’ve got this gigantic ball of plasma that’s only a few tens of millions of kilometers away? Isn’t that kind of scary?” It’s not scary.
Dr. Pamela Gay: It’s basically 150 million kilometers, if you get the rounding correctly. So, it’s not quite as bad as you’re letting on. We can add another million.
Fraser Cain: It’s far enough away, and in fact, without the sun we wouldn’t have any life at all.
Dr. Pamela Gay: We’d be dead.
Fraser Cain: So, thank you, sun. We appreciate everything you’ve ever done for us. But, when we see, we imagine the sun, we just imagine this great big ball of just roiling plasma, but in fact, if we sort of peel back these layers, what’s [crosstalk] [00:05:33]
Dr. Pamela Gay: It’s worse than that.
Fraser Cain: It’s worse than that. So, let’s kind of, as I mentioned, let’s start from the inside out. What if we were to get right down to the very heart of the sun, what would we find?
Dr. Pamela Gay: So, if we go all the way down to the solar core where the juicy stuff is happening, we’d be able to experience first-hand as our bodies completely disintegrate and becomes nothing except for a plasma of atoms, we’d be able to experience densities of 160 grams per cubic centimeter, which is kind of a lot for a cubic centimeter. In fact, it’s ten times denser than lead. At this density and at the temperature of the heart of the sun, which is 15 million degrees Kelvin, which is 27million degrees Fahrenheit, at those temperatures, is possible for nuclear reactions to occur. It’s possible for neutrons and protons to come together to form a heavy nuclei of hydrogen that collide together, eventually leading helium, working their way up.
We have a good proton-proton chain going. And all of these things lead to vast amounts of gamma rays and neutrinos being released, and it’s –
Fraser Cain: It’s – I was gonna say, it’s kind of amazing that you’ve got, at the very center of the sun, this environment that is denser than lead. Because I don’t think that’s sort of what you would imagine; that you would imagine the very core of the sun, you imagine this intense heat and intense pressure, but in fact, it’s as dense as lead.
Dr. Pamela Gay: And I think this is one of those things where I’ve just been studying astronomy so long that the, that I actually came at it from the exact opposite – it’s only 10 times denser than lead? That seems not that dense at all because I think white dwarfs, it’s like a tablespoon is the weight of an elephant –
Fraser Cain: Right. A neutron star, the weight of a mountain
Dr. Pamela Gay: Yeah, yeah. Here we have a tablespoonful that’s only 160 grams, but, yeah, it takes intense pressures, intense energies, in order to get this sort of nuclear fusion taking place. One of the reasons we can’t readily get sustainable fusion here on Earth is, well, the sun has gravity to create this amazing pressure and the ideal gas law and other gas laws to happily provide the high temperatures that comes out of taking a large ball of mass and crushing it with gravity. It’s this combination of pressure from the gravity and temperature from just the way materials work that drives these nuclear reactions.
The amount of energy it takes to artificially create that sort of temperature/energy combination is something that we just can’t generate here on Earth so that the amount of energy you get out of the reaction is greater than the amount of energy you put in to the reaction.
Fraser Cain: Right, right. So, how big is this region of the sun? How much of the sun is this, the core?
Dr. Pamela Gay: It’s only the inner few percentage of the core, so, it’s, or of the sun. The vast bulk of the sun actually goes into regions that radiatively transfer this energy to the surface, that convectively transfer this energy to surface. The inner section, it’s not a lot of the sun that’s able achieve the needed pressures and densities for all that vast, nasty amount of convec – nasty amount of fusion to be taking place.
Fraser Cain: Right, and I think, again, that’s something that a lot of people are quite surprised about: the fact that the part of the sun that’s actually generating the energy is really just this few percent. That the rest of the sun doesn’t have this density and pressure and heat required to allow this fusion to take place.
Dr. Pamela Gay: And to give you scale of the size of the sun because, again, this is one of those things that people, I think, really struggle with, the sun’s basically 700,000 kilometers in radius. So, we’re looking at a couple of times the Earth/moon diameter when you start looking at the – or Earth/moon distance – when you start looking at the size of the sun. But the Earth is still awful small. Those sun spots that you see on the sun, those sun spots are the size of the Earth, in many cases.
Fraser Cain: Right. Okay, so I think we’ve got a pretty good idea at the core. We’ve got these intense pressures, intense temperatures, we’ve got this proton-proton fusion reaction that’s creating gamma rays, and I think one of the, one of those amazing stories is how those gamma rays have to get from there to the surface, but –
Dr. Pamela Gay: And how they actually transfer in what they are. It’s not like the Earth is getting bombarded with pure gamma rays coming at us from the sun. If that were the case, we would, again, probably be dead.
Fraser Cain: So that leads us to the next layer of the sun.
Dr. Pamela Gay: Yes. So, beyond that layer, and the way the sun works, and Chandrew Seycars is one of the scientists who worked really hard to figure this out. Before that, people had been looking at all sorts of different matters and Addington came forward and was like, “No, no, no, no, no. It’s nuclear reactions guys.” Not coals, not chemistry, not any normal chemical reaction. We’ve got fusion. And then Chandrew Seycars wrote a really awesome, small book on stellar atmospheres that details all of the math in a beautifully straightforward way, and I highly recommend trying to find this old Penguin book at a used bookstore, if you can.
The way it works is, as you move outward from the center of the sun, there’s clearly less stuff pushing down on you. So, the density decreases as you move from the center of the sun out towards the edges, but at the same time that the density is decreasing, the temperatures also decreasing until physics breaks when you get to the outermost layers of the sun, which we’ll get to. So, as you move outward you eventually reach this point where it’s no longer dense enough, and it’s no longer hot enough for these fusion reactions to continue. At that point, you end up with a level of radiation transfer taking place, an envelope of radiation transfer. In this area, the light that’s passing out, those gamma rays, those high-energy photons, as they pass out they’re getting absorbed and reemitted and absorbed and reemitted, over and over and over again. But, all the material that is still extremely dense, it’s still extremely opaque, and each time one these poor, innocent photons of deadly energy levels gets reemitted, it is going to excite an electron often to the point of escape.
But, the electron doesn’t stay at that high energy level, and if it’s a free electron that absorbs the photon it doesn’t stay at that high energy level. Those extremely energetic electrons, they end up releasing energy in lower-energy photons, so, one gamma ray absorption might lead to, well, over the course of time, a thousand lower energy photons being given off. It’s these lower energy photons then move out, get reabsorbed, move out, get reabsorbed. And this just goes on and on and on and it, depending on who’s paper you read, takes anywhere from 17,000 to a million years for that photon to escape.
More realistically, it probably takes order of 40,000 years for a produced gamma ray photon to end up becoming a thousand lower energy photons that finally escape the sun.
Fraser Cain: So, I imagine that with the density of the material in this radiative zone, the journey that these photons have to take from the one particle to the next one, is really short. It’s literally nanometers to then, to get reabsorbed. I mean, you can’t go very far, right?
Dr. Pamela Gay: But it’s also completely random walk. So, photon might start out heading straight towards the surface, but then it gets absorbed and gets resent out on its journey in a completely random direction. It might head towards the surface, head towards the center; head to the left, head to the right, head in any possible direction and in three-dimensional space there’s a whole lot of directions to choose from.
Fraser Cain: Right, but just imagine that – if you had one of these photons and you just let it go in one direction n, it would go for a 1,000,000 years or 100,00 years, let’s say 100,000 years, it would be 100,000 light years away. It would’ve essentially crossed the entire galaxy if it was allowed to just go on a straight line. But because it’s having to do this completely random walk, and it might get almost to the surface or almost out of the radiative zone, and then get turned around and go back the other way, and just bounce around inside until finally, randomly makes it outside of the radiative zone.
Dr. Pamela Gay: And this is called Brownian motion or drunk-walking – Brownian motion’s a much more politically correct term – and, yeah, it’s a completely random path that, luckily, on average leads to the photon getting to escape the sun. And as it continues to go it eventually reaches the point where the amount of time that it takes for each of these progressively lower and lower energy photons to get emitted gets longer. This sounds like, “Okay, yeah, and…?” Well, when an atom’s holding on or an electron is holding on to that higher energy photon, it’s at a higher energy state. That’s another way of saying that sucker’s hotter.
In a gas, which is what our sun is, it’s another way of looking at hot ball of deadly plasma, our gaseous sun, it becomes a lava lamp at a certain point because these atoms are holding on to that higher energy. They’re staying hot and you’ll end up with a blob of this hotter gas, this lower density gas and lower density material floats. If it holds on to the heat long enough, that blob is going to end up rising to a cooler region, radiative transfer, then giving off its heat to its surroundings and sinking back down. So, you end up with this zone where, because the radiative process isn’t happening fast enough, you instead, end up with a convective process.
And that’s the next region of the sun, the convective envelope.
Fraser Cain: Nice segue. So, you’ve got the core, outside of that is this radiative zone, and so, almost like, when you think about it, it’s like each zone is this transition between what just isn’t possible anymore. When I think about the radiative zone, it really begins where the core, where you just can’t fuse hydrogen anymore and then you‘ve gotta do something different. And then you get to this point where it’s almost like it’s the opposite, where the convectionable will actually function. Below that, convection isn’t gonna work and that’s’ why you have this radiative zone in between. So, what are we seeing inside the convective zone?
Dr. Pamela Gay: In the convective zone, you actually have blobs of hotter material rising and blobs of cooler material sinking. We can actually start to see this when we look at the sun in the correct filters and with high enough resolution you see these solar granules boiling on the surface. These are the convective bubbles. In terms of chemistry and gas laws and thermal dynamics, this is very similar to what happens when you have oil on your stove and you start to see these cells of material rising and sinking in your oil on the stove. Our sun is doing very similar things that you can see when you start looking at it high resolution in h-alpha filters.
Fraser Cain: This is something that amateurs have really gotten good at bringing out in the last few years. If you’ve really seen that, with the kind of astrophotography that people are doing and the kinds of these h-r filters, you can really see these great granules on the surface or these bubbles of plasma are rising to the surface. It feels like they’re popping and then the material’s sinking back down.
Dr. Pamela Gay: It’s actually a very beautiful process. Just the way you described it actually describes how we write the software to start understanding the centers of stars, how we understand why big stars act differently from little stars. We write models that say you have a star with this radius, with this mass, with this density calculated at each layer, and as you calculate the densities you get to the temperatures and vice versa. It’s multiple equations you’ll have to solve together. When we write the software and we step through the different layers, we see what physics is allowed and which physics isn’t and how the energy is transferred and we balance the equations until we balance a star.
It’s the type of project that’s just within reach of an undergraduate astronomy student and is the standard homework assignment of the graduate student. But to do it fully in three dimensions starts to become a lifetime career.
Fraser Cain: Yeah, you’ve described this as the hardest science you know of.
Dr. Pamela Gay: Well, it’s because you have to start dealing with magnetospheres, this is where you get into magneto hydrodynamics, you have to deal with mass loss with cool stars, you have to deal with dust formation. It’s a complicated process that requires physics and chemistry and nuclear physics all getting balanced one against the other.
Fraser Cain: I think as the stars, we’ve talked about dwarf stars and other kinds of stars and I don’t wanna sort of go too deeply on this episode, but I think as a star gets bigger those zones change, and the same thing is as a star gets smaller, right?
Dr. Pamela Gay: Right. In fact, in some cases you can actually get a fully convective star. This is one of the really neat things about the smallest red dwarf stars. They’re still stars. Save a fully convective envelope, they never have sufficient nuclear burning that you end up with that radiative zone. And because they’re fully convective, they end up burning the entirety of the composition of the star through the nuclear processes. In a star like the sun, for the most part atoms that are in the outer layer of the envelope, they’re happy to stay there. So they’re not going to end up being part of the nuclear burning that goes on in the center.
Fraser Cain: Right. So, it’s like with our sun, we’ve got that radiative layer, it’s like a firewall that stops the convective material from getting down and mixing in the core. So, you’d love to have that because then the sun would last for trillions for years, but it can’t.
Dr. Pamela Gay: And so, instead what we see is with the material that’s down in the core, at a certain point you’re gonna run out of the hydrogen. When that happens, the star’s initially going to crush down because you don’t have the light pressure pushing outwards, supporting the outer layers. As it starts to crush down, you’re going to end up with a shell around that core flashing into hydrogen burning eventually. You’re gonna end up with helium burning in the core, not necessarily in that order. It’s a dynamic process where you’re constantly rebalancing these equations and looking to see, “Well, at this temperature and density, what nuclear possibilities exist?”
Then looking at the amount of light that’s giving off, measuring how much light pressure in the equations is predicted, figuring out how that balances out against the energy being released. You’re constantly, well, dealing with light pressure out, gravitational pressure in, density, temperature and that mass, which doesn’t bother to stay constant because you have mass loss going on throughout the lifetime of the star.
Fraser Cain: So, then we’ve got this sort of the three major internal layers of the sun. So, then we’ve got the surface.
Dr. Pamela Gay: Well, and the surface is one of the most annoying parts of the sun. At the top you have the photosphere. This is that nice 6,000 Kelvin degree surface that we see when we look at the sun. My brain is happy to consider this a nice, cool temperature because I’ve been doing astronomy for far too long. This photosphere, when you use a white light filter to look at the sun, that’s that area that you’re looking at that contains the even cooler sun spots that are Earth-sized and indicate the places where you have magnetic fields looping through the surface creating their differently-polarized spots on the surface.
Fraser Cain: So, we’ve got the photosphere, this is where we see that the color, the photons that have finally been able to escape out into space and this is what they look like. The final result of it.
Dr. Pamela Gay: Then the sun gets confusing. Everything that we’ve done up until this point is easy to simplify and explain at a level that you can get an undergraduate to write computer software for. Everything above that, it’s –
Fraser Cain: It’s weird.
Dr. Pamela Gay: Yeah, so for reasons that we don’t know, although there’s usually a new paper every double AS that gets a big press release, the chromosphere starts to get hotter at 7,000 degrees Kelvin and as you go even further out to the corona, which is that beautiful gas flowing out to space bit that you see during a solar eclipse, and that every little kid draws when they draw the sun, that corona is not a cool, refreshing beverage. It’s actually 3 million Kelvin plasma that is deadly.
Fraser Cain: So we’re into the atmosphere now of the sun. We’ve got that, the surface, and then as we move up, is the corona kinda that first layer of the atmosphere?
Dr. Pamela Gay: No, that would make more sense. The photosphere is the first layer-
Fraser Cain: Right, we’re talking about the photosphere, which is the surface, so above that –
Dr. Pamela Gay: Is the chromosphere, which is that 7,000 degrees Kelvin –
Fraser Cain: What distinguishes the chromosphere?
Dr. Pamela Gay: You start to get a transition at just how dense things are. As you go to lower and lower densities, the photosphere’s kind of that nice, contiguous band of red glow-y stuff above the granulations that you see with h-alpha. Then above that is the chromosphere and then above that is the corona, which is where you see the coronal mass ejections, where you see the coronal loops, and all of those bits of gas that are streaming in all directions.
Fraser Cain: You said that the corona is hotter than the surface of the sun by a long shot.
Dr. Pamela Gay: Yeah. It sorta goes from 6,000 Kelvin to 3 million. Reasons for this goes to the energy being released when the magnetic field lines break and rearrange themselves, to pie-in-the-sky ideas and it’s, no one’s completely certain why, but it has to do with somehow energy’s getting released at this high level.
Fraser Cain: Yeah, I’ve heard there’s waves moving through the corona and could be releasing energy that way, or they’ve got –
Dr. Pamela Gay: They talk about acoustic waves colliding, all sorts of different things. Now, for better or worse, at least this is an extraordinarily a low density environment, so if you were to take the Starship Enterprise and fly it into the corona, because it’s so low density, the energy transfer isn’t particularly effective and you’re not going to get a lot of transfer to your ship’s hull. It will kill you eventually, just not quickly.
Fraser Cain: Right. It’s not the same as you attempting to fly your spaceship through the core of the sun. We’ve got the –
Dr. Pamela Gay: You’d just die.
Fraser Cain: Yeah, you’ve got the pressure and the temperature. In this case you’ve just got the temperature, you don’t have the pressure. Okay, so we’ve got the corona. Is there anything outside of the corona?
Dr. Pamela Gay: That’s pretty much it. What we do pay attention to is, well, you wouldn’t necessarily call it a layer of the sun, you have the solar wind which is streaming material throughout our entire solar system, pushing out, defining where the boundary of our solar system is, you have coronal mass ejections which periodically fling large bursts of material toward the planet Earth and other points in the solar system, to be fair. So those are the layers of the sun, but that doesn’t mean that the sun keeps its hands to itself. It does like to wreck havoc on the rest of the solar system and push out define our boundaries and the boundaries between us and interstellar space.
Fraser Cain: Yeah, I guess you could sort of imagine the heliosphere is like the outer layer of the sun, right? The point at which the solar wind hits the cosmic wind and the interstellar wind and they kind of balance each other out. That’s really the end of the influence of the material streaming off the sun, but not necessarily its gravity.
Dr. Pamela Gay: Right.
Fraser Cain: Cool. All right. Well, thank you very much, Pamela.
Dr. Pamela Gay: It’s my pleasure. Thank you so much, Fraser.
Male Announcer: Thanks for listening to Astronomy Cast, a non-profit resource, provided by Astrosphere New Media Association, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astronomycast.com. You can email us at firstname.lastname@example.org. Tweet us @Astronomycast. Like us on Facebook or circle us on Google+. We record our show live on Google+ every Monday at 12:00 p.m. Pacific, 3:00 p.m. Eastern, or 2,000 Greenwich Mean Time. If you miss the live event, you can always catch up over at Cosmoquest.org. If you enjoy Astronomy Cast, why not give us a donation? It helps us pay for bandwidth, transcripts, and show notes.
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Duration: 31 minutes