1 00:00:00,000 --> 00:00:04,658 Okay, so we've solved the fundamental problem that the sun posed. 2 00:00:04,658 --> 00:00:08,080 We now know what the sun's source of energy is. 3 00:00:08,080 --> 00:00:14,122 Now we trust our models and we try to use this understanding to figure out what we 4 00:00:14,122 --> 00:00:17,259 can about how sun, how the Sun is constructed. 5 00:00:17,259 --> 00:00:21,253 What its structure is. How do we study the structure of the Sun. 6 00:00:21,253 --> 00:00:26,262 Well, lots of numerical modeling. We measure reaction rates and properties 7 00:00:26,262 --> 00:00:29,813 of hydrogen here on Earth, and then try to apply them. 8 00:00:29,813 --> 00:00:34,188 But it's hard to simulate the conditions of the Sun here on Earth. 9 00:00:34,188 --> 00:00:37,540 another important. Measurement two is helioseismology. 10 00:00:37,540 --> 00:00:42,157 Just as we infer the interior structure of the Earth from the propagation of 11 00:00:42,157 --> 00:00:46,313 sound waves through the Earth. Sound waves propagate through the very 12 00:00:46,313 --> 00:00:50,643 compressed hydrogen that is the Sun. And the properties of that propagation 13 00:00:50,643 --> 00:00:53,817 can be used to infer interior properties of the sun. 14 00:00:53,817 --> 00:00:56,761 How do we generate sunquakes? Well, we don't need to. 15 00:00:56,761 --> 00:00:59,358 The Sun oscillates. The Sun is a fluid system. 16 00:00:59,358 --> 00:01:02,533 It oscillates. And what we do is we measure the Doppler 17 00:01:02,533 --> 00:01:05,169 shift, of radiation from, spectral lines 18 00:01:05,169 --> 00:01:09,278 actually from particular points on the sun and we notice that, that 19 00:01:09,278 --> 00:01:12,918 when we plot this, that the sun is wobbling in particular ways. 20 00:01:12,918 --> 00:01:17,321 And studying the frequency and, spatial structure of these wobbles, these 21 00:01:17,321 --> 00:01:20,373 resonances. The sun is basically ringing like a bell. 22 00:01:20,373 --> 00:01:23,132 And, you can try to hear the shape of a bell. 23 00:01:23,132 --> 00:01:27,594 And that's what we're trying to do. And that's one of the main tools we have 24 00:01:27,594 --> 00:01:32,995 for, verifying our modelling predictions for the structure of the interior of the 25 00:01:32,995 --> 00:01:35,364 sun. And so this is what, how we, we, this 26 00:01:35,364 --> 00:01:39,633 plus modeling is how we understand everything that happens between the core, 27 00:01:39,633 --> 00:01:43,733 that inner, densest part, and hottest part, where fusion is ongoing, and the 28 00:01:43,733 --> 00:01:48,115 photosphere, which is the name we give the outside of the sun, the part that we 29 00:01:48,115 --> 00:01:50,128 see. So the part that we see is the 30 00:01:50,128 --> 00:01:54,860 photo-sphere it makes the photons and now you know, we have our understanding of a 31 00:01:54,860 --> 00:01:58,091 spherical object. It's a fluid object, it's held together 32 00:01:58,091 --> 00:02:00,630 by gravity and it's held up against gravity. 33 00:02:00,630 --> 00:02:05,500 By the intense heat and pressure and radiation pressure generated by all that 34 00:02:05,500 --> 00:02:08,810 energy being released, in the interior in the core. 35 00:02:08,810 --> 00:02:13,306 And, like any object in equilibrium, density and pressure and likewise 36 00:02:13,306 --> 00:02:18,364 temperature increase as you move from the surface of the sun, and the core is the 37 00:02:18,364 --> 00:02:23,235 hottest and densest, and as you move out, it becomes the sun is cooler and less 38 00:02:23,235 --> 00:02:26,545 dense, as one expects for hydro static equilibrium. 39 00:02:26,545 --> 00:02:29,480 This is what holds the sun up against collapse. 40 00:02:29,480 --> 00:02:34,195 That due to gravity is the heat coming in from the core, and here is what we 41 00:02:34,195 --> 00:02:37,173 understand in terms of the structure of the Sun. 42 00:02:37,173 --> 00:02:42,137 So the internal core of the Sun occupies the regions to within a quarter of the 43 00:02:42,137 --> 00:02:45,240 solar radius. Solar radius is the radius of the 44 00:02:45,240 --> 00:02:49,496 Photosphere by convection, and that's about 696,000 kilometers. 45 00:02:49,496 --> 00:02:54,714 And to with, and to, from the center, about a quarter of a solar radius out, 46 00:02:54,714 --> 00:02:58,284 that is the core. That is the region where 99% of the 47 00:02:58,284 --> 00:03:02,746 fusion power is generated. Temperatures at the center of the core 48 00:03:02,746 --> 00:03:05,973 reach about 15,000,000 or 16,000,000 kelvin. 49 00:03:05,973 --> 00:03:09,337 And at the outside of the core, there's 7,000,000. 50 00:03:09,337 --> 00:03:13,594 And by that time, fusion is becoming completely inefficient. 51 00:03:13,594 --> 00:03:17,856 The density two decreases at the center of the sun. 52 00:03:17,856 --> 00:03:22,470 The density of hydrogen is 150 times the density of water. 53 00:03:22,470 --> 00:03:27,012 And by the time you get to the outside of the core it's only twenty times the 54 00:03:27,012 --> 00:03:31,380 density of water and this region, the inner quarter solar radius of the sun 55 00:03:31,380 --> 00:03:35,864 contains 40% of the mass of the sun. This might not sound impressive until you 56 00:03:35,864 --> 00:03:40,349 remember that volume scales by radius Q. So we're talking about a 64th of the 57 00:03:40,349 --> 00:03:43,260 volume of the sun containing almost half the mass. 58 00:03:43,260 --> 00:03:47,686 This is indeed far denser than the rest of the sun, this is the region where 59 00:03:47,686 --> 00:03:52,789 hydrogen is most extremely compressed. And, importantly, the core is in a state 60 00:03:52,789 --> 00:03:58,567 of equilibrium in the following sets. If fusion rates in the core decrease, for 61 00:03:58,567 --> 00:04:01,324 example. Then, the core is producing less energy. 62 00:04:01,324 --> 00:04:04,598 There is less radiation pressure and less thermal pressure. 63 00:04:04,598 --> 00:04:07,304 The core cools a little. What this does under the. 64 00:04:07,304 --> 00:04:11,483 Is that, then, the force of gravity. The weight of the outer layers of the sun 65 00:04:11,483 --> 00:04:15,497 compresses the cores a little bit. And when the core compresses, densities 66 00:04:15,497 --> 00:04:18,851 and temperatures increase. Densities, because it's compressing. 67 00:04:18,851 --> 00:04:22,535 temperatures, because it's now heated by Kelvin Helmholtz heat, 68 00:04:22,535 --> 00:04:25,654 it's contracting. This in turn, increasing the temperature 69 00:04:25,654 --> 00:04:28,111 and the density, increases the rate of fusion. 70 00:04:28,111 --> 00:04:30,677 So there, there's this self-regulating thing. 71 00:04:30,677 --> 00:04:34,827 If the, on the other hand, the rate of fusion were to increase, the core would 72 00:04:34,827 --> 00:04:38,759 heat up, expand a little the expansion would cool it down, and this would 73 00:04:38,759 --> 00:04:43,529 decrease the rate of fusion. so the rate of fusion is in fact controlled by the 74 00:04:43,529 --> 00:04:45,991 weight of the outer atmosphere of the sun. 75 00:04:45,991 --> 00:04:50,389 In other words, the luminosity is determined by mass of the outer layers of 76 00:04:50,389 --> 00:04:54,728 the sun pushing down on the core, determining how much of the sun is going 77 00:04:54,728 --> 00:04:58,011 to be compressed. When we talk about other stars, this is 78 00:04:58,011 --> 00:05:01,998 going to be very important, the luminosity of the star is determined 79 00:05:01,998 --> 00:05:06,396 essentially by it's total mass, that determines how much of the hydrogen is 80 00:05:06,396 --> 00:05:10,090 compressed enough in the core and hot enough to undergo fusion. 81 00:05:10,090 --> 00:05:13,591 This is the internal, this is sort of the engine of the sun. 82 00:05:13,591 --> 00:05:18,575 This is where the energy gets produced. And then there is what you might call the 83 00:05:18,575 --> 00:05:22,967 analog of the mantle of the Sun, the next layer labeled number two here. 84 00:05:22,967 --> 00:05:27,121 It's called the radiation zone. It extends from a quarter of the, solar 85 00:05:27,121 --> 00:05:30,800 radius out up to about 7.. Temperatures in the, in the radiation 86 00:05:30,800 --> 00:05:35,666 zone decrease from seven million at the outer boundary of the core to only two 87 00:05:35,666 --> 00:05:39,976 million kelvin at the outside of the radiation zone, densities 88 00:05:39,976 --> 00:05:45,841 to decrease, down by the, by the, the outside of the convection zone, the sun 89 00:05:45,841 --> 00:05:49,162 has thinned out to nearly the density of water. 90 00:05:49,162 --> 00:05:52,657 It's merely, but remember, this is, hydrogen gas at 91 00:05:52,657 --> 00:05:56,563 that density. throughout this region, temperatures are 92 00:05:56,563 --> 00:05:59,873 in the millions of Kelvin. There are no atoms here. 93 00:05:59,873 --> 00:06:04,705 The interior of the sun is a plasma. In other words, nuc-, nuclei, mostly 94 00:06:04,705 --> 00:06:07,948 hydrogen nuclei. And, electrons move completely 95 00:06:07,948 --> 00:06:12,450 independent of each other. There's no coherent atomic structure that 96 00:06:12,450 --> 00:06:17,149 survives at these temperatures. The thermal energies of things are much 97 00:06:17,149 --> 00:06:19,930 higher than the binding energies of atoms. 98 00:06:19,930 --> 00:06:23,764 And this region is characterized by the fact 99 00:06:23,764 --> 00:06:28,152 that the way energy is transferred, so of course, there's a net transfer. 100 00:06:28,152 --> 00:06:32,788 Energy is produced in the core, and it flows out, and then is radiated out to 101 00:06:32,788 --> 00:06:35,817 heat us. And the mode of heat transfer in this 102 00:06:35,817 --> 00:06:40,012 part of the sun is radiation. Now, this does not mean that this part of 103 00:06:40,012 --> 00:06:42,825 the sun is transparent, it's radiation diffusion. 104 00:06:42,825 --> 00:06:47,220 In other words, high energy photons produce gamma rays produced in the core. 105 00:06:47,220 --> 00:06:50,996 Are admitted they are immediately absorbed by interaction with the charged 106 00:06:50,996 --> 00:06:52,513 particles, in the plasma. 107 00:06:52,513 --> 00:06:56,386 And then readmitted and reabsorbed and readmitted and reabsorbed, 108 00:06:56,386 --> 00:07:01,093 an average photon, or the energy of an average photon takes 170,000 years to 109 00:07:01,093 --> 00:07:05,503 travel the distance from the core to the outer edge of the radiation zone. 110 00:07:05,503 --> 00:07:08,125 So this is not moving at the speed of light. 111 00:07:08,125 --> 00:07:12,296 This is some very slow diffusion. When you have charged particles in a 112 00:07:12,296 --> 00:07:16,527 plasma, then there's a, a strong interaction between the charges and the 113 00:07:16,527 --> 00:07:19,626 radiation. And radiation does not penetrate a plasma 114 00:07:19,626 --> 00:07:23,440 the way it penetrates a neutral gas like the light in this room. 115 00:07:23,440 --> 00:07:28,748 But, the mode of energy transfer is radiation, diffusion, and, eh, that means 116 00:07:28,748 --> 00:07:34,336 it that, that energy moves up, but the plasma itself, is not in any, under going 117 00:07:34,336 --> 00:07:39,226 any macroscopic, global motion. This changes in the outer mantle of the 118 00:07:39,226 --> 00:07:45,273 sun, that's the convection zone, between. .7 of the solar radius all the way out to 119 00:07:45,273 --> 00:07:50,597 the photosphere and here temperatures decrease very rapidly from two million 120 00:07:50,597 --> 00:07:55,782 degrees in the interior down to the surface temperature of the sun that we 121 00:07:55,782 --> 00:08:00,345 measure, 5,777 Kelvin. densities decrease from about the density 122 00:08:00,345 --> 00:08:04,194 of water to a fifth of that density, still pretty 123 00:08:04,194 --> 00:08:09,536 densed for hydrogen gas its still compressed by gravity and the mode of 124 00:08:09,536 --> 00:08:12,408 heat transfer in this region is convection. 125 00:08:12,408 --> 00:08:18,618 In other words in this region the plasma becomes opaque essentially and the most 126 00:08:18,618 --> 00:08:24,721 efficient mode of heat transfer becomes, hydrogen that is heated at the inner, 127 00:08:24,721 --> 00:08:29,991 bottom of the convection zone by the incoming radiation from the radiation 128 00:08:29,991 --> 00:08:35,260 zone rises, and then cools when it reaches the surface, and then sinks again 129 00:08:35,260 --> 00:08:40,178 so you get these convection cells. This is responsible for the sort of 130 00:08:40,178 --> 00:08:45,658 granular appearance of the photosphere which is very evident in the following 131 00:08:45,658 --> 00:08:49,584 movie. Here we see very high resolution image of 132 00:08:49,584 --> 00:08:54,257 the solar surface. Details as small as 500 kilometers across 133 00:08:54,257 --> 00:08:59,878 can be measured and for proportion. We've indicated here the size of the 134 00:08:59,878 --> 00:09:03,702 earth so that you know the size of the object we're looking at. 135 00:09:03,702 --> 00:09:08,740 We're looking near a sunspot and you see the sort of granular shape of the surface 136 00:09:08,740 --> 00:09:11,290 of the sun. This characterizes the way the 137 00:09:11,290 --> 00:09:14,932 photosphere looks. In the center of a granule you imagine is 138 00:09:14,932 --> 00:09:20,031 where warm gas is welling up and then on the edges, between the granules is where 139 00:09:20,031 --> 00:09:25,251 cooler hydrogen is seeping back down and because it's cooler we observe a lower 140 00:09:25,251 --> 00:09:27,700 temperature. Take t to the fourth, from Stephan 141 00:09:27,700 --> 00:09:31,243 Boltzman, means those region radiates less which is why they look dark. 142 00:09:31,243 --> 00:09:35,090 They are still very warm and they are radiating but they are radiating less. 143 00:09:35,090 --> 00:09:38,829 Then the surroundings. And it's interesting to watch the 144 00:09:38,829 --> 00:09:43,236 dynamics of this process. We see sort of, constantly changing 145 00:09:43,236 --> 00:09:47,443 convection patterns. There's not a stable convection pattern 146 00:09:47,443 --> 00:09:51,250 on the sun, but there's these oozing changing patterns. 147 00:09:51,250 --> 00:09:55,059 So we've gotten all the way out to the outside of the sun. 148 00:09:55,059 --> 00:09:59,656 But of course, like the earth, the sun extends be, beyond its surface. 149 00:09:59,656 --> 00:10:03,597 There is more of the sun outside the photosphere the sun. 150 00:10:03,597 --> 00:10:06,552 What you might call the, the sun's atmosphere. 151 00:10:06,552 --> 00:10:10,260 There's several layers. The density of gas decreases 152 00:10:10,260 --> 00:10:15,364 dramatically, but it turns out that. So there's not much there in the solar 153 00:10:15,364 --> 00:10:18,399 atmosphere. The mass of the sun, is below the 154 00:10:18,399 --> 00:10:19,710 photosphere. But 155 00:10:19,710 --> 00:10:24,217 The temperature it turns out because of various processes some of which we'll 156 00:10:24,217 --> 00:10:28,667 discuss increases with altitude the first lower layer of the atmosphere, it's 157 00:10:28,667 --> 00:10:32,251 called the chromosphere. That's the layer up to an altitude of 158 00:10:32,251 --> 00:10:36,759 2000 kilometers above the photosphere. And in the chromosphere the temperature 159 00:10:36,759 --> 00:10:41,209 rises to about 50,000 kelvin but the density decreases by seven orders of 160 00:10:41,209 --> 00:10:43,520 magnitude. So there's not much gas there. 161 00:10:43,520 --> 00:10:46,514 It's very hot. But we don't see the chromosphere. 162 00:10:46,514 --> 00:10:51,204 We're blinded by the photosphere even thought the chromosphere is hotter, 163 00:10:51,204 --> 00:10:56,343 because the T to the fourth is outweighed by the fact that there's basically 164 00:10:56,343 --> 00:11:01,226 nothing there compared to that. To observe the chromosphere we use the 165 00:11:01,226 --> 00:11:04,760 fact that in the chromosphere there are hydrogen atoms. 166 00:11:04,760 --> 00:11:09,899 hydrogen atoms emit a characteristic frequency spectral line at 656.28 167 00:11:09,899 --> 00:11:12,020 nanometers. It's the H alpha line. 168 00:11:12,020 --> 00:11:16,303 We've talked about it before and will again and so when you take pictures of 169 00:11:16,303 --> 00:11:20,212 the sun using the filter that allows only that wavelength through, you're 170 00:11:20,212 --> 00:11:25,078 essentially taking a picture of the. photosphere and this movie is indeed such 171 00:11:25,078 --> 00:11:28,604 a picture. Those dark finger like shapes are called 172 00:11:28,604 --> 00:11:31,934 spicules. They reach thousands of kilometers up out 173 00:11:31,934 --> 00:11:37,158 of the atmosphere of the sun and their precise dynamics in structure are not 174 00:11:37,158 --> 00:11:41,010 well understood. But again we get this image of broiling 175 00:11:41,010 --> 00:11:44,210 atmosphere of the sun and we'll see more of that. 176 00:11:44,210 --> 00:11:49,530 Above the Chromosphere the temperature jumps very rapidly, as you enter the 177 00:11:49,530 --> 00:11:53,005 corona. And the temperature jumps from the 50,000 178 00:11:53,005 --> 00:11:57,120 in the, at the top of the Chromosphere to 2,000,000 Kelvin. 179 00:11:57,120 --> 00:12:02,108 This is not a fusion going region. The density is down to ten to the minus 180 00:12:02,108 --> 00:12:05,668 twelve. there is again virtually nothing there 181 00:12:05,668 --> 00:12:11,991 and this region extends as you can see in this beautiful coronagraph out to one or 182 00:12:11,991 --> 00:12:16,108 two solar radii. So the blacked out region, a coronagraph 183 00:12:16,108 --> 00:12:21,917 is a telescope that blacks out the blinding illumination of the photosphere 184 00:12:21,917 --> 00:12:27,578 so that you can see the rest. another way to see the corona is wait for 185 00:12:27,578 --> 00:12:31,770 the moon to play the role of a coronagraph and black out. 186 00:12:31,770 --> 00:12:36,790 The central disc of the sun and during the total eclipse indeed, people take 187 00:12:36,790 --> 00:12:40,984 beautiful pictures of the corona, you can find many of them online. 188 00:12:40,984 --> 00:12:46,288 We see here a few spicules, these redish the two grids are spicules in the 189 00:12:46,288 --> 00:12:49,501 chromosphere. I told you that the dynamics are not 190 00:12:49,501 --> 00:12:54,063 understood, but the red color is the region, the reason that part of the 191 00:12:54,063 --> 00:12:58,947 atmosphere is called the chromosphere. because of the high temperatures we 192 00:12:58,947 --> 00:13:04,743 absorb the corona ideally in ultraviolet or X Ray wave lengths where it competes 193 00:13:04,743 --> 00:13:08,508 with the sun and where it emits most of its light. 194 00:13:08,508 --> 00:13:12,934 The fact that the temperatures are millions of Kelvin allows many of the 195 00:13:12,934 --> 00:13:17,730 particles as we did in our, little helium calculation for Earth, the fastest 196 00:13:17,730 --> 00:13:22,356 particles in the corona where they're all ionized because the temperature is 197 00:13:22,356 --> 00:13:27,224 millions of degrees can in fact escape sun, achieve escape velocity, and stream 198 00:13:27,224 --> 00:13:30,589 away from the Sun. In what we call the stream of charged 199 00:13:30,589 --> 00:13:33,927 particles. which achieve escape velocity and move 200 00:13:33,927 --> 00:13:38,767 away from the sun is the solar wind. We've talked about it's effects, this is 201 00:13:38,767 --> 00:13:43,033 where it actually comes from. And the solar corona, as we can see in 202 00:13:43,033 --> 00:13:46,153 this movie, is an active and interesting place. 203 00:13:46,153 --> 00:13:51,183 the white disc here is the photosphere, the corona graph blocks more of that. 204 00:13:51,183 --> 00:13:54,731 We see coronal mass ejections and flares 205 00:13:54,731 --> 00:13:58,255 bursting out from the sun. We'll talk about the mechanism. 206 00:13:58,255 --> 00:14:02,953 And we, interestingly see, the detector, being attacked by streams of these 207 00:14:02,953 --> 00:14:06,415 charged particles that's that sort of staticy stuff. 208 00:14:06,415 --> 00:14:10,371 Those are the streams of charged particles hitting the detector. 209 00:14:10,371 --> 00:14:15,254 And this is an image from 2001, and I believe that if we wait long enough, or 210 00:14:15,254 --> 00:14:20,014 if you watch that image till the end, you will actually see something transit 211 00:14:20,014 --> 00:14:23,414 behind the sun. But, you can look at that at your own, 212 00:14:23,414 --> 00:14:24,280 leisure later.