安娜·弗雷貝爾：宇宙、星系與恆星的起源和演化

Lex Fridman Podcast #378 – Anna Frebel: Origin and Evolution of the Universe, Galaxies, and Stars Machine transcription by Qwen3-ASR 0.6B (MLX, local, Apache-2.0). May contain recognition errors. The following is a conversation with Anna Fribel, an astrophysicist at MIT studying the oldest stars in the Milky Way galaxy in order to understand the chemical and physical conditions of the early universe and how from that our galaxy formed and evolved to what it is today, the place we humans call home. And now a quick just second mention of each sponsor. Check them out in the description is the best way to support this podcast. We got Hexclad for well designed and well engineered cookware, Numirai for the world's hardest data science tournament and House of Academias for delicious snacks. Choose wisely, my friends. Also, if you want to work with our team, we're always hiring. Go to hexfreeman dot com slash hiring. And now onto the full ladreads, never any ads in the middle. I try to make this interesting, but if you skip them, please still check out the sponsors. I enjoy their stuff, maybe you will too. This episode is brought to you by a new sponsor, Hexclad, the maker of well engineered and patented in quotes hybrid cookware. I don't know what that means. All I know it looks badass. I got it a couple of weeks ago. I already cooked a couple of steaks on it. It's just perfect. My goal for this weekend actually is to cook an omelette on it, but it works perfect for steak, nonstick. I don't know how they do it, but as they say, has the performance of a stainless steel, the durability of cast iron, and it has non toxic and nonstick coating. The steaks, in case you are wondering, were delicious. I like it about medium rare, and there is probably a lot more to be said about how I like to cook steak in different conditions when I am traveling, when I am at home, all that kind of stuff. But steak is the simple meal I return to time and time again for source of simple happiness and fulfillment. Almost exclusively, I've been eating one time a day, and so it's steak, big steak with a bunch of veggies on the side if I am going keto and just steak if I am going carnivore. Anyway, go to hexclad dot com and use code Lex to get 10 off your entire order. That's hexclad dot com and use code Lex for 10 off your entire order. This show is also brought to you by Numirai, a hedge fund that I've talked a bunch about in the past. It uses machine learning to make investment decisions, and it's a tournament that challenges data scientists from all over the place to construct predictive models for financial markets and then it uses those models to help people make financial decisions. If you are data scientist and you want to be involved, this is an amazingly interesting, complex, difficult and impactful machine learning tournament and challenge. 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It's 30 less carbs and almonds. It's the only nut rich in omega sevens. I think a long time I was reading what is the healthiest nut, and Academias were up there. I believe they were at number one in the website I was looking at. And I was heartbroken to find out that I think peanuts were at the bottom, and you know so much of my life I've wasted on peanuts when I could have been spending all this valuable time with Academia nuts. Anyway, if you want some more delicious nuts in your life, go to house of academias dot com slash lex to get a free box of their best seller, the Namibian sea salted Academia nuts. They're delicious, plus 20 off your entire order. That's house of academias dot com slash lex. This is the Lex Friedman podcast. To support it, please check out our sponsors in the description. And now, dear friends, here is Anna Fribel. Let's go back to the early days. What did the formation of the Milky Way galaxy look like? Or maybe we want to start even before then. What did the formation of the universe look like? Well, we scientists believe that was the Big Bang, some big beginning. But what is important for my work and I think that's what we're going to talk about is what kind of elements were present at that time. So the Big Bang left a universe behind that was made of just hydrogen and helium and tiny little sprinkles of lithium. And that was pretty much it. As it turns out, it's actually quite hard to make stars or any structure from that. That's very hot gas. And so the very first stars that formed prior to to any galaxies were very massive stars, big stars, hundred times the mass of the sun, and they were made from just hydrogen and helium. So big stars explode pretty fast after a few million years only. That's very short on cosmic timescales. And in their explosions, they provided the first heavier elements to the universe because in their cores all stars fuse lighter elements like hydrogen and helium into heavier ones. And then that goes all the way up to iron, and then all that material gets ejected in these massive supernova explosions, and that marked a really really important transition in the universe because after that first explosion. It was no longer chemically pristine, and that set the stage for everything else to happen, including us here talking today. What do you mean by pristine? So there is a whole complex soup of elements now as opposed to just hydrogen, helium, and a little bit of lithium. Yeah, so after the Big Bang, just hydrogen and helium. We don't really need to talk too much about lithium because the amount was so small. And after these very first stars formed and exploded, they, heavier elements like carbon, oxygen, magnesium, iron, all of that stuff was was suddenly present in the gas clouds. In tiny amounts only, very tiny amounts. But and that actually helped, especially the carbon and the oxygen to to make the gas cool. These atoms are more complicated than hydrogen. That's just a proton, and so it has cooling properties, can send out photons outside of the gas cloud, so the gas can cool. And when you have gas that that gets colder and colder, you can make small and smaller stars. So you can fragment it and clump it and turn it into stars like like the sun. And the cool thing about that is that when you have small stars like the sun, they have a really long lifetime. So those first low mass stars that formed back then are still observable today. That is actually what I do. I try to find these early survivors because they tell us what the gas looked like back then. They have preserved that composition of these early gas cloud, the chemical compositions until today. So I don't need to look very far into the universe to study all the beginnings. I can just chemically analyze the oldest stars and it's like unpacking everything that that happened back then. It's very exciting. So to just reiterate, so in the very early days in the first few million years, there was giant stars that's mostly hydrogen and helium, and then they exploded in these supernova explosions and then they made these clumps. Yeah, so the first stars pristine. Not pristine clumps. Yeah, pretty much fun. So it took a few hundred million years for the first stars to emerge, and then they exploded after a few million years, kaboom, and then it's like I always consider the universe like a you know a nice soup. And then these first supernova explosions kind of provided the salt, you know, just a little sprinkle of heavier elements, and that made it really tasty. Just changed it completely, right? And that changed the physics of the gas. So that meant that these these gas clouds that were you know surrounding the the former first stars they could now cool down and clump and form the next generation of stars that now included also little stars. And as I just mentioned, the small stars have these really long lifetimes. The sun has a lifetime of 10 billion years. Any star that is even less massive will have an even longer lifetime. So that gives us a chance to to still observe some of the stars that formed back then. So we are testing the the conditions, the chemical and physical conditions of the early universe even before the galaxy formed. So what's the timeline that we're talking about? What is the age of the universe and what is the earliest time we got those salty delicious soup clump soups with heavier elements. Well, the universe is 13.8 billion years old. Allegedly, yeah. Is it so? Okay, thirteen. Well, you know when I was in high school, the universe was 20 billion years old. Yeah, so the estimate have changed. Do you think that estimate will evolve in interesting ways or no? Is that is it? I think we have mostly converged. Yes, because the techniques are are very different now, much more precise. The whole business of precision cosmology by mapping out the cosmic microwave background. You know that that's a marvelous feat. Maybe you know the digits will still move around a little bit, but that's all right. Plus the gravitational waves and all that. So all the different sources of data, yeah, kind of mapping out this detailed picture the early universe. Yeah, totally. And so we think the earliest little stars formed. I don't know, maybe half a billion years after the Big Bang, right? Again, a few hundred million years for the first stars to emerge and then you know took some time. So give or take half a billion years. And that was the time when sort of the very first proto galaxies formed early stellar structures, stellar systems from which the Milky Way eventually formed, right? So it was the Milky Way probably a bigger slightly bigger one. And we know today that galaxies grow hierarchically, which means they eat their smaller neighbors. So if you are the bigger one and have a few few friends around, you just gonna eat them, absorb them, and then you grow bigger. And so all these these little early stars you know kind of came into the Milky Way through that kind of process. And that's why we find them in the outer parts of the galaxy today because they just kind of have been just left there since. So the old stuff is on the outskirts of the galaxy and the new stuff is in closer to the middle. Is there broadly speaking? Okay, yes. So that's where you would look for it. So maybe just to step back, like what is a galaxy? What is a proto galaxy? I love that question. So the galaxy is a huge assembly of stars. The Milky Way contains something like 200 to 400 billion stars, and most of the material and the stars are in the disk. And when we look at the night sky, what we see as the Milky Way band on the sky, that is actually the the inner the next inner spiral arm because we actually live in a spiral disk galaxy. So the Milky Way is a spiral disk galaxy. And we are looking actually depends a little bit in the northern hemisphere we're looking out of the galaxy, so we're seeing the next outer spiral arm. And as you can imagine, there is only dark space behind that, so we don't see it all that nice on the sky. But if you travel to south to the southern hemisphere, let's say South America, you see the Milky Way and it looks so different on the sky because that's the next inner spiral arm and that's backlit by the galactic center. The galactic center is is a very big puffy you know region of gas. There is a lot of star formation that the the galactic party is happening there, so it's very bright and it it makes for this very beautiful Milky Way on the night sky that we see. So actually, if you if you ever get the chance to experience that, I encourage you to almost like close your eyes while seeing this and imagining that you are sitting in this kind of disk in this pancake and you are just kind of looking right into it and you can you can really feel that we are in this 2D disk. And then you can imagine that there is a top and a bottom and that that we really part off the galaxy. You can really experience that we're just not not just lost in space somewhere, but we're really a part of it. And you know knowing a little bit about the structure of the Milky Way really helps. Do you feel small when you think about that? When you look on that spiral on the inside of the Milky Way and then you look out to the outside, how are we supposed to feel? I don't know. I don't feel small necessarily. I feel in awe and I feel I am a part of it because I can really feel that I am a part of it. I think for many people they think like oh there is just the planet and then there is nothing and that's almost a little bit sad but that's really not the case right because there is there so much more and I really like to imagine wow I'm I'm sitting in this big galactic merry go round and we're going around the center and I can see the center above me right and I can almost feel like we're going going there. Of course, we can't really feel that, but the sun does circle the galactic center. But there is a kind of sadness to like looking pictures of a nice vacation place. All we get is that light, an old light. Do you feel like sad that we don't get to travel or you and I will not get to travel there and maybe humans will never get to travel there. Yeah, I always wanted to travel to space and see the Earth and other things from from up there. There are certainly that, but I don't know it's it's also okay to just be at our vantage point and and see it from from here. With the sensors, with the telescopes that we have, and explore the possibility. Yeah, I mean there is a kind of wonder to the mystery of it all. What's out there? What interesting things that we can't possibly imagine. You know there could be all kinds of life forms, bacteria, all this kind of stuff. I tend to believe that you know it depends on the day. I tend to believe there is just a lot of very primitive organisms just spread out throughout and they build their little things like bacteria type organisms. And just to think what kind of worlds there are because they're probably really creative living organisms. Because the conditions, I guess the question I am wondering to myself when I look out there to the stars, how different are the conditions on the different planets that orbit those stars. Will definitely be very different. I mean the variety out there is is huge. We know now that I think it's about every other star has at least one planet. I already mentioned the number of stars in the galaxy. I mean, you know, it's that's it's a huge number of planets out there. So who knows what that looks like? All we know is that there is there is a lot of variety. We don't quite yet understand what drives that, what governs that, why that is the case, why is it not all one size fits all. You mean the dynamics of planet formation, like exoplanet formation or star formation? The whole right, all of it. All of it. Star formation is remains a much research topic. We kind of we definitely know that it works because all the stars are there. Same for the planets, but the details are so varied per gas cloud, right? It's very hard to to come up with very detailed prescriptions. Broadly, we have figured it out. You need a gas cloud, you need to cool it, something clumps and fragments and somehow it makes a star with planets or without. But the dynamics of the clumping process is not fully understood. No, no, and and the local conditions are so varied, right? I mean, it's the same with you know all people look like people, but individually we look very different. So even the subtle diversity of the formation process creates all kinds of fun. Yes, so we just don't know how this turned out in an individual case, and it's kind of hard to to figure it all out and and to take a look certainly with planets, right? The chance for ever to ever actually take a picture of a planet is miniscule because they don't shine, so they're really dark. So I'd say there is there is there is a lot of possibility out there, but we have to be a little bit more patient. Or come up with technologies where patience becomes less necessary by extending our lifetimes or or increasing the speed of space travel, all the kind of stuff. Humans are pretty pretty intelligent. They're pretty sometimes, yeah, for the most part. I hope when I'm on the optimistic days. Well, maybe just to linger on the on the what a galaxy is. What should we know about our understanding of black holes in the formation? Is that an important thing to understand in the formation of a galaxy? Like so, all the orbiting, all the spiraling that's going on. How important is that to understand? All of the above, that's what makes astronomy really hard, but also really interesting. Right, no day is like another because we always find something new. I want to come back to the the idea of the proto galaxy because it actually matches or you know relates to to the black hole formation. So most large well, pretty much all large galaxies have a super massive black hole in the center and. We don't actually know. Don't we don't really know where they come from. Again, we know that they are there, but how how do we get there? So if we go back to the to the early universe, right? We had a a little galaxy that just sort of you know I don't know had some small number of stars. It was a first gravitationally bound structure that that was held together by dark matter because dark matter actually kind of structured up you know first before the luminous matter could because that's what dark matter kind of does and it it started to hold gas and then stars sort of together in these first very shallow what we call potential wells so these gravitationally bound systems and then the Milky Way grew from absorbing neighboring smaller even smaller systems and somewhere in that process there must have been a seed for one of these super massive black holes and I'm I'm not actually sure that it's clear right now kind of what was there first the super massive black hole or the galaxy so lots of people are trying to study that and of course the black hole wasn't as massive back then as it is these days but it's that's a it's a big area of of research and the new James Webb the JWST the telescope the infrared telescope in space is is working on many people are working on that to to figure out exactly what what happened and there are some some surprising results that we really don't understand right now so so to solve the the chicken or the egg problem of do you need the super massive black hole to form a galaxy or does the galaxy naturally create the super massive black hole yeah yeah I mean I think to some degree we we can answer that because there are lots of little dwarf galaxies out there you know the Milky Way remains surrounded by many dozens of of small dwarf galaxies I have studied a bunch of them and to the extent that we can tell they do not contain black holes so they are certainly were gravitationally bound structures so either you can call them proto galaxies or dwarf galaxies or first galaxies they were definitely there but there must have been bigger of these things like the proto Milky Way where something was different right what made them more massive so that you know they would gravitationally attract these smaller systems to to integrate them so we'll have to see how do we look into that the into the the dynamics of the formation and the evolution of the proto galaxies is it possible that they shine I mean what what are the set of data that we can possibly look at so we got gravitational waves which is really insane that we can even detect those there's light what what else can we so that that would fall into the category of observational cosmology and the the JWST is is the prime telescope right now to and it promises big big steps forward this is in its early days because it's only been online like a year or so but that collects the infrared light from the farthest like literally proto galaxies earliest galaxies that light has traveled some 13 billion years to us and they are observing these faint little blobs and folks are trying to you know again study the early the onset of these early super massive black holes how they shape galaxies so they they are seeing that they are they were there you know surrounded by already bigger galaxies ideally I'd like for for my colleagues to push a little bit further hopefully that will eventually happen in terms of looking towards older and older ones yeah yeah and more and more sort of primitive in terms of the structure but of course as you can imagine if you make your system smaller and smaller it becomes dimmer and dimmer and it's further and further away so we're reaching the end of the line from a technical perspective pretty quickly but it's dimmer and dimmer means older and older yes in a sense because it all started really small because the smaller smaller which correlates to older in that phase of the universe it would otherwise it it doesn't just to take a small attention about black holes and you know because you do quite a bit of observational cosmology and maybe experimental astrophysics what's the difference to you between theoretical physics and experimental so there's a lot of really interesting explorations about paradoxes around black holes and all this kind of stuff about black holes destroying information do those worlds intermix to you when you especially when you step away from your work and kind of think about the mystery of it all well at first glance there there isn't actually much cross talk personally I mostly observe stars so I don't usually actually think too much about black holes and stars is a fundamentally kind of chemical physical phenomenon that doesn't that's right the physics is is kind of different it's not extreme I mean you know you could consider a nuclear fusion sort of be perhaps extreme you need to tunnel there are some interesting physics there but it's it's just a different flavor and I don't I don't do these kinds of calculations myself either I I very much like to talk with my theory colleagues about these things though because I find there's always an interesting intersection and often it's it's just I've I've written a number of papers with colleagues who do like simulations about galaxies and so they are they are not quite as far removed as let's say the the black hole you know pen and paper folks but even in those cases we had the same interest in the same topics but it was almost like we're speaking two different languages and we weren't even that far removed you know both astronomers and all and it was really interesting just to take the time and really try to to talk to each other and it's it's amazing how how hard that is you know even among scientists we already have trouble talking to each other imagine how hard it is to talk to non scientists and other people to try you know to we're all interested in the same things as humans at the end of the day right but everyone has sort of a different angle about it and different questions and way of formulating things and sometimes it really takes a while to to converge and to to get you know to the common ground but if you take the time it's so interesting to participate in that process and it feels so good in the end to say like yes we tackled this together right we overcame our our differences not not so much in opinion but just in expressing ourselves about this and how we go about solving a problem and these were some of my most successful papers and I certainly enjoyed them the most it can also lead to big discoveries I mean there's a I think you put it really well in saying that we're all kind of studying the same kind of mysteries and problems I mean I see this in the space of artificial intelligence you have a community maybe it seems very far away artificial intelligence and neuroscience you know you would think that they're studying very different things but one is trying to engineer intelligence and in so doing try to understand intelligence and the other is trying to understand intelligence and cognition in the human mind and they're just doing it from a different set of data different set of backgrounds and the researchers that do that kind of work and probably the same is true in observational cosmology and simulation so it's it's like a fundamentally different approach to understanding the universe let me use for simulation let me use the things I know to create a bunch of parameters and create some just play with it play with the universe play God create create a bunch of universes and see in way that matches experimental data it's a it's a fun it's like playing Sims but at the cosmic level yes but and then probably the set of terminology used there is very different and maybe you are allowed to break the rules a little bit more let's have you know yeah take the Drake equation yeah you don't really know you kind of come up with a bunch of values here and there and and just see how it evolves and from that kind of intuit the different possibilities the dynamics of the evolution of a galaxy for example yeah but it's cool to play between those two because we it seems like we understand so little about our cosmos so it's good to play yes it's it's like a big sandbox right and everyone kind of has their little corner and they do things but we're all in the same sandbox together at the end of the day but in that sandbox does have super powerful and super expensive telescopes that everybody's also all the children are fighting for the resources to to make sure they get to ask the right questions using that big cool tool well can we actually step back on the the the big field of stellar archaeology what is this process can you just speak to it again you've been speaking to it but what what is this process of archaeology in the cosmos yeah it's it's it's really fascinating so I mentioned the the lesser the mass of the star the longer it lives and again for reference for the next dinner party the sun's lifetime is 10 billion years so if you have a star that's 0.6 or 0.8 solar masses then its lifetime is going to be 15 to 20 billion years and that's that's an important range for our conversation because again if you assume that such a small star formed soon after the Big Bang then it is still observable today you mentioned old light before yeah that light is like a few thousand years old but compared to the age of these stars is nothing so to me that's young it comes straight from from a galaxy oh you know it's not far these stars are not far away they are in our galaxy in the outskirts they probably did not form in the galaxy because again hierarchical assembly of a Milky Way meant exactly they formed in a little other galaxy in the vicinity and at some point the Milky Way ate that which means it absorbed all the stars including you know these little old stars that are now in the outskirts of the Milky Way that I used to point my telescope to so what can we learn from these stars why should we study them now these little stars are really really efficient with their energy consumption they are still burning for the experts just burning hydrogen to helium in their cores and they have done so for the past 12 13 billion years however all they are and they're going to keep doing that for another few billion years same as the sun the same sun also just does hydrogen helium burning and will continue that for a while which means the outer parts of the star well pretty much actually most of the star that gas doesn't talk to the core so whatever composition that that star has you know in in its outer layers is exactly the same as the gas composition from which the star formed which means it has perfectly preserved that information from way back then all the way to today and going forward so I'm a stellar archaeologist because I don't dig in the dirt to find remnants of past civilizations and and whatnot I dig for the star for the old stars in the sky because they have preserved that information from this first billion year years in their in their outer stellar atmosphere which is what I'm observing with telescopes so I'm getting the best look at the chemical composition early on that you could possibly wish for what kind of age are we talking about here are we talking about something that's close to that you know like a 13 billion 12 13 billion age range that's what we what we think now there's a small caveat here we cannot accurately date these stars but we use a trick to say oh these stars must have formed as some of the earliest generations of stars because we need to talk about the chemical evolution of the universe in the Milky Way for a second so already mentioned the the pristineness of of the universe after the Big Bang right just hydrogen and helium then the first stars formed they produced a sprinkle of heavier elements up to iron then the next generation of stars formed that included again massive stars that they would explode again but also the little ones that keep on living right so and then the massive ones again explode as supernova so they provide again another sprinkle of heavier elements and so over time all the elements in the periodic table have been built up there have been other processes for example neutron star mergers and other exotic supernovae that have provided elements heavier than iron all the way up to uranium from very early on we're still trying to figure out those details but I always say pretty much all the elements were done from like day three so iron is where like once you get to iron you got all the fun you need most of the fun yes I know I I really like the heavier elements you know gold silver platinum that kind of stuff for personal reasons or for star formation well both I mean like what's the importance of these heavier metals in in the evolution of the stars so they have they are the spice of life right so every supernova gives you elements up to iron that's cool but at some point it gets a little bit boring because that always works but that's the baseline we need that and that's certainly what came out of the first stars and then all the other supernova explosions that you know followed with every generation and I talk about a thousand generations give or take until the sun was made so the sun formed from a gas cloud that was enriched by roughly thousand generations of supernova explosions and that's why the sun has its its chemical the the chemical composition that it has including you know and somehow the planets were were made from that as well so the supernova explosions the many generations are creating more and more complex elements no it just goes all the way up to iron and then it just it's a little bit more of of all of these elements just more yeah just yeah it's one sprinkle then another and it just kind of adds up right now the heavy elements form in very different ways they are not fusion made they are made typically through neutron capture processes but for that you need seed nuclei ideally you know iron or carbon or something so the supernova made elements are a very good seed nuclei for other processes they then create heavy elements and because they cannot be made everywhere they when you when you know so I my some of my stars have huge amounts of these heavy elements in them and they tell us in much more detail something really interesting happened somewhere well wait I thought I thought the really old ones we would not have so what does that mean if if the old yes important clarification um so the stars that we are observing today these old ones they formed from the gas and the question is what enriched that gas so it could have been just a first star dumping their elements into that gas all the way up to iron and we have found some stars that we think are second generation stars so they form from gas enriched by just one first star that's super cool yeah then we find other old stars that have a much more complicated um heavy element signature and that means okay they are probably formed in a gas cloud that had a few things going on such as maybe a first star maybe another more normal supernova and maybe some kind of special process like a neutron star merger that would make heavy elements and so they created a local chemical signature from which the next generation star then formed and that is what we're observing today so all these old stars basically carry the signature from all their this these progenitor events and it's it's our job then to unravel okay which processes and which events and how many you know may have occurred in the early universe that led to exactly that signature that we observed 13 billion years later is it possible to figure out like the number of generations that resulted in this in these stars well we can we we think we can sort of say okay this was like second generation or third because the amounts of heavy elements in in the stars that we observe um is so tiny one super one normal supernova explosion is actually already basically too much it would give us too much of it and the thing is you can never take away things in the universe you can only add there's no cosmic vacuum cleaner going around sucking things away the black holes probably the closest to that but they would have taken the whole star yeah they take the whole thing that would have taken stuff out of the gas you know um so we have a maybe 10 stars or so now where we where we are saying they contain so little of these heavy elements that there must be second generation because how else would you have made them and again I want to I want to stress that the elements that we observe in these stars were not made by the stars themselves that we observe they that's just a reflection of the gas cloud so we don't actually I had to say that because I love stars we don't at the end of the day we don't really care for the stars that we're observing we care for the story that they're telling us about the early universe so yeah so the stars are kind of a small mirror into the the the early universe yeah and so what are you detecting about those stars can you tell me about the process of archaeology here like what kind of data can we possibly get to tell the story about these heavy elements on these stars yeah it depends really on what star you find there are many different chemical signatures we actually pair up these days our our our element signatures with also kinematic information how the star moves about the galaxy that actually gives us clues um As to where the star might have come from, because again, all these old stars are in the galaxy, but they are not off the galaxy. That's a small but important distinction. So they all came from somewhere else. You can rewind back in time to kind of estimate where where it came from. Yeah, so we can't really say, "Oh, it came from that in that dwarf galaxy," but interestingly enough, so I'm just just a few days ago I submitted a paper with three women undergrads. That was so good to work together, and we found a sample of stars that have very very low abundances in strontium and barium, so very heavy elements. And I had a hunch for a while that these stars would probably be some of the oldest, because as I said, heavy elements give you extra information about special events. And again, finding something that's really low means it must have for it that must have happened either really early on or in a very special environment, right? Because we can only ever add. So if you find something that's that's incredibly low in terms of the abundance, it maybe just one event contributed that max. So we looked at the kinematics: how are these stars moving? And they're all going the wrong way in the galaxy. How how is that possible? Well, it is possible because consider now we come back to the protogalaxy. The protogalaxy was like a beehive; it just didn't really know what it was or what it wanted to become when it grew up. So and it was absorbing all these little galaxies to grow fast. Some galaxies, some absorbed galaxies were thrown in going the main way, and some came in the wrong way, happens. It happens, but this could only happen early on when you know there wasn't left and right and up and down, so stuff would come in from always. So now, 13 billion years later, we're still doing it. Yeah, a they're still doing it, and B we just looked for stars that have low strontium and barium abundances, and then we look at the kinematics, and lo and behold, there are at hundreds of kilometers per second going the wrong way. It's like, dude, you must have come in really early on from somewhere else. So we call this retrograde motion. That's a clear sign of accretion, so something that has come in to the galaxy, and because they are so fast, um, and it it's really all of them. That that must have happened early on, right? You can't throw a galaxy into the Milky Way right now the wrong way. It eventually will turn around. Can you actually just in a small tangent speak to the the three women undergrads like this little? It's pretty cool that you were able to use a hunch to find this really cool little star. Yeah, what's the process like? Especially with undergrads, I think that would be very interesting and inspiring to people. Yes, it was a wonderful little collaboration that actually emerged in the fall. I so I like I really like working with with undergrads and grad students postdocs, and I came up with a new concept for a class at MIT where I wanted to integrate the research process into the classroom. Because sometimes people find it really hard to call email a professor, hey, you know this I'm this and that person and I'm interested in your research, could have possibly you know come. And I wanted to to streamline that and give and you know just trial how it would work to provide a sort of the safe confines of a classroom where you just sign up and do research in a very structured way. And I developed it was a lot of work, a little bit more than I thought to map up an entire research project basically from scratch in 10 worksheets so that they could do it again in a very structured and organized fashion. Oh, you created this whole framework for for them to do the whole thing. Um, but the promise was, you come sign up for my class in teams of two. You each get your own old star that has not been analyzed before. I don't know what