1 00:00:00,000 --> 00:00:07,829 So we left our star 12.233 billion years old and an interesting situation the core 2 00:00:07,829 --> 00:00:11,097 collapsed. Had stopped because the core had become 3 00:00:11,097 --> 00:00:13,926 degenerate. the outside of the star was still 4 00:00:13,926 --> 00:00:16,460 ballooning out. It was becoming a red giant. 5 00:00:16,460 --> 00:00:19,230 And they said at 12.233 this growth would stop. 6 00:00:19,230 --> 00:00:23,002 What is it that stops it? Well, what's happening is, we still have 7 00:00:23,002 --> 00:00:27,187 hydrogen fusion in the shell. More and more helium being deposited onto 8 00:00:27,187 --> 00:00:30,723 this degenerate core. The degenerate core is becoming hotter 9 00:00:30,723 --> 00:00:33,788 and hotter. And eventually it's temperature hits 100 10 00:00:33,788 --> 00:00:37,088 million Kelvin. When the temperature reaches 100 million 11 00:00:37,088 --> 00:00:39,800 Kelvin something qualitatively new happens 12 00:00:39,800 --> 00:00:44,097 at 100,000,000 kelvin. Two alpha particles can overcome their 13 00:00:44,097 --> 00:00:49,311 electrostatic repulsion and you can get fusion of two alpha particles to form 14 00:00:49,311 --> 00:00:53,122 beryllium. In fact once that happens, pretty rapidly 15 00:00:53,122 --> 00:00:57,735 beryllium fuses with another alpha particle to form carbon twelve. 16 00:00:57,735 --> 00:01:00,810 So this is called the triple alpha process. 17 00:01:00,810 --> 00:01:05,691 This is the fusion of helium that produced predominantly the very stable 18 00:01:05,691 --> 00:01:10,376 carbon twelve nucleus and. Because the helium in the core was 19 00:01:10,376 --> 00:01:16,340 degenerate, this is a very different ignition of fusion that what we've been 20 00:01:16,340 --> 00:01:21,320 used to because as fusion begins. The core starts to heat up, but as it 21 00:01:21,320 --> 00:01:25,197 heats up it does not expand. Remember degeneracy pressure is 22 00:01:25,197 --> 00:01:29,204 temperature independent. The core maintains degenerate density 23 00:01:29,204 --> 00:01:33,533 until it gets hot enough the thermal pressure overtakes degeneracy. 24 00:01:33,533 --> 00:01:38,444 During this time, fusion because it doesn't immediately expand, the fusion 25 00:01:38,444 --> 00:01:43,290 and the heating associated with it, spreads explosively throughout the core. 26 00:01:43,290 --> 00:01:48,007 And for a few seconds, this core produces illuminosities of 100,000 solar 27 00:01:48,007 --> 00:01:50,140 illuminosities. Not for very long. 28 00:01:50,140 --> 00:01:55,707 But there's a huge outcore of energy very dramatic helium flash is what it's 29 00:01:55,707 --> 00:01:58,753 called. this doesn't make it to the outside. 30 00:01:58,753 --> 00:02:03,981 You don't see the star suddenly flare up there is too much star between the core 31 00:02:03,981 --> 00:02:09,080 and the outside the stellar envelope happily absorbs this energy it expands. 32 00:02:09,080 --> 00:02:13,892 What really happens is that this, extra output of energy from the core expands 33 00:02:13,892 --> 00:02:18,189 the hydrogen fusing shell, and so, in fact, the star's luminosity is going to 34 00:02:18,189 --> 00:02:20,767 decrease. Momentarily, when the great pulse is 35 00:02:20,767 --> 00:02:25,178 coming out, the atmosphere might balloon out and, in fact, some of it might get 36 00:02:25,178 --> 00:02:27,928 blown away. You might get a second mass loss, but 37 00:02:27,928 --> 00:02:32,339 expanding the hydrogen fusing shell, which is where most of the star's energy 38 00:02:32,339 --> 00:02:36,350 is coming from, actually decreases the luminosity, in response to which. 39 00:02:36,350 --> 00:02:42,916 A response to ignition of helium infusion is that the star in fact contracts and as 40 00:02:42,916 --> 00:02:48,354 it contracts it heats up so The structure that we're seeing as the 41 00:02:48,354 --> 00:02:54,460 star stops growing bigger and cooler when it hits the helium flash, and, in fact, 42 00:02:54,460 --> 00:02:59,948 it starts heating and contracting. So you see the beginning of the next 43 00:02:59,948 --> 00:03:04,200 phase of the sun's trajectory. And what happens is that. 44 00:03:04,200 --> 00:03:10,624 As the, the envelope. Contracts and heats the deep convection 45 00:03:10,624 --> 00:03:13,320 zone that was, I acted. 46 00:03:13,320 --> 00:03:19,267 Because of the opacity the, the outside of the envelope ceases that rises up, we 47 00:03:19,267 --> 00:03:23,836 get shallow convection. We get convection in the core where now 48 00:03:23,836 --> 00:03:27,100 helium is fusing producing carbon and oxygen. 49 00:03:27,100 --> 00:03:32,612 So we have a helium burning core surrounded by a shell where hydrogen is 50 00:03:32,612 --> 00:03:36,166 fusing. And in fact they're separated by layer 51 00:03:36,166 --> 00:03:42,113 not depicted here of inert helium that's been fused out of the hydrogen, but is 52 00:03:42,113 --> 00:03:46,610 not yet compacted or heated enough to produce to, to be . 53 00:03:46,610 --> 00:03:53,033 Eligible for fusing so we still have a hydrogen fusing shell surrounding a 54 00:03:53,033 --> 00:03:58,743 helium fusing core this a. version of the sun after the envelope has 55 00:03:58,743 --> 00:04:05,323 contracted and the star continues to heat at a pretty much constant luminosity is 56 00:04:05,323 --> 00:04:11,515 what's called a horizontal branch star and notice that the, the horizontal 57 00:04:11,515 --> 00:04:16,470 branch lasts all of a million years it's like the main sequence. 58 00:04:16,470 --> 00:04:19,551 Except for helium burning and much, much faster. 59 00:04:19,551 --> 00:04:23,418 the sun would have contracted to all of ten solar radii. 60 00:04:23,418 --> 00:04:26,893 Its luminosity is down to 41 times its current value. 61 00:04:26,893 --> 00:04:31,613 There is the solar system with mercury long gone, but the sun contracted. 62 00:04:31,613 --> 00:04:36,530 Notice that the organs of the planets have expanded slightly due to the 30% 63 00:04:36,530 --> 00:04:41,577 mass loss in the previous expansion. And when we plotted on the HR diagram, 64 00:04:41,577 --> 00:04:45,773 this is what we're seeing. At the helium flash, the sun begins to 65 00:04:45,773 --> 00:04:49,902 contract and heat up. And this horizontal branch is the analog 66 00:04:49,902 --> 00:04:55,407 for helium burners of the mean sequence there's more variation to start contracts 67 00:04:55,407 --> 00:05:00,510 and heats at almost constant luminosity and the whole process lasts about a 68 00:05:00,510 --> 00:05:03,694 million years. What happens after a million years? 69 00:05:03,694 --> 00:05:08,695 Well, you can imagine what's going on. Over these million years helium has been 70 00:05:08,695 --> 00:05:13,120 very rapidly fusing in the core. The core is becoming now depleted in 71 00:05:13,120 --> 00:05:16,005 helium. We actually have a core that is inert 72 00:05:16,005 --> 00:05:20,135 carbon and oxygen. And, around that, we will acquire a 73 00:05:20,135 --> 00:05:26,488 helium fusing shell surrounded by inert helium, surrounded by a hydrogen fusing 74 00:05:26,488 --> 00:05:30,188 shell. We have this, onion version of a star. 75 00:05:30,188 --> 00:05:36,219 Eventually the inert CO core can no longer support the outside of the star, 76 00:05:36,219 --> 00:05:40,080 and that collapses to degeneracy. We now have, 77 00:05:40,080 --> 00:05:46,094 It collapsing inner core again we have helium fusion in the shell accelerated by 78 00:05:46,094 --> 00:05:50,212 the collapse. This increases the luminosity of the star 79 00:05:50,212 --> 00:05:53,155 that expands again and cools the envelope. 80 00:05:53,155 --> 00:05:58,550 The motion to the left when the star was heating and, contracting is reversed. 81 00:05:58,550 --> 00:06:04,015 The star now starts to move over to the right to lower temperatures and up to 82 00:06:04,015 --> 00:06:07,168 higher luminosities. Again the envelope cools. 83 00:06:07,168 --> 00:06:11,396 We get convective envelope. And, because of the high temperature 84 00:06:11,396 --> 00:06:16,061 dependence of the triple alpha process, we also have convection in the interior 85 00:06:16,061 --> 00:06:19,073 of the star. In big stars, these two regions hook up, 86 00:06:19,073 --> 00:06:23,620 and we get a second dredge-up where carbon and oxygen, the products of helium 87 00:06:23,620 --> 00:06:27,238 fusion, are dredged up. Of the surface of the star we can see on 88 00:06:27,238 --> 00:06:29,670 the absorption lines, this is what happened. 89 00:06:29,670 --> 00:06:35,557 At A mill- 12.365 so a million years have 90 00:06:35,557 --> 00:06:40,925 gone by, my times are not going to shift. Everything now starts happening so fast 91 00:06:40,925 --> 00:06:43,743 that we won't be able to keep track of it. 92 00:06:43,743 --> 00:06:49,044 The sun now balloons even larger than it was and even cooler, and luminosity is 93 00:06:49,044 --> 00:06:52,980 even larger. Notice, if you will, that this diagram, 94 00:06:52,980 --> 00:06:59,474 aside from omitting the inert helium shell in between the hydrogen fusing 95 00:06:59,474 --> 00:07:05,891 shell and the helium fusing shell, is also greatly exaggerating the size of the 96 00:07:05,891 --> 00:07:10,038 core. the entire fusing region, including the 97 00:07:10,038 --> 00:07:16,298 hydrogen fusing shell, is probably about a tenth of a percent of the size of the 98 00:07:16,298 --> 00:07:18,598 star. And so the radius of the star. 99 00:07:18,598 --> 00:07:23,496 So this is a very tiny core on the inside of this hugely bloated envelope. 100 00:07:23,496 --> 00:07:28,711 And because it's so hugely bloated the gas on the outside of the atmosphere is 101 00:07:28,711 --> 00:07:31,108 very. Weakly gravitationally bound. 102 00:07:31,108 --> 00:07:36,412 And, so again, every time the star gets large it loses the top level of its 103 00:07:36,412 --> 00:07:40,139 atmosphere. There's a lot of mass lost in this early 104 00:07:40,139 --> 00:07:44,369 asymptotic giant branch. Here's the picture of the solar system 105 00:07:44,369 --> 00:07:48,550 when the sun has gotten this big. And on the H-R Diagram, what has 106 00:07:48,550 --> 00:07:52,475 happened, as we said, is the leftward motion has been reversed. 107 00:07:52,475 --> 00:07:56,463 We're now moving to the right. And we're on what is called the 108 00:07:56,463 --> 00:08:01,417 asymptotic giant branch because it asymptotes to that old Hioshi Track where 109 00:08:01,417 --> 00:08:05,020 temperature's controlled by negatively ionized hydrogen. 110 00:08:05,020 --> 00:08:12,948 And as the star cools and expands. what's going to happen is that the 111 00:08:12,948 --> 00:08:19,119 hydrogen fusing shell which was temporarily inactive begins to be active 112 00:08:19,119 --> 00:08:25,212 it deposits more and more helium so that we have this structure now that is a 113 00:08:25,212 --> 00:08:32,007 complete onion I don't know if I can draw it but imagine somewhere out there is the 114 00:08:32,007 --> 00:08:38,377 star's envelope and inside it is. A shell where hydrogen is fusing and 115 00:08:38,377 --> 00:08:46,167 inside that is a shell of inert helium that's not doing anything and inside that 116 00:08:46,167 --> 00:08:54,520 is a shell whoops that was helium inside that is a shell where helium is fusing. 117 00:08:54,520 --> 00:09:00,020 And inside all of that is the inert carbon oxygen 118 00:09:00,020 --> 00:09:04,090 core and what's going on is because Hydrogen is fusing. 119 00:09:04,090 --> 00:09:08,791 it is constantly depositing Helium into this inert Helium shell. 120 00:09:08,791 --> 00:09:12,510 And Helium is making deposits into the carbon oxygen. 121 00:09:12,510 --> 00:09:17,422 The entire thing because a number of particles is still decreasing is 122 00:09:17,422 --> 00:09:21,282 crunching down. And the density and temperature of this 123 00:09:21,282 --> 00:09:24,440 inert Helium rise. And over here the boundary. 124 00:09:24,440 --> 00:09:29,984 Between fusing helium and inert helium we have large concentrations of helium which 125 00:09:29,984 --> 00:09:35,330 suddenly become degenerate and hot enough and so there are flashes in the helium 126 00:09:35,330 --> 00:09:39,620 burning shell where the helium burning shell suddenly becomes 127 00:09:39,620 --> 00:09:44,642 fusing and produces a large amount of, large spike in helium luminosity. 128 00:09:44,642 --> 00:09:47,561 What does this do? You got the drill by now. 129 00:09:47,561 --> 00:09:52,923 The extra spike in helium luminosity expands the hydrogen shelf that decreases 130 00:09:52,923 --> 00:09:57,742 the output of hydrogen fusion. Decreasing the output in hydrogen fusion 131 00:09:57,742 --> 00:10:01,340 will call, cause the envelope to contract and to heat. 132 00:10:01,340 --> 00:10:05,318 When this happens, eventually the contraction will reheat the hydrogen. 133 00:10:05,318 --> 00:10:09,695 Hydrogen fusion rates will go back up, the helium rate will have settled down, 134 00:10:09,695 --> 00:10:13,902 the new increased luminosity will cause the atmosphere to balloon back up. 135 00:10:13,902 --> 00:10:17,994 And this happens with a period of like 100,000 years, one, two, three, four, 136 00:10:17,994 --> 00:10:21,534 five times. And of course each time the star balloons 137 00:10:21,534 --> 00:10:26,739 out, it loses a lot of its mass to this now stellar super wind is what it's 138 00:10:26,739 --> 00:10:30,139 called, its luminosity reaches a maximum of 5,000. 139 00:10:30,139 --> 00:10:35,899 Its radius is as large as it's going to get, 213 times the current radius of the 140 00:10:35,899 --> 00:10:40,965 sun out to today's Earth orbit. Again when the atmosphere cools we get 141 00:10:40,965 --> 00:10:45,421 these deep convection. In the atmosphere, we get convection in 142 00:10:45,421 --> 00:10:50,282 between the shells, and, in a star slightly more massive than the sun, say, 143 00:10:50,282 --> 00:10:55,467 two solar masses and above, we find what we call carbon stars, where, as I said, 144 00:10:55,467 --> 00:11:00,133 the carbon, the, and other products of fusion, are being brought up to the 145 00:11:00,133 --> 00:11:05,317 surface and ejected, and we actually see the star ejecting as they get away what 146 00:11:05,317 --> 00:11:09,659 coalesces as actual soot. So we see, carbon solids in the stellar 147 00:11:09,659 --> 00:11:14,078 wind, an example of, Asyntotic giant bran star, is Mira notice 148 00:11:14,078 --> 00:11:19,266 gets harder and harder, to find stars in these existant stages, because these 149 00:11:19,266 --> 00:11:23,908 stages last, less and less. that is why most stars are on the main 150 00:11:23,908 --> 00:11:29,437 sequence, because any star spends most of its time, on the main sequence, mass also 151 00:11:29,437 --> 00:11:35,103 in a carbon star, by the way, can be on the order of, ten thousandth of a solar 152 00:11:35,103 --> 00:11:39,950 mass per year, so in 10,000 years, a star can lose, a complete solar mass, 153 00:11:39,950 --> 00:11:46,124 obviously these are slightly more massive stars and The thermal pulse, asymptotic 154 00:11:46,124 --> 00:11:51,290 giant phase lasts only about half a million years and so 155 00:11:51,290 --> 00:11:56,076 The star does not completely evaporate, this is what the solar system will look 156 00:11:56,076 --> 00:11:58,741 like. And what we are seeing in terms of the 157 00:11:58,741 --> 00:12:03,104 evolutionary track is this part, the latter part of the asymptotic giant 158 00:12:03,104 --> 00:12:05,951 phase. And then we see something really bizarre, 159 00:12:05,951 --> 00:12:09,586 we see the star appearing to heat up at constant luminosity. 160 00:12:09,586 --> 00:12:12,030 What is going on there. Well. 161 00:12:12,030 --> 00:12:15,332 There is no more fusion. Carbon will not fuse in the sun. 162 00:12:15,332 --> 00:12:20,108 There's not another phase of fusion that would cause us to move to the left like 163 00:12:20,108 --> 00:12:24,943 hydrogen did initially and helium later. What's really going on is that with these 164 00:12:24,943 --> 00:12:27,891 successive pulses, the star is losing its envelope. 165 00:12:27,891 --> 00:12:32,782 And we are seeing what we call the star. Is deeper and deeper until eventually 166 00:12:32,782 --> 00:12:36,736 what we're looking at is the bare carbon oxygen degenerate core. 167 00:12:36,736 --> 00:12:41,555 And, of course, that makes it look like we're seeing things that are hotter and 168 00:12:41,555 --> 00:12:44,335 hotter. So, the motion here is a little bit 169 00:12:44,335 --> 00:12:47,610 sporadic and not regular. But this, what makes this 170 00:12:47,610 --> 00:12:52,908 motion work to higher temperature is basically peering deeper and deeper into 171 00:12:52,908 --> 00:12:56,713 the star. the last observation about the ABG phase 172 00:12:56,713 --> 00:12:59,770 is that during this asintonic giant phase, 173 00:12:59,770 --> 00:13:05,882 We have other, sort of processes that are enough neutrons floating around the core 174 00:13:05,882 --> 00:13:11,370 of the star that, there are neutron capture processes that allow the star to 175 00:13:11,370 --> 00:13:16,996 synthesize elements heavier than, carbon or oxygen, so we get, things, like 176 00:13:16,996 --> 00:13:22,275 technetium being formed in stars with only slightly more than solar mass 177 00:13:22,275 --> 00:13:26,165 because of this sort of slow process of neutron capture. 178 00:13:26,165 --> 00:13:31,444 We'll see what happens at this very end in our last, clip about the sun's 179 00:13:31,444 --> 00:13:32,695 torturous history.