WEBVTT 1 00:00:03.399 --> 00:00:07.719 Welcome to Bedtime Astronomy. Explore the wonders of the cosmos 2 00:00:07.759 --> 00:00:12.279 with our soothing Bedtime Astronomie podcast. Each episode offers a 3 00:00:12.359 --> 00:00:16.320 gentle journey through the stars, planets, and beyond, perfect for 4 00:00:16.399 --> 00:00:20.239 unwinding after a long day. Let's travel through the mysteries 5 00:00:20.239 --> 00:00:22.440 of the universe as you drift off into a peaceful 6 00:00:22.480 --> 00:00:23.800 slumber under the night sky. 7 00:00:26.920 --> 00:00:28.320 I want you to just take a second and look 8 00:00:28.320 --> 00:00:31.280 around you. Think about the device you're listening on, the 9 00:00:32.399 --> 00:00:35.000 coins in your pocket, maybe the ring on your finger, 10 00:00:36.200 --> 00:00:38.880 every single atom of the heavy elements in those things, 11 00:00:39.000 --> 00:00:43.960 you know, copper, nickel, gold, silver, anything heavier than iron. Really, yeah, 12 00:00:44.119 --> 00:00:45.079 is an ancient relic. 13 00:00:45.320 --> 00:00:47.920 It is it was forged inside a star, or maybe 14 00:00:47.960 --> 00:00:50.039 a dying star, or in some cases a star that 15 00:00:50.119 --> 00:00:52.880 exploded in a just a cataclysmic event. 16 00:00:52.960 --> 00:00:55.719 We're literally made of stardust, and we're standing on stardust. 17 00:00:55.840 --> 00:01:00.320 That's not just poetry, it's literal truth. And for physicists, 18 00:01:00.399 --> 00:01:03.200 you know, figuring out the exact recipes for that start 19 00:01:03.280 --> 00:01:06.280 us the cosmic mechanisms that build everything from the ground 20 00:01:06.280 --> 00:01:09.079 beneath our feet to the phone in our hands. That's 21 00:01:09.239 --> 00:01:11.760 one of the big fundamental quests. 22 00:01:11.879 --> 00:01:13.799 And for a long long time it felt like we 23 00:01:13.840 --> 00:01:16.040 had most of that recipe book figured out. We had 24 00:01:16.079 --> 00:01:19.439 our two main characters, right, the two protagonists of element creation. 25 00:01:19.280 --> 00:01:22.359 Right, the s process and the process, the slow and 26 00:01:22.400 --> 00:01:23.560 the rapid exactly. 27 00:01:23.920 --> 00:01:27.239 We thought we understood the slow, steady way elements were built, 28 00:01:27.560 --> 00:01:30.959 and we also thought we understood the violent explosive way. 29 00:01:31.280 --> 00:01:33.719 And they did a remarkable job. I mean, they accounted 30 00:01:33.719 --> 00:01:35.640 for a stunning amount of what we see out there. 31 00:01:36.000 --> 00:01:38.599 If you looked at the sort of the chemical inventory 32 00:01:38.640 --> 00:01:42.879 of our own solar system, those two processes gave you 33 00:01:43.000 --> 00:01:47.359 this beautiful, almost textbook explanation for how you get from 34 00:01:47.480 --> 00:01:49.200 iron all the way up to uranium. 35 00:01:49.319 --> 00:01:54.480 That's where things get interesting because modern science, specifically modern astronomy, 36 00:01:54.959 --> 00:01:58.280 got a lot more precise, it really did, and these 37 00:01:58.319 --> 00:02:02.799 new incredibly detailed observation of certain types of stars started 38 00:02:02.840 --> 00:02:05.799 sending back data that well, it just didn't add up. 39 00:02:05.840 --> 00:02:06.840 The numbers were just wrong. 40 00:02:07.120 --> 00:02:10.319 Yeah, the ratios of elements. They were seeing, things like strontium, 41 00:02:10.360 --> 00:02:13.439 ettrium's or conium, they just couldn't be explained by the 42 00:02:13.479 --> 00:02:16.199 slow process or the rapid process. It was like finding 43 00:02:16.199 --> 00:02:18.879 a dish that had ingredients from two completely different recipes 44 00:02:19.159 --> 00:02:20.120 that shouldn't work together. 45 00:02:20.199 --> 00:02:22.960 It was a huge signal, a persistent anomaly that told 46 00:02:23.039 --> 00:02:27.000 us our cosmic recipe book was well incomplete. Astronomers were 47 00:02:27.000 --> 00:02:29.159 seeing the final products of a process that had to 48 00:02:29.199 --> 00:02:31.879 be something in the middle, not slow, not rapid, but 49 00:02:31.919 --> 00:02:34.479 some kind of middle ground our models just weren't ready for. 50 00:02:34.840 --> 00:02:37.199 And that's exactly what we're going to be diving into today. 51 00:02:37.479 --> 00:02:41.599 We are unpacking the proposed solution to this cosmic puzzle, 52 00:02:41.919 --> 00:02:45.960 the intermediate or as it's known, the eye process. We're 53 00:02:46.000 --> 00:02:48.199 going to get into the physics that makes it so unique, 54 00:02:48.560 --> 00:02:52.919 explore the really mind bending experimental challenges of trying to 55 00:02:52.919 --> 00:02:56.240 measure it here on Earth. And this is the amazing part. 56 00:02:56.639 --> 00:03:01.520 Talk about why solving this mystery have these surprising, profound 57 00:03:01.599 --> 00:03:03.879 implications for technology we use every day. 58 00:03:03.960 --> 00:03:06.439 It really does. I mean, we are talking about designing 59 00:03:06.439 --> 00:03:08.639 the next generation of nuclear reactors, for one. 60 00:03:08.759 --> 00:03:09.479 It's incredible. 61 00:03:09.599 --> 00:03:13.400 This whole field requires this amazing convergence of expertise. You've 62 00:03:13.439 --> 00:03:16.000 got the biggest telescopes in the world, the fastest supercomputers 63 00:03:16.080 --> 00:03:19.879 running simulations, and the most powerful particle accelerators all working 64 00:03:19.919 --> 00:03:22.039 on this one question, you know, where did all the 65 00:03:22.039 --> 00:03:25.199 elements heavier than iron really come from? It's still one 66 00:03:25.199 --> 00:03:28.479 of the biggest unanswered questions in physics, and this eye 67 00:03:28.560 --> 00:03:32.080 process might just be the key to unlocking the final chapters. 68 00:03:32.360 --> 00:03:34.800 Okay, so before we get to the eye process, let's 69 00:03:34.840 --> 00:03:37.240 just lay the groundwork. Let's talk about the core mechanism. 70 00:03:37.280 --> 00:03:42.039 Because when we say creating elements heavier than iron, we 71 00:03:42.120 --> 00:03:46.360 are almost always talking about one thing. Neutron capture. 72 00:03:46.599 --> 00:03:50.159 Yes, that is the engine absolutely, you know, for everything 73 00:03:50.240 --> 00:03:53.479 lighter than iron, carbon, oxygen, the stuff of life, that's 74 00:03:53.520 --> 00:03:57.280 all built through stellar fusion. Just burning hydrogen into helium, 75 00:03:57.319 --> 00:03:58.560 helium into carbon, and so on. 76 00:03:58.680 --> 00:03:59.439 That's like a furnace. 77 00:03:59.479 --> 00:04:02.240 It's a furnace exactly. But once you get to iron, 78 00:04:02.479 --> 00:04:06.840 that furnace shuts down. Iron is the ultimate ash. Fusing 79 00:04:06.879 --> 00:04:09.599 iron actually costs you energy instead of releasing it, so 80 00:04:09.639 --> 00:04:11.879 the star hits a wall. To get any heavier, you 81 00:04:11.879 --> 00:04:14.479 have to switch to a completely different mechanism. 82 00:04:14.199 --> 00:04:16.959 And that mechanism for over ninety nine percent of the 83 00:04:17.000 --> 00:04:20.800 heavy elements is neutron capture. Could you just walk us 84 00:04:20.839 --> 00:04:22.680 through the basic physics of that. How does it work? 85 00:04:22.759 --> 00:04:24.759 Of course, so you start with what we call a 86 00:04:24.800 --> 00:04:27.680 seed nucleus. Think of it as an iron nucleus, which 87 00:04:27.680 --> 00:04:30.560 is very stable. This seed is sitting deep inside a 88 00:04:30.600 --> 00:04:33.399 star in an environment where there's a flow of free 89 00:04:33.439 --> 00:04:37.360 neutrons floating around. The iron nucleus will every so often 90 00:04:37.680 --> 00:04:40.199 grab one of those neutrons. It captures it. Now it's 91 00:04:40.199 --> 00:04:42.720 a little heavier. Maybe a bit later it grabs another one. 92 00:04:42.879 --> 00:04:45.519 So its mass is going up step by step. 93 00:04:45.319 --> 00:04:48.040 But it's still iron, right, because the number of protons 94 00:04:48.040 --> 00:04:48.720 hasn't changed. 95 00:04:48.879 --> 00:04:51.680 That is the absolute key. It's just a heavier isotope 96 00:04:51.680 --> 00:04:54.759 of iron. But if you keep packing more and more 97 00:04:54.839 --> 00:04:58.680 neutrons into that nucleus, eventually you reach a tipping point 98 00:04:58.720 --> 00:05:02.839 where it becomes unstable. It's radioactive. It has too much tension. 99 00:05:02.720 --> 00:05:04.959 And it needs to relieve that tension. 100 00:05:04.639 --> 00:05:08.120 Somehow exactly, and it does that through a process called 101 00:05:08.160 --> 00:05:08.879 beta decay. 102 00:05:09.240 --> 00:05:11.759 And this is where the magic happens. This is the 103 00:05:11.759 --> 00:05:13.879 transmutation what happens in beta decay. 104 00:05:13.959 --> 00:05:17.639 So in beta decay, one of those extra neutrons inside 105 00:05:17.680 --> 00:05:21.879 the nucleus spontaneously transforms. It turns into a proton, and 106 00:05:21.959 --> 00:05:24.439 it spits out an electron to keep the charge balanced. 107 00:05:24.639 --> 00:05:25.879 So you've just added a proton. 108 00:05:26.079 --> 00:05:28.800 You've added a proton, and when you change the number 109 00:05:28.800 --> 00:05:31.720 of protons, you change the fundamental identity of the element. 110 00:05:32.160 --> 00:05:35.879 You've climbed one rung up the periodic table. Iron becomes cobalt, 111 00:05:36.079 --> 00:05:39.879 Cobalt becomes nickel, and so on. It's this beautiful step 112 00:05:39.879 --> 00:05:43.399 by step ladder, all powered by the cycle of capturing 113 00:05:43.439 --> 00:05:46.040 neutrons and then undergoing radioactive decay. 114 00:05:46.199 --> 00:05:49.040 And you mentioned earlier that the speed of this process 115 00:05:49.120 --> 00:05:51.839 is what really matters. That's what separates the two classical 116 00:05:51.879 --> 00:05:52.519 ways of doing this. 117 00:05:52.879 --> 00:05:55.800 Precisely, it all comes down to the neutron density and 118 00:05:55.800 --> 00:05:59.279 the timescale. They dictate which path you take up that ladder. 119 00:05:59.560 --> 00:06:02.079 So's talk about the first path, the slow one, the 120 00:06:02.199 --> 00:06:03.279 S process. 121 00:06:02.959 --> 00:06:05.839 Right, the S process or slow neutron capture. This is 122 00:06:05.879 --> 00:06:10.040 happening inside certain types of evolved stars, specifically in what 123 00:06:10.079 --> 00:06:13.199 we call asymptotic giant branch stars AGB stars. 124 00:06:13.480 --> 00:06:15.360 So what are the conditions like in there? When we 125 00:06:15.360 --> 00:06:18.160 say slow, what does that actually mean in cosmic terms? 126 00:06:18.279 --> 00:06:21.399 We're talking about a process that unfolds over thousands of years. 127 00:06:21.439 --> 00:06:26.240 It's a very leisurely paced The environment has a neutron 128 00:06:26.319 --> 00:06:29.120 density that's well. It sounds high to us, but it's 129 00:06:29.199 --> 00:06:32.600 relatively low for a star. We're talking maybe tens of 130 00:06:32.600 --> 00:06:36.720 millions up to a few hundred billion neutrons per cubic centimeter. 131 00:06:36.519 --> 00:06:39.920 Which on a nuclear timescale is an eternity. It means 132 00:06:39.920 --> 00:06:42.759 the nucleus has plenty of time between one neutron capture 133 00:06:42.759 --> 00:06:43.319 and the next. 134 00:06:43.480 --> 00:06:46.519 That's the crucial part. Because the captures are so infrequent. 135 00:06:46.560 --> 00:06:49.879 If a nucleus becomes unstable, it has all the time 136 00:06:49.920 --> 00:06:52.480 in the world to undergo that beta decay and stabilize 137 00:06:52.480 --> 00:06:54.319 itself before the next neutron comes along. 138 00:06:54.399 --> 00:06:57.680 So it's always staying close to the most stable configurations exactly. 139 00:06:57.839 --> 00:07:00.319 It walks a very predictable path right along we call 140 00:07:00.399 --> 00:07:02.720 the valley of stability on the chart of all nuclei, 141 00:07:03.439 --> 00:07:07.199 and this clean, steady process is responsible for creating a 142 00:07:07.240 --> 00:07:09.879 lot of the stable heavy elements we know, all the 143 00:07:09.920 --> 00:07:12.439 way up to bismuth, which is element eighty three. 144 00:07:12.639 --> 00:07:15.519 But bismuth isn't the end of the line. If we 145 00:07:15.560 --> 00:07:19.079 want the really good stuff, the gold, the platinum, not 146 00:07:19.160 --> 00:07:23.240 to mention radioactive elements like uranium, the s process just 147 00:07:23.319 --> 00:07:26.319 can't get us there. For that, you need do I 148 00:07:26.399 --> 00:07:27.000 need violence. 149 00:07:27.079 --> 00:07:30.279 You need the R process, the rapid neutron capture process. 150 00:07:30.360 --> 00:07:34.240 This is the complete opposite extreme. It requires environments with 151 00:07:34.519 --> 00:07:37.680 just I mean apocalyptic neutron densities. 152 00:07:37.720 --> 00:07:39.560 We're talking about a huge jump here. 153 00:07:39.519 --> 00:07:42.000 I'm mind boggling jump. For the S process, we were 154 00:07:42.040 --> 00:07:45.279 talking about billions of neutrons per cubic centimeter. For the 155 00:07:45.439 --> 00:07:48.199 R process, we're talking about densities well over ten to 156 00:07:48.279 --> 00:07:52.160 the power of twenty one, a trillion trillion neutrons per 157 00:07:52.240 --> 00:07:53.040 cubic centimeter. 158 00:07:53.439 --> 00:07:56.079 That is just an impossible number to comprehend. What kind 159 00:07:56.079 --> 00:07:59.680 of cosmic event could possibly create that level of neutron saturation. 160 00:08:00.000 --> 00:08:02.839 Only the most catastrophic events we know of the leading 161 00:08:02.879 --> 00:08:06.000 candidates for a long time, where the core collapse supernovae 162 00:08:06.000 --> 00:08:09.600 of massive stars. But more recently the evidence is pointing 163 00:08:09.720 --> 00:08:13.120 very strongly toward the merger of two neutron stars. 164 00:08:12.959 --> 00:08:15.480 When two of the densest objects in the universe. 165 00:08:15.160 --> 00:08:19.279 Collide exactly in that kind of environment. The neutron capture 166 00:08:19.360 --> 00:08:23.600 is so unbelievably fast that the nucleus has absolutely no 167 00:08:23.720 --> 00:08:26.160 time to decay and stabilize. It can't keep up, not 168 00:08:26.199 --> 00:08:28.879 even close. The whole thing happens in less than a second. 169 00:08:29.120 --> 00:08:33.519 A nucleus just gets flooded, swallowing dozens of neutrons one 170 00:08:33.559 --> 00:08:37.360 after another, pushing it way way out into the most exotic, 171 00:08:37.519 --> 00:08:41.559 unstable neutron rich territory on the nuclear chart. These are 172 00:08:41.600 --> 00:08:44.960 isotopes that might only exist for milliseconds. And then what 173 00:08:45.440 --> 00:08:47.679 And then after the explosion is over and the neutron 174 00:08:47.720 --> 00:08:52.240 flood subsides, this whole chain of incredibly unstable nuclei starts 175 00:08:52.279 --> 00:08:56.039 to decay back towards stability. It's this rapid accumulation followed 176 00:08:56.039 --> 00:08:59.440 by a cascade of decays that forges the heaviest possible elements, 177 00:08:59.679 --> 00:09:02.159 including the actinides like uranium and plutonium. 178 00:09:02.240 --> 00:09:04.480 So for a very long time that was the complete story. 179 00:09:04.559 --> 00:09:07.799 You had the slow, steady S process giving us elements 180 00:09:07.879 --> 00:09:11.000 up to bismuth, and the fast violent R process giving 181 00:09:11.039 --> 00:09:13.320 us the really heavy, rare stuff. It seemed to cover 182 00:09:13.399 --> 00:09:14.080 all the bases. 183 00:09:14.320 --> 00:09:16.840 It did. It was a really elegant picture. But then 184 00:09:17.120 --> 00:09:21.159 as astronomers started using these new high resolution spectrographs on 185 00:09:21.360 --> 00:09:26.840 very old, very pristine stars, they started seeing cracks in 186 00:09:26.879 --> 00:09:31.080 that picture. Ani the anomalies they were finding elemental signatures. 187 00:09:31.120 --> 00:09:34.639 These abundance patterns that were clearly made by neutron capture, 188 00:09:35.080 --> 00:09:37.279 but the ratios were all wrong. They didn't fit the 189 00:09:37.320 --> 00:09:40.240 slow path and they didn't fit the rapid path. It 190 00:09:40.279 --> 00:09:42.600 was too fast for one, but not extreme enough for 191 00:09:42.639 --> 00:09:44.879 the other. It was a clear sign that there had 192 00:09:44.879 --> 00:09:47.559 to be a third way, a mechanism that fell squarely 193 00:09:47.600 --> 00:09:48.360 in the middle. 194 00:09:48.360 --> 00:09:51.159 Which is the perfect setup for what was really a 195 00:09:51.240 --> 00:09:54.240 rediscovery of this missing link. The eye process. 196 00:09:54.360 --> 00:09:57.519 Yes, and structurally it's very simple to define. The eye 197 00:09:57.600 --> 00:09:59.799 stands were intermediate and it sits, you know, right in 198 00:10:00.000 --> 00:10:02.559 between the sn R processes, both in terms of how 199 00:10:02.559 --> 00:10:05.000 many neutrons are available and how fast it all happens. 200 00:10:05.039 --> 00:10:07.519 So we're talking about neutron densities that are way higher 201 00:10:07.559 --> 00:10:09.919 than what you'd find in a typical one of those 202 00:10:09.919 --> 00:10:13.120 AGB stars, high enough that the captures happened pretty quickly, 203 00:10:13.480 --> 00:10:17.360 but still what many many orders of magnitude less than 204 00:10:17.360 --> 00:10:19.279 the insane conditions of a neutron. 205 00:10:18.960 --> 00:10:21.960 Star Merger exactly. The density is sort of in this 206 00:10:22.080 --> 00:10:25.879 sweet spot, roughly ten to the power of fourteen, maybe 207 00:10:25.960 --> 00:10:29.480 up to ten to the sixteen neutrons per cubic centimeter. 208 00:10:29.720 --> 00:10:32.200 Okay, so what does that density mean for the timing. 209 00:10:32.600 --> 00:10:35.279 It means it's fast enough to build up some really 210 00:10:35.399 --> 00:10:39.080 neutron rich nuclei, pushing the reaction path a good way 211 00:10:39.120 --> 00:10:41.960 off that value of stability we talked about. But it's 212 00:10:42.000 --> 00:10:46.080 slow enough that the whole process takes say hours or 213 00:10:46.120 --> 00:10:48.759 maybe even a few days instead of being over in 214 00:10:48.799 --> 00:10:49.559 a single second. 215 00:10:49.919 --> 00:10:52.279 Now, the history of this idea is fascinating to me. 216 00:10:52.360 --> 00:10:55.080 This isn't some brand new concept that someone dreamed up 217 00:10:55.120 --> 00:10:58.159 last year. The idea was first proposed what back in 218 00:10:58.200 --> 00:10:59.919 nineteen seventy seven, that's right. 219 00:11:00.080 --> 00:11:03.240 It was a compelling theoretical idea even then, but the 220 00:11:03.279 --> 00:11:07.200 problem was there was no solid observational evidence to back 221 00:11:07.240 --> 00:11:10.000 it up, so it kind of well was almost forgotten about. 222 00:11:10.039 --> 00:11:12.639 It remained this niche theory because we just didn't have 223 00:11:12.679 --> 00:11:14.679 the tools to go out and find its signature in 224 00:11:14.720 --> 00:11:15.399 the cosmos. 225 00:11:15.480 --> 00:11:17.639 You couldn't drive the research because it was nothing. 226 00:11:17.399 --> 00:11:19.840 To measure precisely. You can have a great theory, but 227 00:11:19.879 --> 00:11:21.679 if you can't test it, it's hard for it to 228 00:11:21.679 --> 00:11:23.399 gain traction in the wider community. 229 00:11:23.639 --> 00:11:26.559 So what change What brought the eye process back from 230 00:11:26.600 --> 00:11:30.080 the dead in the last say ten or fifteen years. 231 00:11:30.000 --> 00:11:36.159 In a word technology, specifically, huge advancements in our telescopes 232 00:11:36.200 --> 00:11:39.360 and our detectors. We're talking about new generations of both 233 00:11:39.440 --> 00:11:44.159 space based observatories like Hubble and massive ground based telescopes 234 00:11:44.200 --> 00:11:47.080 that can now analyze starlight with a level of precision 235 00:11:47.120 --> 00:11:49.559 that was unimaginable in the seventies. 236 00:11:49.600 --> 00:11:52.919 And the key technique here is absorption spectroscopy, right. 237 00:11:52.840 --> 00:11:54.720 Yes, that's the one. You look at the rainbow of 238 00:11:54.759 --> 00:11:56.919 light coming from a star and you see these dark 239 00:11:56.960 --> 00:12:00.519 lines like a barcode. Each dark line core responds to 240 00:12:00.559 --> 00:12:04.679 a specific element in the star's atmosphere, absorbing that particular 241 00:12:04.720 --> 00:12:05.720 color of light, and. 242 00:12:05.679 --> 00:12:07.639 The darker the line, the more of that element there. 243 00:12:07.559 --> 00:12:10.360 Is, exactly. And with these new instruments, we can measure 244 00:12:10.399 --> 00:12:13.360 that barcode with incredible detail. And that's when we started 245 00:12:13.399 --> 00:12:17.639 finding these really hard anomalies in very specific types of stars. 246 00:12:17.759 --> 00:12:20.200 Stars that were perfect laboratories for this kind of thing. 247 00:12:20.480 --> 00:12:24.720 The research you're referencing specifically calls out carbon enhanced, metal 248 00:12:24.720 --> 00:12:27.679 poor stars. Now that's a mouthful. Can you break that 249 00:12:27.720 --> 00:12:30.000 down for us? Why is that type of star so important? 250 00:12:30.159 --> 00:12:33.000 Okay, so that name is basically a description of a 251 00:12:33.039 --> 00:12:38.120 cosmic fossil metal poor is astronomer speak for a very 252 00:12:38.240 --> 00:12:39.200 very old star. 253 00:12:39.440 --> 00:12:42.799 Right, because in astronomy, metal is anything heavier than hydrogen 254 00:12:42.799 --> 00:12:44.039 and helium exactly. 255 00:12:44.159 --> 00:12:46.720 So a metal poor star is one that formed early 256 00:12:46.759 --> 00:12:49.480 in the universe's history, when it was still mostly just 257 00:12:49.600 --> 00:12:52.799 hydrogen and helium. It's a pristine sample. It hasn't been 258 00:12:53.320 --> 00:12:57.080 polluted by generations of other stars exploding and seeding the 259 00:12:57.120 --> 00:12:59.039 galaxy with heavy elements. 260 00:12:58.840 --> 00:13:01.919 So it's chemical make is a much cleaner record of 261 00:13:01.919 --> 00:13:05.600 whatever nucleosynthesis happened inside of it, or to it correct. 262 00:13:05.960 --> 00:13:08.399 And the carbon enhanced part is the other key. It 263 00:13:08.440 --> 00:13:11.120 means that at some point these old stars got a 264 00:13:11.159 --> 00:13:13.879 big dump of carbon onto their surfaces, most likely from 265 00:13:13.879 --> 00:13:16.919 a companion star that evolved and puffed away its outer layers. 266 00:13:17.159 --> 00:13:20.440 And when astronomers pointed their new powerful spectrographs at these 267 00:13:20.440 --> 00:13:22.559 specific old pristine stars. 268 00:13:22.399 --> 00:13:23.559 They found the smoking gun. 269 00:13:23.840 --> 00:13:27.480 They found the smoking gun the ratios of certain heavy 270 00:13:27.519 --> 00:13:33.080 elements barium, lanthanum, europium. They just could not be made 271 00:13:33.120 --> 00:13:36.360 by any combination of the S and R processes. It 272 00:13:36.440 --> 00:13:40.240 was the definitive evidence that a third mechanism, an intermediate one, 273 00:13:40.320 --> 00:13:42.039 had to be at work in these environments. 274 00:13:42.120 --> 00:13:44.639 So let's talk about the physics of why. How does 275 00:13:44.679 --> 00:13:47.879 that intermediate timing hours or days create a different set 276 00:13:47.919 --> 00:13:49.080 of elements than the other two. 277 00:13:49.399 --> 00:13:51.519 This is where it gets really cool. It's all about 278 00:13:51.600 --> 00:13:54.080 hitting what we call branching points on the nuclear chart. 279 00:13:54.840 --> 00:13:59.039 These are specific unstable nuclei where there's a competition between 280 00:13:59.279 --> 00:14:03.000 capturing another neutron and undergoing beta decay. The fork row 281 00:14:03.039 --> 00:14:05.759 a fork in the road exactly. In the S process, 282 00:14:05.879 --> 00:14:08.759 it's so slow you almost always take the decay path. 283 00:14:09.039 --> 00:14:11.720 The nucleus stabilizes before another neutron. 284 00:14:11.440 --> 00:14:13.559 Arrives, And in the R process, it's so fast you 285 00:14:13.639 --> 00:14:16.159 just blow right past the fork. You capture another ten 286 00:14:16.200 --> 00:14:18.759 neutrons before decay is even an option precisely. 287 00:14:19.120 --> 00:14:22.039 But the EYE process, because of its intermediate speed, it 288 00:14:22.120 --> 00:14:24.279 hits some of these forks where the half life of 289 00:14:24.320 --> 00:14:27.720 the unstable nucleus is on the order of hours or days, the. 290 00:14:27.679 --> 00:14:29.679 Same time scale as the process itself. 291 00:14:30.000 --> 00:14:32.440 Yes, so the nucleus gets to that fork and it 292 00:14:32.480 --> 00:14:35.559 sits there. There's a real competition does it decay or 293 00:14:35.639 --> 00:14:39.559 does it capture another neutron, And that specific timing allows 294 00:14:39.559 --> 00:14:43.639 the reaction path to zigzag in a way that's totally unique. 295 00:14:43.679 --> 00:14:46.279 It can bypass some elements that the cess process makes 296 00:14:46.320 --> 00:14:49.440 a lot of and create other elements in ratios that 297 00:14:49.559 --> 00:14:51.240 neither of the other two can explain. 298 00:14:51.759 --> 00:14:55.559 So it's this unique timing that produces that specific anomalous 299 00:14:55.679 --> 00:14:57.759 fingerprint that astronomers were seeing. 300 00:14:57.919 --> 00:15:01.240 That's it. It's the convergence of the app sstronomical observations 301 00:15:01.279 --> 00:15:04.320 with the nuclear physics theory that told us, yes, the 302 00:15:04.399 --> 00:15:08.120 eye process isn't just a possibility, it's a necessity. It's 303 00:15:08.159 --> 00:15:10.639 a required piece of the cosmic puzzle. 304 00:15:10.840 --> 00:15:13.080 Okay, so it seems like there's a strong consensus that 305 00:15:13.120 --> 00:15:16.200 the eye process is real and necessary, which brings us 306 00:15:16.240 --> 00:15:18.480 to the next, much more complicated part. How do you 307 00:15:18.519 --> 00:15:21.519 actually solve it. Let's talk about this research ecosystem, because 308 00:15:21.600 --> 00:15:23.919 it's not one person in a lab. It sounds like 309 00:15:23.960 --> 00:15:27.600 a massive, coordinated effort across completely different fields of science. 310 00:15:27.960 --> 00:15:32.559 It's a fundamentally iterative process. It's complex, it's expensive, and 311 00:15:32.600 --> 00:15:37.200 it absolutely relies on constant, detailed communication between these different groups. 312 00:15:37.639 --> 00:15:39.519 I like to think of it as a three leg stool. 313 00:15:39.799 --> 00:15:43.000 You have observation, theory, and experiment, and. 314 00:15:42.960 --> 00:15:44.919 If any one of those legs is weak, the whole 315 00:15:44.960 --> 00:15:46.519 thing falls over it does. 316 00:15:46.720 --> 00:15:49.320 It all starts with the observers, the astronomers. They're the 317 00:15:49.360 --> 00:15:52.080 ones at the telescopes pointing at these stars and collecting 318 00:15:52.080 --> 00:15:54.600 the light. They provide the ground truth. 319 00:15:54.759 --> 00:15:57.399 They're providing the what and the where. They're saying, look 320 00:15:57.399 --> 00:16:00.639 at this carbon enhanced metal poor star. It has one 321 00:16:00.759 --> 00:16:03.840 hundred times more europium relative to iron than our sun. 322 00:16:03.919 --> 00:16:05.480 Does explain that exactly. 323 00:16:05.679 --> 00:16:08.600 They provide the target. They give us the final abundance 324 00:16:08.639 --> 00:16:12.039 pattern and say to the other groups, your models, your experiments, 325 00:16:12.279 --> 00:16:15.159 they must reproduce this specific fingerprint. 326 00:16:15.279 --> 00:16:18.159 So once they have that target, the baton gets passed 327 00:16:18.159 --> 00:16:22.120 to the theoretical physicists and the modelers, and in. 328 00:16:22.039 --> 00:16:25.600 Some ways they have the hardest job. Their task is 329 00:16:25.639 --> 00:16:29.440 to build a virtual star inside a supercomputer. They have 330 00:16:29.480 --> 00:16:32.799 to model everything, the star's temperature, its pressure, convection, these 331 00:16:32.879 --> 00:16:36.559 things called thermal pulses and AGB stars, and simulate its 332 00:16:36.600 --> 00:16:40.200 evolution and its nucleosynthesis over millions or billions of years. 333 00:16:40.320 --> 00:16:42.639 I can't even imagine the complexity of those simulations. 334 00:16:42.639 --> 00:16:46.519 They are monstrously complicated, and to run they rely on 335 00:16:46.559 --> 00:16:50.360 a huge library of pre existing nuclear data. Things like 336 00:16:50.440 --> 00:16:54.720 reaction rates, decay, half lives for thousands of different isotopes. 337 00:16:54.759 --> 00:16:56.559 But this is where the problem starts, and where the 338 00:16:56.600 --> 00:16:57.879 feedback loop begins. 339 00:16:58.000 --> 00:17:01.240 This is it. When they run these dudably complex models, 340 00:17:01.279 --> 00:17:04.160 they'll find that the final outcome, the amount of barium 341 00:17:04.240 --> 00:17:08.000 or lanthanum produced, is extremely sensitive to a handful of 342 00:17:08.039 --> 00:17:11.759 specific nuclear reactions along the eye process path. Okay, And 343 00:17:12.000 --> 00:17:14.160 often when they look up the data for those critical 344 00:17:14.160 --> 00:17:17.359 reactions in the nuclear databases, they find that the numbers 345 00:17:17.400 --> 00:17:21.119 have what we call large uncertainties. The value might be known, 346 00:17:21.200 --> 00:17:23.720 but only to within say, a factor of five or ten. 347 00:17:24.039 --> 00:17:26.599 So the model basically says, okay, the final abundance of 348 00:17:26.640 --> 00:17:30.400 barium should be x, but because we're not sure about 349 00:17:30.440 --> 00:17:33.920 this one key reaction, X could be anything between ten. 350 00:17:33.759 --> 00:17:37.680 And one thousand precisely, And that kind of uncertainty is 351 00:17:37.799 --> 00:17:41.039 completely useless if you're trying to match the very precise 352 00:17:41.119 --> 00:17:43.920 data coming from the telescopes. It's at that point that 353 00:17:43.960 --> 00:17:47.279 the modelers pick up the phone figuratively and call the 354 00:17:47.359 --> 00:17:49.839 experimental nuclear physicists. 355 00:17:49.640 --> 00:17:53.319 Like Mathiswheediking, the physicists whose work we're drawing on today. 356 00:17:53.200 --> 00:17:56.640 People exactly like him. The modelers go to the experimentalists 357 00:17:56.640 --> 00:18:00.279 and they say, listen, our models are completely stuck. The 358 00:18:00.319 --> 00:18:03.880 outcome is dominated by the uncertainty in this one specific 359 00:18:03.960 --> 00:18:07.640 neutron capture reaction on this one specific unstable nucleus. We 360 00:18:07.680 --> 00:18:09.039 need you to go to your lab and measure it 361 00:18:09.079 --> 00:18:10.759 for us. We need better data. 362 00:18:10.839 --> 00:18:13.440 So this massive cosmic question gets boiled down to a 363 00:18:13.559 --> 00:18:17.640 very specific, actionable challenge for a team at a particle 364 00:18:17.640 --> 00:18:18.519 accelerator here on. 365 00:18:18.480 --> 00:18:22.200 Her Yes, their job is to design an experiment that 366 00:18:22.240 --> 00:18:25.799 can reduce that uncertainty, to shrink those error bars. And 367 00:18:25.839 --> 00:18:29.759 it's this constant back and forth. The experimentalists provide new, 368 00:18:30.039 --> 00:18:33.519 more precise data, the modelers plug it into their simulations. 369 00:18:33.559 --> 00:18:37.039 The simulations get better, which then often highlights another reaction 370 00:18:37.119 --> 00:18:39.880 that needs to be measured even more precisely. It's a 371 00:18:39.920 --> 00:18:43.160 collaborative dance that's pushing our knowledge forward, one reaction at 372 00:18:43.200 --> 00:18:43.519 a time. 373 00:18:43.640 --> 00:18:45.839 So let's zoom in on that experimental work, because this 374 00:18:45.880 --> 00:18:49.279 is where the difficulty just goes off the charts. What 375 00:18:49.400 --> 00:18:52.720 is that single most fundamental piece of data that the 376 00:18:52.759 --> 00:18:54.480 modelers are always asking. 377 00:18:54.200 --> 00:18:58.599 For, the absolute number one most critical ingredient for any 378 00:18:58.640 --> 00:19:02.519 neutron capture model, whether it's sr or I is something 379 00:19:02.559 --> 00:19:04.279 called the neutron capture cross section. 380 00:19:04.359 --> 00:19:07.599 Okay, that sounds pretty technical. Is there a more intuitive 381 00:19:07.599 --> 00:19:08.960 way to think about what a cross section is? 382 00:19:09.119 --> 00:19:11.319 Absolutely the easiest way to think about it is as 383 00:19:11.359 --> 00:19:13.559 the size of the nucleus from the neutron's point of view. 384 00:19:13.599 --> 00:19:15.759 It's the probability that if you shoot a neutron at 385 00:19:15.759 --> 00:19:18.400 a nucleus it will actually get captured, that it will stick. 386 00:19:18.920 --> 00:19:21.599 So a big cross section means it's a big, easy target. 387 00:19:22.039 --> 00:19:24.440 A small cross section means it's a tiny target that 388 00:19:24.480 --> 00:19:25.960 the neutron will probably just miss. 389 00:19:26.160 --> 00:19:28.440 That's a perfect analogy. And if you don't know that 390 00:19:28.440 --> 00:19:30.839 cross section, you have no idea how often the capture 391 00:19:30.880 --> 00:19:33.599 will happen. And if you don't know that, you can't 392 00:19:33.640 --> 00:19:36.839 predict how fast you climb the periodic table, your whole 393 00:19:36.880 --> 00:19:37.799 model falls apart. 394 00:19:37.839 --> 00:19:40.400 Okay, so we need to measure these cross sections. But 395 00:19:40.519 --> 00:19:42.559 this is where the huge problem comes in, especially for 396 00:19:42.599 --> 00:19:43.400 the eye process. 397 00:19:43.759 --> 00:19:47.079 This is the massive challenge. Measuring a cross section is 398 00:19:47.559 --> 00:19:51.200 I won't say easy, but it's relatively straightforward. If you're 399 00:19:51.279 --> 00:19:54.039 working with a stable isotope, you can make a physical 400 00:19:54.079 --> 00:19:57.279 target out of the material like a thin foil, stick 401 00:19:57.319 --> 00:19:59.440