08:08:58 Well anyway, it's great to talk to you guys. 08:09:01 I'm really excited to be here and like Mateo says, I remember a conference we had at KITP back in 2012, the first basic conference, and it's something that he and Lars have been really involved in over the last, well, 10 years plus and I'm very happy 08:09:18 to be a part of that. I'm here, I'm a research scientist at the University of Texas at Austin. Here is the Hobby Elberly telescope here's a real picture time expose, you can see the Milky Way in the background of the 82 inch telescope and here is a student's 08:09:35 artist impression of the Z machine that we're using an Albuquerque to make stars in the lab. 08:09:44 So, let me get started so just like atoms are the building blocks of all matter in the universe stars are kind of the building blocks of everything we see in the universe. 08:09:55 So when we look at a galaxy, the light that we're seeing is mostly coming from the stars in the galaxy and where it's brighter, there's more stars and where it's Denver there's few stars, and of course when Galileo first took a telescope and looked at 08:10:08 the milky way he realized oh my goodness it's full of stars, they're just so many and so close together that you can't see them without a telescope. 08:10:17 So stars really are fundamental if you want to understand galaxies, you need to understand stars. 08:10:23 But of course stars aren't exactly like atoms, all hydrogen atoms are the same, but all stars are a little bit different. They have different masses they have different chemicals. 08:10:34 They have different ages and they change as they age, unlike. Unlike atoms so they're born, they have an age. They transmute hydrogen, they burn hydrogen and helium and other elements. 08:10:45 They then run out of that fuel and on the way through their lives they change size and they change mass, so they can lose mass as the age, so they're not like atoms at all in that respect. 08:10:56 So here is stellar evolution in a nutshell. 08:11:00 The two minute version. So there's really two different tracks there's high mass stellar evolution down here and there's low mass stellar evolution then what do we mean by high and low mass well stars that are below about eight to 10 times the mass of 08:11:15 the Sun, we believe will become a white dwarf star. And it turns out that's really most stars 97 to 98% of all stars, we think will become white dwarfs, and they go through this sequence where they burn hydrogen, helium, then they swell up and become 08:11:31 a red giant, then they lose a lot of their mass, and then a white dwarf is what's produced on the other track. 08:11:40 Are the high mass stars above eight to 10 solar masses. They also burn hydrogen the helium they also become red giants they're just bigger and brighter. 08:11:48 We believe they become supernovae, which are big intense explosions that are visible from great distances. And we believe that you can either have a neutron star at the end of that or a black hole at the end of that. 08:12:00 And these particular kind of supernovae are called type two supernovae. 08:12:05 These aren't the kind of supernova you've heard about in the news where we're figuring out how far distant galaxies are away from each other. Those are actually a different kind of supernovae called type one a supernovae, and those we think are exploding 08:12:19 white dwarf stars. And so the supernovae that are the most useful for cosmological understanding, are the type one supernovae of white dwarfs, but like the beetle song says they need a little help from their friends we don't think a white dwarf star that's 08:12:34 isolated can become a supernova. Or at least that's not the standard model. but if they have a companion and they can exchange mass than we think it's more possible and Dr Ken Shen at the, the last talk today is going to tell you about the most recent 08:12:48 research in that field. 08:12:51 So just to get a feel of how big white dwarf gets, I mean how big the sun will get as it ages here's the sun over here. And as time goes on, it's more or less constant but toward the end it starts to become a red giant and it gets much, much bigger. 08:13:07 So, the sun will become hundreds of times larger in diameter than it currently is. and it will become thousands of times brighter. 08:13:17 So, This is what all stars do. 08:13:21 Now we don't know exactly how big it's going to get, but it's going to swell up to be approximately the size of the Earth's orbit, it may be a little inside the Earth's orbit or maybe a little outside, if it's a little outside it'll vaporize the earth. 08:13:34 If it's a little inside it'll just bake the dickens out of the earth. It isn't going to make a large difference either way because it's going to destroy all life on Earth. 08:13:44 Either way, but this is a generic feature of all stars as they age. 08:13:55 So, 08:13:55 astronomers like to classify things it's one thing that we're, we really are good at, because you know you've heard astronomy is the oldest science. And what that means is, we've been doing astronomy for thousands of years before we had any clue about 08:14:10 what the underlying physics was. And so when you really don't know what the important physical quantities are you, you simply break things into different categories and see what patterns emerge. 08:14:21 So for instance, one thing we like to do with stars is just say, Well, what color are they, and how bright are they. And so we can we can break them into what's called an HR diagram where we plot them along here based on their color blue being over here 08:14:37 and red being over here, and how bright they are being up here. 08:14:41 And so if we do that if we take this field of stars and sift them along those axes. This is what it looks like. 08:14:52 So it turns out in this diagram that you don't get stars everywhere. You get stars along this thing that's called the main sequence and along this part of the main sequence stars are still burning hydrogen in their cores just like the sun. 08:15:05 These stars here starting to run out of hydrogen, and these stars here have already run out of hydrogen in their cores, and they're on the way to becoming a red giant, so they're, they're still burning hydrogen but not in the center they're doing it in 08:15:17 a shell around the helium core that they've generated. 08:15:21 Now when all stars are first born, they're all on the main sequence. So that's represented here, where we've looked at a population that stars, all born at the same time, and a billion years after they've been born most stars are still on the main sequence 08:15:37 and become red giant already. But as we run the clock forward. 08:15:42 And I just want to mention the low mass stars are the red stars here, the only difference between stars is these are low mass they're determined red. 08:15:54 These are high mass, they're bluer and hotter and brighter. And as time goes on. These stars died first and become red giants. And this is how they move in the HR diagram. 08:16:04 And the reason for that is a star which is 10 times the mass of the Sun is more like 3000 times brighter than the sun. 08:16:14 So if it's brighter. The reason is brighter is because it's burning energy 3000 times faster. 08:16:26 Now it has 10 times the energy because it's 10 times the mass of the sun but it's burning it 3000 times faster. So it only lives a tiny fraction of the life of the sun. 08:16:36 So high mass stars live fast die young and low mass stars, the ones that were down here. They live essentially forever, you know, a point five solar mass star may live for 50 billion years and the universe is only 13.7. 08:16:45 billion years old. And so these guys, they they're just barely entering middle age, they're going to be there forever. 08:16:52 And so there's this huge diversity and stellar evolution based mainly on the mass of the stars that are present. 08:16:59 Now after the red giant phase stars lose a lot of mass. 08:17:11 An eight solar mass star may lose 6.9 solar masses and become a 1.1 solar mass white dwarf. Our Sun we think will lose about 40 to 45% of its mass and become a point six or a point five six solar mass white dwarf. 08:17:22 And so stars are losing a significant fraction of their mass, and they do so in a very entertaining way. Here's a bunch of stellar mass loss scenarios captured with the Hubble Space Telescope. 08:17:34 Now of course this is the famous Crab Nebula, which is the supernova. That's an extreme form of mass loss and this didn't result in a white dwarf there's a neutron star down there at the center, but most of these others. 08:17:46 For instance, this one the little white dot is going to become a white dwarf. 08:17:51 This little white dwarf is also going to become a white dwarf. 08:17:57 These are the kinds of pictures that scare you, because you're looking at it going, Oh my god, I'd like to predict this but look at the diversity of everything that's here. 08:18:08 This is a very complicated phases stellar evolution and it depends on a lot of factors. 08:18:14 But people like us obviously the four of us who are getting together today, like studying white dwarf stars because after all this is finished, we can then look at the end product and so we can say oh well it looks like this star, probably started out 08:18:29 as a two solar mass star and look, it's a point six five solar mass white dwarf. And so you can quantify how much math it lost during its life. And if you look at the end points in the beginning points you can start to infer things about these phases of 08:18:43 stellar evolution. The other problem with these phases of stellar evolution is they're very rapid, so it's hard to catch stars in the act, so we don't have nearly as many examples of this as we would like to build up the statistics, but we've got tons 08:18:57 of white dwarfs and we've got tons of stars, before they become white dwarfs. Okay. Well, let's look at the most famous white dwarf in the sky potentially here's serious it's the brightest star in the sky. 08:19:10 And it turns out it's actually a binary system. So back in the 1800s people plotted the position of serious on the sky over decades, so they had a great deal of patience, and they saw that a wobble perceptibly in fact kind of scientists so Italy on the 08:19:26 sky. And, of course, we'd already had Newtonian mechanics for hundreds of years so the obvious explanation was well it's orbiting something, and the pair of stars is orbiting but we only see one of the pair. 08:19:40 And so, scientists back then knew that the other one was an invisible star of some sort, which was puzzling, because from the wobbling of serious you can say well, but it seems as if the other star is comparable in mass to it so why can't we see it. 08:19:57 So this is really quite a puzzle. 08:20:00 Nowadays we know that serious is about to solar masses, and this other star this little tiny star which is 110 thousandth as bright as this star in the visible light is actually about one solar mass. 08:20:20 So, the ratio is only a factor of two, and yet it's 10,000 times fainter, that's really hard to process. So, I feel sorry for the guys back in the late 1800s thinking about this because the things that they were missing were really quite significant. 08:20:31 First of all, nobody back then knew vice stars shine, nobody didn't know about nuclear reactions didn't know about it. 08:20:39 Nobody knew anything about quantum mechanics it hadn't been invented yet. And thirdly, I'm not going to talk about this much but you actually need special relativity to understand the stars that hadn't been discovered yet either. 08:20:53 So, but quite rightly they said you know this is some kind of this is something different this, if we ever understand this, we're going to understand something important, and it was really true. 08:21:03 so white dwarfs the the endpoint and 97% of all stars. The reason that I and a lot of others like them is that they are simpler to model than other physical systems. 08:21:14 So if you can build a model of a system. And it's not too complicated. Then you have some confidence that the results you get out of that model may match reality. 08:21:23 And then you can go and make small additions to that model and measure various effects, so they're uncomplicated in structuring composition, and they make ideal cosmic laboratories and I think that even the subtitle of this conference today is that white 08:21:39 dwarf stars are excellent for getting a handle on other physics that are difficult to achieve on earth. 08:21:45 And of course, other, we care about astronomy too not just physics. So here's a generic picture of a white dwarf. Most white dwarfs have carbon and oxygen in their core pretty much just carbon and oxygen, and it may be about 60% oxygen and 40% carbon. 08:22:01 And that's 99% of the mass of the star about 1% of the mass of the star is helium, and about point 1% of the mass of the star is hydrogen. 08:22:12 Now since white dwarfs are really dense, they have strong gravity's and the gravity is between 10 and 100,000 times the Earth's gravity. So you know if you have a balloon of helium and you let it go and rises up. 08:22:25 And that's because it's more buoyant than the surrounding air so the air in it is actually lighter, the helium is lighter than the surrounding nitrogen atmosphere. 08:22:34 But the reason it goes up is because of gravity. So if you made the Earth's gravity stronger the balloon would rise even faster. Well if we make on a white dwarf the gravity is 10,000 times stronger and so the balloons would just shoot up at a huge rate. 08:22:49 So that's basically what's happening here, the lightest element hydrogen is bubbling up to the surface and forming a pure layer. The next slide is the element helium is bubbling up and forming a nearly purely or helium. 08:23:02 And then you have the remnant carbon and oxygen that was produced when helium was burning in the core producing carbon and oxygen. And again, they're about the size of the Earth, and about a lot of them are about 60% the mass of the Sun, but there's actually 08:23:17 a whole range down from about point two solar masses up to about 1.3 or 1.4 solar masses in white horse. So they span a large mass range, so they're small, about the size of the Earth, their faith because they're small, they're dense, because you're cramming 08:23:34 over half the mass of the Sun into the volume the size of the Earth. 08:23:39 And if you were to take a cell phone sized amount from the center of a white dwarf, it would weigh over 200,000 pounds, which is the equivalent of about 15 African elephants elephants are 11 large t Rexes I learned a little about T rex fossils doing this, 08:23:55 this, this is the really big ones they found in the 90s. 08:23:58 Otherwise it's more like 20 t Rex's. So it's very dense. They are supported from collapsing by electron degeneracy pressure. This is the quantum mechanics. 08:24:11 You may have heard of something called the Pauli exclusion principle. There was a physicist, Wolfgang Wolfgang Pauli, and he basically said the basis of the principle is electrons don't like to be in the same place. 08:24:24 If you put two electrons in the same place. One of them has to be moving faster than the other one. And if you put a whole bunch of them in the same place, they have to have a whole distribution of energies and velocities and all that motion leads to 08:24:38 a pressure, and this pressure doesn't depend on the temperature. So as a white dwarf cools off the electron to the electronic degeneracy pressure doesn't change and that story is stable. 08:24:49 If you were take to take the sun and cool it off, then it's going to shrink and shrink and shrink, because the sun, the pressure comes from like the ideal gas law, where pressure is proportional to temperature times density, and as you decrease the temperature 08:25:03 the pressure just gets less and less, but in a white dwarf that doesn't happen. So, even at infinite time our white dwarfs are going to still be about the same size so they are ultimately stable in that sense. 08:25:16 And there's simple, they have pure surface layers when you look at him you see appear hydrogen spectrum. 08:25:24 And the nuclear reactions are either not present or at a very low level that just doesn't matter so they really just cool off, they cool off with time. 08:25:34 So, you might think well if they pull off with time is their temperature related to their ages in some way. 08:25:42 And just to segue a little bit, getting ages of stellar objects or any objects at all is really hard. 08:25:49 You know I often have asked my students you know well how do we know that this is this age or this is that age and of course the answer is Wikipedia, but you know the stars don't actually have their ages printed on them, ages a derived quantity. 08:26:02 It turns out in the solar system, our best estimate for the age of the sun comes from the estimate of ages of other things in the solar system such as meteorites, we can build a model of the sun and get an age from it but we could probably make the age 08:26:15 age come out to a lot of different values because we need to calibrate that we could get anywhere from 3 billion years to 6 billion years, unfortunately we've got meteorites and other things to tell us that the solar system is like 4.5 to 4.6 billion 08:26:28 And so that's how we calibrate our models for other stars, we can measure the meteorites and those systems we actually need to understand stellar evolution. 08:26:29 years old 08:26:37 Now in the mid 80s, the age of the universe was believed to be anywhere from 10 to 25 billion years. And that's largely because we didn't have a handle on the expansion rate of the universe, some people claimed it was fast. 08:26:51 Some people claim slow and this lead, lead to older new ages for the universe, older or younger agents. 08:26:58 But people are also making models of stars and many of those models had the oldest stars in our galaxy being at least 18 billion years old, the oldest stars and some globular clusters around our galaxy. 08:27:10 Being 18 billion years old or older. And so there was this conundrum of, of people astronomers starting to think that the universe wasn't that old, but then some of the stars were still believed to be older than that. 08:27:23 Well, maybe white dwarfs have something to add to this discussion because the models of stars that I was talking about were really main sequence evolution and how long they spend in all these phases burning hydrogen the helium and then on the red giant 08:27:36 branch. 08:27:37 So let's make an analogy. If you make a pot of coffee, it comes out hot you set it on the table. And if you measure that temperature over time it'll go down with time, then somebody else makes a pot of coffee. 08:27:50 You don't know when you come in but you measure how hot it is. And you've already done this calculation and calibrated it and you say well it's been out 15 minutes, you know, and then you come back another time during the day and it's down here and you 08:28:02 you say oh it's been out 45 minutes. So, based on the fact that you can model how fast the coffee cools off you can figure how long it's been sitting there. 08:28:11 Well you can do the same thing of white dwarfs started too like I said they just cool off, that's what they do. Except now instead of measuring and hours, you're actually measuring and billions of year. 08:28:21 So here's a white dwarf. 08:28:25 And when they're born they're several hundred thousand degrees, but at that temperature they cool really quickly. And so very quickly they're down below 100,000 degrees, and it takes them about half a billion years to get down to 12,000 degrees, and then 08:28:40 to get down to 4000 degrees takes about 10 billion years. 08:28:44 So, if you know this, then you can look at a white dwarf, use its temperature and work backwards and try to find out what its agents, that's in the ideal sense of course it's not nearly as simple as that and practice, but to get down to 3800 Kelvin, it 08:29:00 would take about 10 billion years for your average white dwarf. To do that, well in the late 80s Don Winget was looking at cooling a white dwarfs and realize that we can actually estimate the age of our of our galaxy from the cooling of white dorks. 08:29:17 So what they did, they use data taken by others. People measured how many white dwarfs were at different luminosity. So you know how the coffee cools off really fast when it's hot. 08:29:28 white dwarfs cool I'm really fast when they're hot. And so this is basically hot or bright over here and this is cool and dim over here. And so there's very few white dwarfs here because they cool quickly and become white dwarfs over here. 08:29:40 As you look cooler and cooler. You see more and more white dwarfs to a point where you don't see any at all. Now for our galaxy had been forming stars continuously for the last hundred billion years, you'd keep seeing cooler and cooler white dwarfs over 08:29:53 So it's a really nice demonstration that our galaxy has a real finite age, and that age they found back then was a hoarder nine to 10 billion years. And of course, that clashed with the 18 billion years for the oldest stars found by other techniques, 08:30:18 but now we know that when now we think we know that the age of the universe is like 13.7 billion years. And so this 10 billion your age for the disk of our galaxy makes sense if it took about 1 billion years for our galaxy to start forming, and then the 08:30:41 disk of the galaxy formed another billion years after that, then the disk of our galaxy is maybe it makes sense for it to be 10 to 11 billion years old. All right, but if you're going to get. 08:30:44 All right, but if you're going to get. You're going to really measure white dwarf ages, then you have to really understand all the physics that goes into how they cool. And one of the things they do as they cool off, we're pretty sure now is they freeze in their centers, well what does that 08:30:59 their centers, well what does that mean when you've got really dense plasma you know like 10 to the six grams per cubic centimeter. All the electrons are squeezed off the atoms so you just have barren nuclei, so you've got a carbon nucleus that's plus 08:31:14 six charge, and a sea of electrons that are just doing their own thing. So each of these dots is a carbon nucleus. So imagine, and they're all moving around really fast which corresponds to the white dwarf being hot in the center, and they cool off with 08:31:27 time which corresponds to the white dwarf cooling off. And as they cool off they can't move as fast, and you see they have trouble getting past each other. 08:31:37 And as they cool off even further than they can't get past each other at all and they kind of trying to get as far away from each other as they can. 08:31:44 And so what happens as is as they cool the carbon atoms the carbon nuclei settled into lattice sites, so there's still vibrating you can see that. But, um, they sell in the lattice sites so let's. 08:31:58 I just like watching this, so let's watch it again. And as they cool off you can see they can't swim past each other anymore. 08:32:06 What we call this, if this were a terrestrial plasma if it's just for water we'd say oh it's freezing is crystallizing here, we're talking about nuclei in a plasma but the same physics supplies. 08:32:18 So when this happens just like water, you get out the latent heat of freezing for water you know you have to take out a certain number of joules per gram to get water to freeze, you have to take out a certain number of jewels program here in order to 08:32:34 get these guys to settle in the lattice sites so it's a first order phase transition, just like freezing the water on the earth. Well, imagine you have a little thing of water that you're, you're taking energy out of it the standard rate and it's cooling 08:32:49 off and so the temperature is going down linearly with time, but then it hits zero Celsius and it starts to turn into eyes, it sits at zero Celsius as it goes from zero percent water. 08:33:00 I mean zero percent ice to 100% eyes, and then once it's all 100% is it starts cooling off again, so it sits at the temperature where it's crystallizing for a longer period of time, so your odds of catching it in that state are better. 08:33:16 The same thing is true of white dwarf stars, they as they crystallize they should slow down their cooling and you should find more of them than you expect. 08:33:24 So here's the Winget et al data from 1987, but it was averaged over all stars, all masses from point for solar masses up to 1.1 solar mass we didn't know the masses of the stars, it was just an average over it. 08:33:39 And this downturn was due to the finite age of the gods. 08:33:42 But in the age of space astronomy, we have the Gaia satellite which people will talk about later today, that's getting better and more data, and we can actually say well let's just look at the white dwarfs that are high mass between 1.9 and 1.1 solar masses. 08:33:59 And now we still see this downturn here. 08:34:02 And that's due to the finite age of the galaxy and the fact that it's not old enough to have white dwarfs down here, but now we have this other bump. 08:34:10 And this is the bump that we think is due to crystallization. So this happens where the models say by George should be crystallizing. 08:34:19 Now, I think this is really quite a success. But before we get too excited. 08:34:24 The models of what this should look like, are the black curve so the data are the red points here that are peaked right here, and the black curve says yeah you know if I, if I do my best and make them all up a crystallizing star should have this bump. 08:34:38 So you see this bump is here but it's a lot wider than it is here. So we still have some physics missing in our models, there's there's something going on that we aren't quite capturing, but I wouldn't claim that this bump is probably due to the stars 08:34:52 crystallizing. 08:34:54 All right, let's see I wish I knew how much time I have left I think I have five minutes Okay great, but still we have problems. 08:35:04 We need. So Gaia is great for getting us thousands of white dwarf stars and getting us estimate to their parameters, but it always, it doesn't always get us the most accurate estimate so those parameters. 08:35:18 If we look at a given white dwarf, our best bet for measuring how massive it is, is to take a spectrum of it, and analyze that spectrum. but unfortunately when we look at the spectrum. 08:35:30 It's harder to analyze than you would think. So anyway, when we take a spectrum we spread it out into the light, and these dark lines here, if this is a hydrogen atmosphere white dwarf are where the hydrogen atom absorbs light. 08:35:44 And if we were to look at two different white dwarfs, and one of them had wider lines that we would say the one with wider lines is a higher mass white dwarf and the narrower lines is a lower mass white dwarf. 08:35:57 But in order to really be able to do this quantitatively. 08:36:02 We need to be able to model how atoms absorb light, and the hydrogen is the simplest atom in the universe and so if we have one hydrogen atom in isolation we essentially know almost everything there is to know about it, we can predict how it will absorb 08:36:17 like perfectly. The problem comes in a wide North atmosphere, it's really dense, and they the, the different atoms and electrons and protons are kind of elbowing each other. 08:36:29 And so it's kind of like rebounding the basketball I don't know if you all play basketball but it's a lot easier to jump straight up when you're not getting elbowed in the ribs. 08:36:37 And so these atoms are elbowing each other as they're trying to absorb photons, and that makes the lines broader, we need to model that process in order to be able to use the broadness of the lines to actually get how dense those atmospheres are. 08:36:51 So if we analyze the spectrum. This is the amount of light that you get. And so that lack of light here in this line and this line is is due to the atoms absorbing it. 08:37:03 The sad fact is if we were to just use the H beta line here, and analyze it, we might get a massive a white dwarf of point six. If we were to use this line, and do the same thing we might get a massive point six eight. 08:37:16 And if we were to use this line we might get a massive point five, four, and you notice that those are not the same. If our models were perfect we would get the same mass no matter which lines we use. 08:37:27 So of course being pragmatist what we do is we say well we just average them together. We'll just average them all together, and we'll get some average and I'm sure that's good enough. 08:37:36 But of course that isn't the best way to do it, and you're sweeping under the rug, the things that you very well might want to understand in order to make a real positive breakthrough, and understand things better. 08:37:49 So wouldn't it be great if we could generate these lines in a laboratory at the same conditions that the stars have. So now here's the the the last part of the talk, I'm going to, this is laboratory astrophysics. 08:38:02 This is the Z machine at Sandia National Laboratories, the most powerful x resource in the world, and basically allows us to create a white dwarf planet, a white dwarf atmosphere on the earth, and then measure that plasma. 08:38:18 So here's a picture of it with the lights on so you won't see the lightning. And here's a shot as it goes off. 08:38:34 And I took, I took this video standing outside the door, obviously they don't let you stand inside the door to take the video. 08:38:41 And it really is massive the whole building shakes, occasionally the door blows open but that didn't happen this time. 08:38:49 And if you're out in the parking lot you can feel the parking lot wave, and if you look out toward the mountains, you can see a wave of dust rising from the ground as it comes out. 08:38:59 So what this device does is it stores up energy, actually not as much as you would think. 08:39:05 It only takes about 15 minutes to charge it up. 08:39:08 But when it releases the energy it's releasing it has a power output that's more than five times all the power plants in the world combined but of course that's for like 10 nanoseconds. 08:39:20 But it allows you to produce an intense bursts of X rays which heats up our plasma and enables us to produce a spectrum like this, so this is actually a laboratory plasma. 08:39:32 And presumably if we've done our experiment correctly, it's at a single temperature in a single density. Now the reason this is a good thing is because when we look at the surface of a star, you know, the surface of the stars not like the surface of the 08:39:44 earth. It's not hard, you're actually looking into the atmosphere and so you're seeing outer layers that are cooler and less dense, and you're seeing hotter layers that are that are that are denser and hotter and it's an average of all that so you've 08:39:57 got an average of plasma conditions here in the lab you actually have a chance to say none Oh, I'm really at about 12,000 degrees plus or minus you know 5%, and the density of the same thing plus or minus 5% whereas in a white dwarf you're averaging over 08:40:12 temperature changes of a factor of two and density changes of a factor of two. So you can model things better. 08:40:19 Here's an example of some data we took a couple of years ago, it's the blue points. And we're actually fitting it with a model atmosphere code that was designed to fit stars, but we repurposed it to fit in the lab so we're actually using the same tool 08:40:34 for the stars as we are the data in the lab. But of course it's a model so it has three parameters, there's something called a pseudo continuum which you can take various parts of and use or not. 08:40:46 And if you don't include the pseudo continuum you find a mismatch here in the wings, and if you do include the pseudo continuum you get the red car which matches pretty well. 08:40:55 It doesn't match this peak very well but we think that's because we haven't got the quantum mechanics right there. We are developing a actually we're just getting ready to publish our first round of calculations of the quantum mechanics of these lines, 08:41:08 and indeed it does make this a lot better. And it agrees here in the center of the age beta line better too. 08:41:16 And so yeah these calculations are using the code developed by Ivan Hubeny of the Tlusty suite, which is used to fit specter of white dwarfs. 08:41:25 So anyway, I hope I've given you a taste of some of the things you can do with white dwarf stars and why they're interesting, I had to stay away from a lot of stuff that I would have talked about if we didn't have so many other experts on the call with 08:41:37 us today, but I want them to be able to tell you all their exciting things themselves and spend and spend time discussing the things that they know so much about. 08:41:47 So white dwarfs make great chronometer, and they're independent they're different, they have different physics that goes into figuring out how old the sun is from a model. 08:41:57 It's a totally different set of physics and so it's an independent way of getting temperatures and it showed up problems in the past, and we still think it has value nowadays, and laboratory astrophysics experiments. 08:42:08 This is an emerging field and astronomy. 08:42:10 When President Obama became president in 2008 he said you know we have the national labs, we want them to commit 10% of their time to doing fundamental science and fundamental research, and that we are the beneficiaries of this, this 10% of time on the 08:42:27 national facilities. 08:42:30 One thing I didn't mention it all is white dwarf stars also pulsate when they reach certain temperatures and this instead of having to look at their surfaces and figure out everything from it, it gives us a key to their interiors. 08:42:44 So, Barbara Castanheira is going to talk about that in her talk. I didn't want to say anything on that because I don't want to spoil anything. And they also teach us about the eventual fate of planetary systems like ours and other stars and JJ Hermes 08:42:59 is going to talk about that in his talk and type one supernovae which we believe are exploding white dwarf stars are absolutely vital for understanding cosmological distances and the overall evolution of our universe. 08:43:13 And Dr Ken Shen is going to talk about that in his talk. So hopefully I haven't gone over too much, and I'd be happy to take your questions. Thanks. 08:43:22 Great, thanks so much Mike actually, if the timing is good, and we have plenty of time for questions. I'm someone Open it, open it up to all the teachers. 08:43:37 Yeah, I see Vince. 08:43:40 Oh yeah, I was wondering about what temperatures, does the crystallization happen. 08:43:46 Um, for the average white dwarf when they reach about between 50 506,000 degrees at the surface they start crystallizing. 08:43:56 And it's been a little while I couldn't tell you exact numbers but by the time they're 4500 degrees or so they're more or less fully crystallized. 08:44:05 And they don't crystallize it exactly a single temperature because in the center they crystallized and it's denture there, but then as they crystallized more the less than three regions or crystallizing and they crystallize it different temperatures. 08:44:20 But I love the tabletop aspect of it. I love being able to think hey this is high school physics I learned in chemistry which is frankly the last chemistry I ever had. 08:44:29 And it's neat to be able to use it here to reason. 08:44:38 Okay, more questions for Mike. 08:44:44 So while people may be thinking of, Sean. Okay. 08:44:47 Go for it. 08:44:55 Sean, are you there 08:45:02 cannot hear you. 08:45:07 Okay, better. You were talking about the Pauli exclusion principle relating to the, the electron degeneracy pressure. Can you just read remind us or tell us a little bit more about that degeneracy and, and how this fits in. 08:45:22 I actually in our textbook that we use for astronomy, they bring in they don't talk about pauli they talk about Heisenberg. So I would really love to know what you would think is a better way to talk about it. 08:45:35 Well that's a great point. 08:45:39 They're really both aspects of the same thing. 08:45:44 I, I just like. 08:45:47 Basically it comes, I don't know the right word to use for this, but you can't put two electrons in exactly the same quantum state. So for instance you, and that means that the same location and the same energy. 08:46:03 So, electrons have spin right so you can put one electron in this place at a low energy, and you can put the next electron there too, but it has to have opposite spin. 08:46:12 But then the third electron you put there can't have either of those spins, so if it's at the same location, then it has to be slightly higher in energy, and whether we think of that as the Pauli exclusion principle or the Heisenberg uncertainty principle 08:46:26 they're both relevant. 08:46:29 I think the Heisenberg is more known, but I prefer to think of it as the Pauli exclusion principle because you really can't put too many electrons in those days, so when you start piling electrons in it's like you're filling up a box and energy is the 08:46:42 dSo when you start piling electrons in it's like you're filling up a box and energy is the vertical direction and location is the horizontal one so when you start piling electrons in, they have to be more and more energetic to force them in. 08:46:50 And in a white dwarf the most energetic electrons the last ones in the box are actually going a significant fraction of the speed of light. 08:46:58 There may be going 60% of the speed of light. So to really understand why to our structure you also need special relativity, because they can't go any faster than the speed of light right you know you keep piling them in but they can get faster than that. 08:47:13 And that means that this this electron degeneracy pressure can't grow too fast as you jam more of a mass. 08:47:21 And one thing I wish I had mentioned but you know there's just not time for anything, white dwarfs are kind of backwards. If you have a point six solar mass white dwarf and you don't attend to the solar mass on it, you might think it gets bigger, but 08:47:33 doesn't it shrinks is smaller, and as you pile on more mass, it gets smaller still. And one of the reasons for that a simple reason of why there is a limit, as you approach one point for solar masses the white dwarf gets smaller and smaller and mathematically, 08:47:53 as you hit the Chandrasekhar mass around 1.42 solar masses the size of the white dwarf goes to zero. Now of course it doesn't really go to zero something bad happens first. 08:48:04 But one of the reasons that it electronic GMC pressure doesn't work forever is that the electrons can't actually move any faster than the speed of light. 08:48:13 And so there's a limit to how much pressure they can, they can contribute per unit volume. 08:48:20 And so hopefully yet they're kind of backwards stars and you may think guy says backwards but I like to think of it as electron generously pressure is like a spring. 08:48:30 You know if you keep dumping mass on the spring it compresses it. So that's kind of what's happening to white dwarf you don't want mass on they they compress it was a little confused because I thought I understand the idea of energy levels being a part 08:48:44 of a balanced object like the electrons within an atom, for example, but what happens to the electrons that are no longer on atoms that are free. Are they still going to have that limitation. 08:48:55 Yeah, I'm sorry there's a lot of things going on. Basically the electrons I'm talking about are all free. 08:49:02 And so I'm really only talking about free electrons. 08:49:06 But the box is containing of is really due to gravity so think of just a box, and you keep piling electrons in that box, then they can, they have to occupy that box but in order to put more and more of them in there, they have to be a higher energies, 08:49:24 so I'm really thinking of these are free electrons that are at high energies. And so really, I'm thinking of them in the box but they're really going back and forth a lot and they're hitting the wall, and that hitting of the walls is creating the pressure 08:49:37 that we're calling electron degeneracy pressure. 08:49:41 I know these concepts are not necessarily the simplest lens. 08:49:53 And perhaps other people here have other ways of thinking about this that might resonate more if you'd like to share. 08:49:56 Thank you. 08:50:00 Okay, so it was a very interesting conversation, Cheryl. 08:50:07 So, I'm just, uh, I hope that quick question but the Z machine that was fascinating to see that and to see that you can actually recreate in a lab. 08:50:17 Any history of the Z machine what else is it used for when did you start using it for this. 08:50:23 Yeah, I cut out those slides because there's only so much time, actually the very first pulse power machine goes back to the Nikola Tesla. He's the very first person to do this. 08:50:37 I don't remember the chronology but it goes back to like 1918 or 1928 or something like that. the Z machine had its roots in the 1980s. 08:50:49 And it was refurbished in the early 2000s into its present form. 08:50:55 It's about two thirds the size of a football field, and it has giant capacitors you know what capacitors are but it's capacitors other size of refrigerators, and as a dielectric in there, it uses either oil or water, so it's kind of low tech in that 08:51:13 way it's a bunch of things submerged in a ring and water and oil, and they all discharge at the same time and the current rushes in toward the center. 08:51:23 And so the currents all flowing in, and it has a width in time of about one microseconds so a millionth of a second. But then it goes through various staging switch sharpen the current so that it's, it, it sharpens it to where it has a width of about 08:51:35 a nanosecond. So all this current is hitting the center in about one nanosecond of time, and it can actually raise temperatures up to two to 3 million degrees. 08:51:49 That's actually too hot for us, so we actually set back about a foot to reach temperatures of only about 15,000 degrees, but there's other experiments. 08:51:59 And the neat thing is is it's like this little camp fire. So we actually have four experiments, they're all sitting around getting warm at the same time, there's one on top that's getting to 2 million degrees and there's others that are getting to a few 08:52:11 hundred thousand degrees, and there's us who are getting to maybe 15,000 degrees, and we simply did that because our white dwarfs that we care about are only about 15,000 degrees on their services. 08:52:24 All right, thank you. That is honestly such a great application for physics as well to think about that as an extension of some of the things that the students learn so thank you that's awesome. 08:52:35 Sure, and there's a there's a video on on the slack I think of a story they did on this, I was a little embarrassed to mention it sooner because it's targeted at teenagers so they're going for the gee whiz factor. 08:52:50 But, but it shows you film and footage of the inside of the Z machine so that's pretty cool. Oh good, thank you. 08:52:58 Yeah and Mike just said you know there was a lot of action going on in the slack on the chat a little chat so there's some question I've been asking, actually. 08:53:12 Some of the scientists answer some of those questions. 08:53:12 And some of the request actually came up and where the roundel to the material you share. And I think there are people who are interested in maybe accessing that material. 08:53:25 After, so maybe there will be a way to for you to share some of the videos and some of the slides for the Z machine especially. 08:53:35 I'm sure you know there'll be a way to share that material with everyone. 08:53:40 Later on, just wanted to mention and make it the point for that later. 08:53:45 Yeah, I'm very happy to share everything I have. I'm just afraid to look at the chat because you know I don't worry about that day I'm going to mess up all my windows. 08:53:57 Yeah, I will take care of that. 08:53:59 Hey thanks great course, Robert. 08:54:04 Do protons and neutrons also have an exclusion principal. 08:54:10 Yes, they do. 08:54:14 But they're much more massive. And so for them it happens, the relevant scales happen in a much smaller size. So you have to pin them into a much smaller volume before you start to get degeneracy pressure from them. 08:54:27 But that leads naturally into neutron stars. So electron degeneracy pressure electrons are you know like one 2,000th the mass of a proton and neutron. 08:54:39 And so electron degeneracy pressure hold up a whiteboard will neutron degeneracy pressure holds up a neutron star but due to that difference in mass, it only kicks in when you get a lot denser. 08:54:51 So while a white dwarf might be 6000 miles across a neutron star is only like 10 kilometers across. So it's a huge difference. And that's because neutrons are so much heavier than the electrons that you have to jam them into an even smaller volume in 08:55:07 order to get the neutron degeneracy pressure to kick in. 08:55:12 Yeah, that's a good. That's a really good question and that that shows particle physics right there. The difference in scales of the two stars. 08:55:24 Does that help. 08:55:26 Oh, yeah, thank you, is, is that all a different form of those are different states right of matter. Yeah, yeah, totally different so electron degeneracy pressure works for white jobs up to about 1.4 solar masses. 08:55:45 Above that, say 1.5 up to, we don't know exactly maybe three solar masses we think neutron degeneracy pressure can hold up a star. 08:55:53 But, but, five solar mass object couldn't be held up by a neutron degeneracy pressure for instance, so if you had a really small star and you didn't know what it was and it was five solar masses. 08:56:05 Pretty much everybody would say it's a black hole. So it's interesting that the three ultimately stable objects black holes neutron stars and white dwarfs their characteristics are determined by quantum mechanics and particle masses. 08:56:22 So it's really like a macroscopic demonstration of particle physics. So that's kind of cool. 08:56:33 And if you have any general questions about him I think I'll refer you to Lars. 08:56:43 Thank you. 08:56:44 Sure. 08:56:46 Okay, so any more questions for Mike. 08:56:53 Can I ask one or two Maggie. Okay. Yeah and sorry if I missed this I like going back and forth between administration and listening, um, you mentioned that this that like the sun was going to lose about 45% of its mass, how do you estimate that or is 08:57:08 that like implicit in some other calculation you show it. 08:57:12 Oh, it's because we make stellar evolution miles and we look at other stars, so you know we can't see one star do anything right we just get snapshots, but we get millions of snapshots, and we try to put them together. 08:57:25 And so we can without getting more technical we can actually say, if we look at a cluster. We can see for instance that the stars up to three solar masses of all left and become white dwarfs, and we could say, well, the most massive star the star on the 08:57:40 Main Sequences three solar masses. And the most massive white dwarf that we see in the cluster is point eight solar masses say, so we can kind of say empirically that oh it looks like three solar mass stars become point eight solar mass white dwarfs, so 08:57:59 we can, we can do that and then we can make our models match that stuff. 08:58:03 But there's almost no clusters. 08:58:06 But there's almost no clusters, where they're 10 billion years old, where it actual one solar mass star has actually become a point five six solar mass white dwarf, and we've actually caught in the act. 08:58:18 Maybe there is a few but we're really relying on our models of stars at that point which we've calibrated through these other observations. 08:58:26 But that is, I would claim that that is mostly a model dependent statement, but we think that's what's going to happen. 08:58:38 Mark on the chat, just asked, Kim the rotation peers of wide works, be used to estimate their ages. 08:58:48 That's a good question, you know, but I'm just going to defer to JJ on now and you can do that for main sequence stars. I'm really not aware of, of being able to do that with white dwarfs but it's a very reasonable question. 08:59:01 It's a very reasonable question. 08:59:04 We wish, but fortunately there, it's it's not so simple with white dwarfs. 08:59:12 Yeah, I think if I may have the main issues that we don't understand fully how rotation propagates if you want inside the star, so how angular momentum technical leads is transported within is still a plasma. 08:59:29 So as the star, expand and trains in different regions, we don't really know how much the core which is in this case what end up being part of the white dwarf. 08:59:40 how fast it rotates. 08:59:42 We can observe it, but I definitely we cannot predict it very well. And so any hope doing gyro chronology, which is the, the astrophysical effort to try and to age Stars by looking at how rapidly they rotate doesn't seem to be applicable yet to very late 09:00:02 phase of evolution like white dwarfs, but now as Mike said we can do it four stars on the main sequence. 09:00:10 But it's certainly an interesting question about rotation and white dwarfs because not, not only do stars lose a lot of their mass they also lose a lot of their angular momentum and rotation and becoming a white dwarf so understanding why you are, how 09:00:22 they rotate tells us something about that loss process as well. And I'll just advertise Barbara's talk because one of the best ways of getting rotation rates of white dwarfs is looking at their pulsations.