1 00:00:00,000 --> 00:00:02,139 Okay. We figured out when it happened. 2 00:00:02,139 --> 00:00:04,916 Now what? we're going to go back four and a half 3 00:00:04,916 --> 00:00:08,154 billion years ago. The atoms that currently comprise the 4 00:00:08,154 --> 00:00:11,567 solar system are already around in this part of the cosmos. 5 00:00:11,567 --> 00:00:15,500 How those atoms were created, where they came from, we'll think later. 6 00:00:15,500 --> 00:00:19,280 The question now is, in what form were they likely to be found? 7 00:00:19,280 --> 00:00:24,037 And, over the past few decades we have a pretty good idea of what that form is. 8 00:00:24,037 --> 00:00:28,976 the material that currently forms the solar system was probably found in a 9 00:00:28,976 --> 00:00:31,843 large molecular cloud. What's a molecular cloud? 10 00:00:31,843 --> 00:00:36,355 Oh, it's a great big collection of, as we know, mostly hydrogen with some 11 00:00:36,355 --> 00:00:41,478 contamination, traces of other elements. it's called molecular if it's cool enough 12 00:00:41,478 --> 00:00:46,417 that hydrogen atoms actually form stable bonds and we have hydrogen molecules in 13 00:00:46,417 --> 00:00:49,040 it. And inside this molecular, wait a minute. 14 00:00:49,040 --> 00:00:52,634 This is already a problem. And it's a problem that I want to pay 15 00:00:52,634 --> 00:00:56,193 attention to, because this is a problem that will haunt us through the rest of 16 00:00:56,193 --> 00:00:58,240 the class. The problem is gravity. 17 00:00:58,240 --> 00:01:03,633 And the problem is that the idea of a large cloud of hydrogen floating around 18 00:01:03,633 --> 00:01:06,536 in space, on the face of it, makes no sense. 19 00:01:06,536 --> 00:01:11,605 Because if you're an atom in the center of a cloud of hydrogen, then of course 20 00:01:11,605 --> 00:01:14,637 you're attracted to all other atoms in the cloud. 21 00:01:14,637 --> 00:01:19,338 And the net attraction on you if the cloud is reasonably symmetric might be 22 00:01:19,338 --> 00:01:20,390 zero. Fair enough. 23 00:01:20,390 --> 00:01:25,339 But if you're an atom near the edge of the cloud then clearly the net effect of 24 00:01:25,339 --> 00:01:28,123 gravity is to attract you towards the center. 25 00:01:28,123 --> 00:01:32,206 Gravity applies a pressure, a contracting pressure to every object. 26 00:01:32,206 --> 00:01:36,721 And whenever we see something in the universe that is not collapsing and 27 00:01:36,721 --> 00:01:40,310 crunching down to a point, something must be holding it up. 28 00:01:40,310 --> 00:01:45,322 What holds up a molecular cloud? Well, it turns out that a molecular cloud 29 00:01:45,322 --> 00:01:50,637 is held up simply by thermal pressures. This molecule near the edge of the cloud 30 00:01:50,637 --> 00:01:54,773 may indeed be accelerated towards the center, but on its way there, it 31 00:01:54,773 --> 00:01:59,329 encounters other molecules which have random thermal velocities, it scatters 32 00:01:59,329 --> 00:02:02,146 off of them, and eventually never gets anywhere. 33 00:02:02,146 --> 00:02:06,702 The cloud, the general shape of the cloud can sustain itself, and we can do a 34 00:02:06,702 --> 00:02:10,659 really nice order of magnitude calculation to understand what the 35 00:02:10,659 --> 00:02:14,495 conditions in a cloud must be in order for it to sustain itself. 36 00:02:14,495 --> 00:02:19,231 And the way we think about it is that if we have a cloud, then we know that the 37 00:02:19,231 --> 00:02:23,960 average kinetic energy if a molecule say, so m would be the mass of a hydrogen 38 00:02:23,960 --> 00:02:28,894 molecule and the cloud is on the order, there may be factors of two or three that 39 00:02:28,894 --> 00:02:32,280 I'm skipping, of k Boltzmann times the temperature. 40 00:02:32,280 --> 00:02:37,808 And on the other hand, the typical gravitational velocity associated to this 41 00:02:37,808 --> 00:02:43,408 cloud can be estimated from the escape velocity, if a molecule is moving with 42 00:02:43,408 --> 00:02:48,791 the escape velocity then it's not really bound to the cloud it can escape. 43 00:02:48,791 --> 00:02:54,756 And the escape velocity we computed is again on the order of newton's constant 44 00:02:54,756 --> 00:02:59,754 time the mass of the cloud. Divided by, the year distance from the 45 00:02:59,754 --> 00:03:02,230 effective center. So R might be. 46 00:03:02,230 --> 00:03:05,655 The radius of the cloud would be a typical number to enter. 47 00:03:05,655 --> 00:03:09,602 And now if this velocity and this velocity are of similar orders of 48 00:03:09,602 --> 00:03:14,130 magnitude then one can imagine that the thermal fluctuations in the cloud are 49 00:03:14,130 --> 00:03:17,091 about the same order as the gravitational pressure. 50 00:03:17,091 --> 00:03:20,981 This will tell us that the cloud can sustain itself under pressure. 51 00:03:20,981 --> 00:03:25,102 This is how molecular clouds survive. The denser a cloud, the higher its 52 00:03:25,102 --> 00:03:28,818 temperature must be in order to prevent the gas from collapsing. 53 00:03:28,818 --> 00:03:33,700 So now we understand that you can have a molecular cloud sitting there. 54 00:03:33,700 --> 00:03:38,799 There is still a problem with gravity and this is a problem that is unique to 55 00:03:38,799 --> 00:03:43,646 gravity in contrast to other forces. And the problem that occurs with gravity 56 00:03:43,646 --> 00:03:48,368 is that imagine that through some reason and we'll talk about it later, this 57 00:03:48,368 --> 00:03:52,837 region of the cloud becomes denser. If this region becomes a little bit 58 00:03:52,837 --> 00:03:57,496 denser than other regions, then of course the net gravitational attraction 59 00:03:57,496 --> 00:04:01,840 everywhere is slightly more directed toward this region of the cloud. 60 00:04:01,840 --> 00:04:08,145 If the net fluctuation is large enough compared to the temperature, that in fact 61 00:04:08,145 --> 00:04:15,081 some significant infall occurs, well the net, then more mass will fall onto this 62 00:04:15,081 --> 00:04:18,550 region. This region will become denser still. 63 00:04:18,550 --> 00:04:22,676 Becoming denser still, the collapse will become more significant. 64 00:04:22,676 --> 00:04:27,318 In other words, once you have a fluctuation or some reason, an additional 65 00:04:27,318 --> 00:04:32,090 den- extra dense region in a cloud. Because unlike, say, electromagnetism, 66 00:04:32,090 --> 00:04:36,990 where if you charge some region with positive charge, it'll attract negative 67 00:04:36,990 --> 00:04:41,369 charge and neutralize itself. And enhanced mass density attracts mass, 68 00:04:41,369 --> 00:04:44,005 which increases the enhanced mass density. 69 00:04:44,005 --> 00:04:46,766 Gravitational collapse is a runaway process. 70 00:04:46,766 --> 00:04:51,723 Even if the cloud is holding itself under thermal pressure, the situation is 71 00:04:51,723 --> 00:04:54,358 unstable. If you make a sufficiently large 72 00:04:54,358 --> 00:04:58,750 perturbation, the cloud will collapse. And this is what happened to our 73 00:04:58,750 --> 00:05:02,641 molecular cloud that led to the formation of the solar system. 74 00:05:02,641 --> 00:05:05,841 So what caused one region of the cloud to be dense? 75 00:05:05,841 --> 00:05:10,546 Well, we think it was the shock wave created by the explosion of a supernova. 76 00:05:10,546 --> 00:05:14,950 In fact, probably more than one nearby and we'll see how that might have 77 00:05:14,950 --> 00:05:19,537 happened, again, later in the class. This causes the great big cloud to break 78 00:05:19,537 --> 00:05:23,391 up into fragments and triggers the collapse of these fragments. 79 00:05:23,391 --> 00:05:27,794 The fragment that created our solar system was probably on the order of 80 00:05:27,794 --> 00:05:28,406 magnitude of 20, 2 to 20,000 astronomical units in size abd with a mass of about 81 00:05:28,406 --> 00:05:28,406 3,000 solar masses. this did not create the solar system, in 82 00:05:28,406 --> 00:05:28,406 fact, it created a cluster between 1 and 10,000 stars. 83 00:05:28,406 --> 00:05:28,406 So, the condition that we see in the solar system suggest that the sun was 84 00:05:28,406 --> 00:05:48,440 formed as part of an open cluster like the Pleiades. 85 00:05:48,440 --> 00:05:52,757 It's now not bound gravitationally to nearby stars and so it's not part of a 86 00:05:52,757 --> 00:05:55,392 cluster. This cluster has dispersed and in fact, 87 00:05:55,392 --> 00:05:59,821 this is the, this leaves a reasonably recent insight that the sun forms as part 88 00:05:59,821 --> 00:06:04,855 of a cluster and the search is on in the neighboring part of the Milky Way to find 89 00:06:04,855 --> 00:06:07,981 the stars that were once members of our cluster. 90 00:06:07,981 --> 00:06:11,172 I do not know that there has been success so far. 91 00:06:11,172 --> 00:06:15,340 And again the shock wave triggers. Remember, it's a sound wave. 92 00:06:15,340 --> 00:06:18,401 There's a region of high pressure in the cloud. 93 00:06:18,401 --> 00:06:23,546 That region becomes sufficiently dense that gravity overcomes thermal pressure. 94 00:06:23,546 --> 00:06:28,104 This is called the Jeans Instability. It's the fact that if you have a 95 00:06:28,104 --> 00:06:33,840 fluctuation, it will grow, and as we saw, If you have a fluctuation whos mass 96 00:06:33,840 --> 00:06:38,803 relative to the is related to the temperature by this equation where big M 97 00:06:38,803 --> 00:06:43,766 is the mass of the cloud say, or the region that's collapsing and little M the 98 00:06:43,766 --> 00:06:48,530 mass of the constituent particles. If you satisfy this condition, 99 00:06:48,530 --> 00:06:52,687 then the region will collapse. And this is the Jeans condition. 100 00:06:52,687 --> 00:06:58,031 And any object that is not collapsing that's made of gas, will have to obey 101 00:06:58,031 --> 00:07:00,803 stability condition against this collapse. 102 00:07:00,803 --> 00:07:05,883 And larger objects, the condition might change, but everything in the universe 103 00:07:05,883 --> 00:07:10,040 that is not collapsing is held up against gravity by something. 104 00:07:10,040 --> 00:07:14,132 That's the light motif that I want you to take away from this. 105 00:07:14,132 --> 00:07:17,467 So. we have this, large molecular cloud and 106 00:07:17,467 --> 00:07:20,720 it initiates a collapse. In this simulation, 107 00:07:20,720 --> 00:07:24,932 what we're going to see is how the collapse of a molecular cloud looks. 108 00:07:24,932 --> 00:07:28,649 On the left, we see the density. On the right, the temperature. 109 00:07:28,649 --> 00:07:33,418 There's some region of increased density. The cloud is collapsing towards the 110 00:07:33,418 --> 00:07:37,941 region of increased density. localized regions of high temperature are 111 00:07:37,941 --> 00:07:41,038 forming. The timescale here is tens or hundreds of 112 00:07:41,038 --> 00:07:44,507 millions of years. We see the beginning of stars forming. 113 00:07:44,507 --> 00:07:48,781 We see these explosives, explosive supernovae, and we see that after a 114 00:07:48,781 --> 00:07:52,701 supernova, there's local, flux of sudden star formation. 115 00:07:52,701 --> 00:07:57,854 Look at that big super nova and then the sudden concentration of star formation 116 00:07:57,854 --> 00:08:00,881 near it. So this is presumably what happened to 117 00:08:00,881 --> 00:08:03,329 us. The sun was formed as one of these 118 00:08:03,329 --> 00:08:08,482 clusters that were generated or triggered by an explosion of a nearby supernova. 119 00:08:08,482 --> 00:08:13,952 We'll get back to the star part of it. But first, we have our cloud, it's about 120 00:08:13,952 --> 00:08:18,833 200, 2000 astronomical units to aside, and it starts to collapse. 121 00:08:18,833 --> 00:08:24,723 But now, let us focus our attention on the region immediately near us in the 122 00:08:24,723 --> 00:08:30,925 cloud, immediately near where we are now. So somewhere there, we will zoom in, and 123 00:08:30,925 --> 00:08:36,397 look at a region of maybe 200 astronomical units or 1,000 astronomical 124 00:08:36,397 --> 00:08:41,725 units around where we are now, that region begins to collapse under the 125 00:08:41,725 --> 00:08:47,278 weight of it's own gravity because the fluctuation makes it too dense for 126 00:08:47,278 --> 00:08:51,542 temperature to sustain it. What happens when a cloud starts to 127 00:08:51,542 --> 00:08:54,625 collapse? Well, first thing we understand is that 128 00:08:54,625 --> 00:08:59,442 as the size of the cloud gets smaller, any random rotation that the thermal 129 00:08:59,442 --> 00:09:03,939 motion, or whatever, might have included is going to get sped up by the 130 00:09:03,939 --> 00:09:08,050 conservation of angular momentum as this demo brilliantly shows. 131 00:09:08,050 --> 00:09:14,101 So what the demo showed us, is that whatever random rotation was part of the 132 00:09:14,101 --> 00:09:19,277 thermal motion of this huge 20,000 astronomical unit sized cloud. 133 00:09:19,277 --> 00:09:25,807 as the cloud collapses, is accelerated. And what we find is that the whole cloud 134 00:09:25,807 --> 00:09:29,791 is spinning rapidly. Now it's not spinning rigidly, because 135 00:09:29,791 --> 00:09:34,638 it's not a piece of plastic hooked up. These are independent particles of gas. 136 00:09:34,638 --> 00:09:38,862 But the net result is the same. The rotation is enhanced. This is the 137 00:09:38,862 --> 00:09:43,960 rotation that makes the planet's orbit, the sun in the same direction as the sun 138 00:09:43,960 --> 00:09:48,751 rotates there was an overall rotation present and it was enhanced by collapse. 139 00:09:48,751 --> 00:09:54,034 This is another important result which is that objects that are collapsing as they 140 00:09:54,034 --> 00:09:59,070 come closer to the center and that such a direction that they're coming closer to 141 00:09:59,070 --> 00:10:04,046 the axis of rotation by the conservation of angular momentum, NVR being conserved 142 00:10:04,046 --> 00:10:08,530 means that as R shrinks their velocity, their speed has to increase, these 143 00:10:08,530 --> 00:10:12,882 objects have to move faster. This creates a barrier that prevents the 144 00:10:12,882 --> 00:10:17,375 cloud from collapsing into its axis. On the other hand, collapse in the 145 00:10:17,375 --> 00:10:21,933 direction along the axis, the other direction is uninhibited by this. 146 00:10:21,933 --> 00:10:26,941 And so, the cloud collapses far more readily along the axis than perpendicular 147 00:10:26,941 --> 00:10:30,086 to the axis. The net result is, that what was once 148 00:10:30,086 --> 00:10:35,030 probably a roughly spherical cloud is flattened into a pancake shape. 149 00:10:35,030 --> 00:10:40,563 This is the reason that we find the planets all orbiting the Sun in roughly a 150 00:10:40,563 --> 00:10:46,238 plane. The plane of the ecliptic is the plane perpendicular to the original axis 151 00:10:46,238 --> 00:10:51,062 that was randomly the axis of rotation, the neck rotation of a cloud. 152 00:10:51,062 --> 00:10:56,737 And then within about 100,000 years of the initial super nova trigger, the cloud 153 00:10:56,737 --> 00:11:00,852 is contracted to a radius of about 200 astronomical units. 154 00:11:00,852 --> 00:11:05,534 And it's now flattened into what we called a proto planetary disc. 155 00:11:05,534 --> 00:11:10,373 A cloud of dust and gas Here is a nice infrared image of such a 156 00:11:10,373 --> 00:11:16,000 protoplanetary disk in the Orion nebula. And here's a sequence of nice Hubble 157 00:11:16,000 --> 00:11:22,285 images that show us how this takes place. So here we see an image of Orion's dagger 158 00:11:22,285 --> 00:11:27,619 with the Orion nebula in the center. And as I click, we will go through 159 00:11:27,619 --> 00:11:30,521 increasing magnifications. we magnify. 160 00:11:30,521 --> 00:11:36,101 Here's a magnified view of the nebula. In the center of the nebula we find all 161 00:11:36,101 --> 00:11:40,036 these dust clouds. And in them we find these cocoons in 162 00:11:40,036 --> 00:11:44,686 which stars are formed. And upon magnification the infrared image 163 00:11:44,686 --> 00:11:50,910 shows us very nicely the nascent star in the center surrounded by a protoplanetary 164 00:11:50,910 --> 00:11:52,770 disk. It's happening there. 165 00:11:52,770 --> 00:11:57,020 physics is universal, this is the way it happened here. 166 00:11:57,020 --> 00:12:02,984 In addition to spinning up, another process that begins to take it place as 167 00:12:02,984 --> 00:12:08,490 the solar nebula collapses, is a process called Kelvin-Helmholtz heating. 168 00:12:08,490 --> 00:12:12,727 This involves the fact that some of the gravitational potential energy, 169 00:12:12,727 --> 00:12:17,442 liberating by all this stuff falling in towards the center is indeed converted 170 00:12:17,442 --> 00:12:21,262 into rotational kinetic energy. But most of it is distributed by 171 00:12:21,262 --> 00:12:25,917 collisions into heat and into the random motion of the components around them. 172 00:12:25,917 --> 00:12:30,752 So as an object contracts under the form of gravity, it's gravitational potential 173 00:12:30,752 --> 00:12:34,990 energy's being converted into heat, and the temperature therefore rises. 174 00:12:34,990 --> 00:12:40,813 this sets up a profile, a distribution around the disc where of course the 175 00:12:40,813 --> 00:12:45,305 density is largest at the center of the disk and tapers off to the edges. 176 00:12:45,305 --> 00:12:49,797 And likewise the temperature, the most compression has taken place in the 177 00:12:49,797 --> 00:12:54,290 center, so the temperature near the center of the disk eventually reaches 178 00:12:54,290 --> 00:12:57,270 2,000 Kelvin. And when it does, we'll see that a 179 00:12:57,270 --> 00:13:02,207 surface, next week we'll see that a surface with a temperature of about 2,000 180 00:13:02,207 --> 00:13:06,374 kelvin sort of stabilizes the material inside it is the protosun. 181 00:13:06,374 --> 00:13:11,568 Most of that will eventually be the Sun, what's left outside is the remnants that 182 00:13:11,568 --> 00:13:16,825 will become the rest of the solar system. And so in the center the temperatures 183 00:13:16,825 --> 00:13:21,893 very high and the densities are rising and then both density and pressure 184 00:13:21,893 --> 00:13:26,394 decrease as you move outside. And so you have this disk with hydrogen, 185 00:13:26,394 --> 00:13:29,264 helium and some of 'em, some other materials. 186 00:13:29,264 --> 00:13:34,320 in this region where both temperature and density. 187 00:13:34,320 --> 00:13:39,970 Decrease with R with distance from the center, and this will have some serious 188 00:13:39,970 --> 00:13:45,765 impact on the state in which we might find different components of the solar 189 00:13:45,765 --> 00:13:48,952 nebula. Farther out we have essentially very 190 00:13:48,952 --> 00:13:53,371 small density, very small pressure, and very cold temperatures. 191 00:13:53,371 --> 00:13:56,848 Near the middle it is hot and relatively denser. 192 00:13:56,848 --> 00:14:00,614 Let's see interesting demonstration, of how this 193 00:14:00,614 --> 00:14:04,272 effects the situation. One of the important ingredients in the 194 00:14:04,272 --> 00:14:07,539 solar nebula is water. Hydrogen and oxygen find to form water. 195 00:14:07,539 --> 00:14:11,450 And remembering that it's despite, the increased density from collapse. 196 00:14:11,450 --> 00:14:15,627 Under very low pressure compared to the atmospheric pressure we feel on earth. 197 00:14:15,627 --> 00:14:19,913 We need to understand how water behaves in space and so what we've done here is 198 00:14:19,913 --> 00:14:24,091 we've taken some water colored blue so we can see it and place it in a vacuum 199 00:14:24,091 --> 00:14:27,572 chamber and essentially we're simulating the conditions in space. 200 00:14:27,572 --> 00:14:30,840 The temperature is room temperature and the water is boiling. 201 00:14:30,840 --> 00:14:34,749 We know, anybody that cooks knows, that as you go to higher altitude, it's lower 202 00:14:34,749 --> 00:14:36,954 pressure. Water boils at a lower temperature. 203 00:14:36,954 --> 00:14:39,260 Here the water is boiling at room temperature. 204 00:14:39,260 --> 00:14:42,155 Boiling water is converting liquid to water vapor. 205 00:14:42,155 --> 00:14:46,499 That requires an input of latent heat. When we boil water on the stove, that 206 00:14:46,499 --> 00:14:50,264 heat comes from the stove. Here, as when our sweat evaporates from 207 00:14:50,264 --> 00:14:54,840 our bodies, it's coming from the water itself and that's cooling the water down. 208 00:14:54,840 --> 00:14:59,299 We see that the boiling is slowing down, because, as more water evaporates, the 209 00:14:59,299 --> 00:15:03,759 water is in fact, becoming quite cold. And when it becomes cold enough, what we 210 00:15:03,759 --> 00:15:07,640 observe is that the boiling completely ceases, the water has frozen. 211 00:15:07,640 --> 00:15:11,472 what we've produced, when we take it out, is an ice pack. 212 00:15:11,472 --> 00:15:16,759 Essentially what we're learning, is that, at the low pressures obtaining in space, 213 00:15:16,759 --> 00:15:21,054 water can exist as ice, or as water vapor but not as liquid water. 214 00:15:21,054 --> 00:15:26,473 The answer to the question is there water on Mars or on the moon or in space in a 215 00:15:26,473 --> 00:15:30,306 silly sense is no. You can have ice and you can have water 216 00:15:30,306 --> 00:15:35,328 vapor, you can not have liquid water at the low pressures obtaining in space. 217 00:15:35,328 --> 00:15:40,285 This is, something we're familiar with for example carbon dioxide, exists on 218 00:15:40,285 --> 00:15:43,171 earth as vapor, and is solid, but not as a liquid. 219 00:15:43,171 --> 00:15:46,755 It's true for any substance that's sufficiently low pressure. 220 00:15:46,755 --> 00:15:50,280 And if the pressure's in space, it's true for all substances. 221 00:15:50,280 --> 00:15:55,159 Cannot resist the temptation to demonstrate that with nitrogen, a major 222 00:15:55,159 --> 00:15:58,527 component of air, so here we have liquid nitrogen. 223 00:15:58,527 --> 00:16:03,750 It is boiling because the boiling point of nitrogen is negative 100 degrees 224 00:16:03,750 --> 00:16:06,705 centigrade. We put it in the vacuum chamber. 225 00:16:06,705 --> 00:16:09,914 We evacuate. And again, we see that the nitrogen boils 226 00:16:09,914 --> 00:16:14,508 even more vigorously and as it boils even more vigorously at the low pressures, 227 00:16:14,508 --> 00:16:18,695 it's cooling itself down and like the water, eventually, nitrogen freezes. 228 00:16:18,695 --> 00:16:23,463 The freezing point is only a few degrees lower than the boiling point, but what we 229 00:16:23,463 --> 00:16:27,260 have there is solid nitrogen. Where we see in this simulation, 230 00:16:27,260 --> 00:16:34,220 is the impact that the temperature gradient through the nebula, will have on 231 00:16:34,220 --> 00:16:40,048 the state of various materials or various of the metals that are present in the 232 00:16:40,048 --> 00:16:43,279 nebula. We see that, as we said, the temperature 233 00:16:43,279 --> 00:16:49,108 near the center near the, the outside of the proto sun, reaches close to 2,000 234 00:16:49,108 --> 00:16:52,484 degrees. On the other hand, as one moves further 235 00:16:52,484 --> 00:16:56,571 out, the temperature decreases. And what we see is that as the 236 00:16:56,571 --> 00:17:02,534 temperature decreases some materials over here on the right, transition from gas to 237 00:17:02,534 --> 00:17:07,895 solid, notice there are no liquids, many oxides and metallic iron for example are 238 00:17:07,895 --> 00:17:13,054 solid at temperatures below 1300 degrees and so that is the temperature that 239 00:17:13,054 --> 00:17:16,070 obtains near the current position of mercury. 240 00:17:16,070 --> 00:17:21,040 How farther out where Earth is located. other minerals 241 00:17:21,040 --> 00:17:27,290 made of, iron and, sulfur, for example, are, solids, but water is still a gas. 242 00:17:27,290 --> 00:17:33,094 Water becomes a solid at a temperature of 175 degrees K, and the line at which 243 00:17:33,094 --> 00:17:38,451 water, a much more plentiful material than, say, iron or nickel, because 244 00:17:38,451 --> 00:17:44,106 remember that, graph of abundance is, was declining on a logarithmic scale. 245 00:17:44,106 --> 00:17:49,017 Water, made up of hydrogen and oxygen, is relatively light elements. 246 00:17:49,017 --> 00:17:54,523 They're relatively plentiful. water becomes solid just inside Jupiter's 247 00:17:54,523 --> 00:17:57,568 orbit. Perhaps not a coincidence, and then 248 00:17:57,568 --> 00:18:03,804 farther out, ammonia and methane, become solid somewhere between Saturn's orbit 249 00:18:03,804 --> 00:18:09,298 and Uranus' orbit, and argon and neon noble gases are solid farther out. 250 00:18:09,298 --> 00:18:15,237 What we see is that as you go farther out into the nebula, there are more solids, 251 00:18:15,237 --> 00:18:19,840 and this will have an important impact on, planet formation. 252 00:18:19,840 --> 00:18:25,287 What we've seen is that, the fact that temperature and density depend on 253 00:18:25,287 --> 00:18:30,660 distance out in the nebula, will mean that the form in which various 254 00:18:30,660 --> 00:18:35,950 materials or constituents of the nebula are likely to be found will change closer 255 00:18:35,950 --> 00:18:40,556 to the center we only find gases. As you move farther and farther out, more 256 00:18:40,556 --> 00:18:45,474 and more of the materials are going to be solids and we saw that in the center 257 00:18:45,474 --> 00:18:49,706 we're forming this protosun. The protosun will eventually become the 258 00:18:49,706 --> 00:18:54,375 sun the region enclosed within this boundary of temperature 2000 Kelvin. 259 00:18:54,375 --> 00:18:59,418 Most of that will turn into the sun and that in addition to providing us life 260 00:18:59,418 --> 00:19:03,197 further on, such as ticking time clock for processes 261 00:19:03,197 --> 00:19:08,939 going on in this disc, full of, gas and in the outer reaches will be called dust, 262 00:19:08,939 --> 00:19:14,318 microscopic particles of solids. the ticking clock is, that within about 263 00:19:14,318 --> 00:19:17,807 ten million years, the sun will ignite as a star. 264 00:19:17,807 --> 00:19:23,519 When that happens, the stellar wind, the solar wind that we discussed undergoes a 265 00:19:23,519 --> 00:19:27,880 very intense period where, for a few million years, the solar wind is 266 00:19:27,880 --> 00:19:31,086 extremely intense, a few tens of millions of years. 267 00:19:31,086 --> 00:19:35,639 This is called the T-Tauri winds. Star in this stage are called T-Tauri 268 00:19:35,639 --> 00:19:38,076 stars. The sun was once a T-Tauri star. 269 00:19:38,076 --> 00:19:43,078 And, the stellar wind, the pressure of this stream of charge particles is so 270 00:19:43,078 --> 00:19:48,144 large that any remaining gas and dust is completely blown away from the system. 271 00:19:48,144 --> 00:19:52,889 And so, that gas and dust is to form planets and whatever else it wants to 272 00:19:52,889 --> 00:19:55,157 form. The timing is 10,000,000 years. 273 00:19:55,157 --> 00:19:59,240 What hasn't formed in 10,000,000 years is going to be blown away. 274 00:19:59,240 --> 00:20:04,453 Gas and dust cannot survive the solar wind although more heavier more robust 275 00:20:04,453 --> 00:20:08,160 objects are able to survive. Here's a nice demonstration. 276 00:20:08,160 --> 00:20:12,725 Of how things look from the point of view of producing the 277 00:20:12,725 --> 00:20:17,451 star, what we are going to do again is right density here, density is common 278 00:20:17,451 --> 00:20:21,845 density, so it is density throughout the depth of the nebula. 279 00:20:21,845 --> 00:20:27,633 We're seeing a two dimensional image and as time increases we see both density and 280 00:20:27,633 --> 00:20:32,235 temperature increase. And we see in the middle the formation of 281 00:20:32,235 --> 00:20:37,047 the protosun the red line indicates a temperature of 2000 degrees. 282 00:20:37,047 --> 00:20:42,556 This is where the protosun move forward and it's still surrounded by the proto, 283 00:20:42,556 --> 00:20:46,989 planetary disc where soon our planets and us will come into being. 284 00:20:46,989 --> 00:20:49,740 We'll start that process in the next clip.