Okay. Good evening, ladies and gentlemen. Thank you for coming to this 14th fancy lecture. I don't know whether you're aware of this, but the planning for these lectures is done in great detail, and the dates are chosen very carefully. So today we are going to have a lecture from a very eminent European astrophysicist, and today is stone. So that's very appropriate. Thank you for coming on Star Wars Day.
So I particularly like to welcome the pupils and teachers and say parents I understand from Parker School near Bristol, thanks for coming. I think some of you have been before, so that's terrific. And it's great to have you here. You've come back a long way, so thank you for coming. So let me introduce the speaker. So our 14th lecturer is Professor Connie S, who's the director of Institute of Astronomy at the Catholic University in London.
She also holds an appointment at the University of Nijmegen in the Netherlands. And in addition to her role as director of institute astronomy, she's also the vice dean for communication and outreach in the Faculty of Science. So you can imagine that she's pretty used to giving talks in formats like this. Connie is a world expert on the structure and evolution of stars.
She has some remarkable qualities. She was the first person to ever be awarded consecutive senior research grants from the European Research Council. I think she's probably the only person to have that honour. But that's a slightly larger claim that I could actually prove, so I put it that way.
She's also a champion of women in science, and so she has that in common with a previous intellectual, McGorry, from Yale, who was the American Astronomer Society president when she gave a lecture about eight months ago. Connie has been widely recognised for the for her accomplishments most recently, and I'm going to read this, I'm afraid. Last year she was made a commander of the Order of Leopold by the King of Belgium, King Felipe.
This is the highest civilian recognition that's offered by the for services to the Kingdom of Belgium. In 2012, she was awarded the Franky Prize, which is like the Nobel Prize in Belgium, but she was awarded that by King Albert. She's also an honorary fellow of the Royal Astronomical Society and does a host of other things that I could say, but there isn't time. So it's a great pleasure for me to introduce her to give the 14th NC lecture. Star Quakes Exposed. Stellar Heartbeats. Colony Arts.
Thanks a lot for this introduction and thank you all for coming and giving me the opportunity to give a public lecture on my passion, which is astronomy in general, but stars particularly, and I wanted to study them ever since I was in primary school. So I hope I can convey some of the recent findings that we have in this field. And I'm also very happy to see young people in the audience and less young people.
So I'm used to ages between four and 99 when I give lectures in school and all other types of fora. So let's see how stars live. And I'm trying to explain to you why that's a very interesting field in astronomy, particularly in this area. And so usually I begin by explaining that stars are really hot gaseous spheres and they radiate, and they do that thanks to nuclear fusion in their core. So in order to be able to do that, we cannot do that safely on these planets.
We need a high temperature. And so to set the scene, the sun, the surface to see here has a temperature and its surface of about 5500 degrees, but in its interior, it's millions of degrees. And that's what you need to do. Nuclear fusion and to create radiation from planets cannot do that. So here you see an image of the planets of our solar system, and you do see them shining in the night sky, even though this may not so be obvious in Oxford and it's even less obvious in Belgium.
So called for two. So they shine. But that's not because they create nuclear energy. They are not massive enough for that, but they reflect the solar, right? So this is reflection of the host star's radiation. That's something different. And that tells you immediately why we have a hard time to search planets in the galaxy and it's much easier to see the stars. Okay, so that's just the difference. Keep that in mind.
I will come back to that. So far, the first a lesson on the difference between stars and planets. That's a question that tends to pop up when we give public those. Why do we want to bother about stars? Well, for me, stars are the building blocks of universe, and some other experts in astronomy might say no galaxies are the building blocks of the universe, but galaxies are made of stars.
And so if we want to understand how they live, they have billions and billions of stars, and there are billions of galaxies in this expanding universe where we live in. So one of the issues is that we better understand how the stars live, because then we can understand better how galaxies see. And so in that sense, stars are, for me, really the bricks of the universe. Stars get born out of very cold, dense clouds in our Milky Way. So our Milky Way's one galaxy, it has a galactic plane.
And we see inside that plane mainly it's also a little bit above it, really dark areas that you see here. And these give rise to names that people use their fantasy to name these clouds. And this is dark here. Not because there are no stars or there's nothing. No, because there's material dust mainly that blocks the light from the stars behind it.
And so these clouds, they when they get disturbed by one or the other reason they can collapse, and when they collapse, they form stars many, many stars at the same time. So stars are born in groups. We call them clusters. So our sun was also born in such a group, but now our sun is isolated. Luckily for us, it's a stable environment here because when these clusters pass through the Milky Way, they get disrupted depending on how close they are together.
So this stellar birth is something that we have yet to understand better because it happens in very cold environments. And so cold environments are difficult to probe with our eyes mainly. And we need specific instrumentation but in any way. Stars get born. They live their life and they die. And we want to understand how that works. Nowadays we can see that stars when they get born. So each of these, let's say, entities in such a cloud collapses under its own gravity and it contracts.
And when you contract things, when you put things together, you heat up. Yeah, I shall not do the experiments here in the audience. But if I were to squeeze you in a tiny area, you would also heat up, so to speak. Right. And they do that until they reach a very high temperature, a temperature high enough to create fusion. And at that point, we call it the star. Now, when they do that, there's always some leftover material that's left behind, and that's what created us.
So we could say that we are sort of the rubbish that was left over when the sun formed. That's perhaps not so positive way of looking at us, but that's the way it is. Yeah. So stars have here is a model theoretical computation of a star that gets born and it has some surrounding material that then is being put in a disk and planets can form out of that material. We can nowadays see that by infrared instruments, either in space missions or from the ground.
This is an image of the VLT, the very large telescope in the Atacama Desert. That's a very pleasant place to go if you ever get the chance and haven't done that. It's a magnificent observatory. And so we see that, in fact, these are stars where the central star is blocked to show that there is remnant material that then may create planets. So the message is stars of planets are formed together so they have more or less the same age.
And one of the key issues that we want to understand is how old stars are and how we can deduce their age. So as an asset, you immediately get the age of the planetary system around the star, around the host. Okay. Now how do stars work? So here I'm explaining how the nuclear fusion inside the stars typically stars like the sun. Let's take our own sun. You know we best why they radiate.
And so a nuclear reactor inside the inner parts of the star is way more efficient than what we can do here on Earth. So stars are super experts in nuclear fusion, and the sun is doing the simplest nuclear fusion one can imagine. And that is starting from the very simplest chemical element that we know, which is hydrogen. So the hydrogen atom consists of an inner proton and an electron circling around it.
And when you push four of these very, very close together, so you create a very high temperature, pressure and density, then these can melt together and that creates energy. Why is that? Well, four of those together make up one helium atom. And you see that from here. And in this helium atom, you will have four nuclear particles, two electrons. And so the mass of this helium atom is lower than the sum of the masses of these four. And so then we can use Albert Einstein, most famous formula.
He invented general relativity. But I think if you ask people in the streets, where do you what's the famous formula of Albert Einstein? They would say yes and see squared. Yeah. So a little bit of mass is lost from this towards this and that mass multiplied by the speed of light squared, gives you energy. And that's the energy that the stars radiate.
Yeah. So in that sense, stars can create nuclear energy and planets cannot because they do not have a high enough temperature in the centre while the stars still have it. How long can a star do that? Because this in fact sets the life of a star. It must be able to create energy to counterbalance gravity that wants to, you know, push the layers together.
Well, it can do that as long as it has hydrogen, of course, because if there is no hydrogen left in the region where the density and temperature is high enough, then the nuclear reactor suddenly stops. I call that's an energy crisis. And so. It depends on the amount of matter and the amount of gas that the star got at its birth. On how long it can live through nuclear fusion. I want you to appreciate the stars, as I did from very early on, though not because I realised all this.
Not at all. But stars created all elements of the chemical table. So here you see all the elements that we know. At the early universe there was only hydrogen and helium. So these two boxes and a tiny little bit of lithium and all these other chemical elements were made by stars. So I always say medical doctors look at your body in a different way than I do because you're just stardust. Yeah, that's what we are made of. This is a bit weird to think of us like that, but that's how it is.
So of course we want to appreciate that, but we also want to understand exactly how that works. And so the nuclear fusion of hydrogen turned into helium happens in the in the sun. Right now, the sun is doing that in the area where it's hot enough to do this nuclear burn. Okay. And so we want to know how long the sun can still provide us with that energy so that we know what's going to happen to our solar system. Because stars get born, live their life, and then they die.
Yeah. And so this is a timeline of the solar life. After its birth, it lives its life quietly, regularly fusing hydrogen into helium. And it can do that for a long time. In the case of the sun, about 10 billion years, as you can see here on this graph. And we are about halfway, let's say, with the sun. Yeah. Now, the more mass the star has at birth, the shorter its life. That's a bit weird, perhaps, but that's because it consumes more of the energy because it radiates much stronger.
Okay. So it doesn't last that long. That's it. But for the sun, we know we're about halfway, which is smooth with its nuclear burning. And what will happen then? Well, if there is no longer any hydrogen in the in the core of the sun, which is the nuclear reactor, then the sun is it's an energy crisis. It can no longer produce the counterbalancing force of gravity. So it has to come up with something different. And what can it do? Well, it can try to fuse helium and create heavier elements.
Yeah, that's the only thing it can do. But to do that requires a temperature that's ten times higher than fusing hydrogen. So how can you make sure that it's hot enough to fuse helium? Well, how do you make things hotter? You push things together so the sun will start constructing its inner parts. That's at that point, helium. And as a as a counter reaction, it will start expanding. And this expansion will mean that the sun will become what we call a red giant star.
And that's what you see indicated here. This is not pleasant for us as an outlook because the sun will become really big and we know more or less when it will happen, you know, more or less how big it will be, but not very precisely. And that's one of the things we want to learn better. So that's not something to worry about because it will not happen in your lifetime, not the one of your grandchildren or their grandchildren. It will take a billion years.
So we have some time to do some thinking about how to say what to do now in order to improve the models that we have and the predictions, we need to know the details about how the physics in the interior of the sun is to a better precision that we can do now. And so one of the key issues is how stars die. We know that more or less also for the sun and when it will take place. We know it more or less, but not very precisely. And stars can die, in fact, in two ways.
And it can lead to three products. And so stars like the Sun will become a red giant, will expel the material, and will give rise to a dying star called the White Dwarf. Which is, in fact a carbon oxygen bowl at the size of the Earth, more or less. And this goes quietly so it will not explode. Stars that get born with a lot more mass than the sun. They live much shorter lives, and they end their life explosively.
They manage to burn helium into carbon and then carbon into heavier elements until they reach iron and then they explode. Yeah, these stars are very different from our sun. So I'm not bothered today with these stars because they don't have planets and we don't have to worry about the sort of future being along this path. So in the end they explode and they can give rise to a neutron star or a black hole. But this is a very different type of life that they lead compared to our sun.
Now, at the moment when the sun hits the energy crisis, it will sort of look a bit like this. This is an artist's impression. Just to clarify, this is not a real image. And so the sun will somehow come very close to earth, and the predictions are that its radius will pass to Earth. So it will eat us, if you like. And then we'll end it somewhere with this radius between the Earth and Mars. Where exactly? We don't know. So we have a desire, so to speak, to try and understand how large stars are.
These things easy, but it's not. Because if you look at the night sky, even with our big telescopes, stars are not resource. We can't resolve the surface. They are tiny little dots that's shiny, right? And so we have to come up with a way to estimate the size or the radii of stars in a better way. I can also not wait and do the experiments until the sun dies. We don't want to do that. We want to be a bit more proactive. Let's say no. And so stars live millions to billions of years.
We can't wait and see what happens to improve our theory. That's far too slow. And what do we do to solve that? Well, we take stars like the sun in different stages of their life, and we try to put the pieces of the puzzle together to improve our theory. But then, of course, we also want to test these theoretical findings, so to speak. And so that's one of the issues where our field of research has made a lot of progress the last decade now.
And so I always come up with Eddington, who was sort of considered the father of stellar structure, who was very frustrated in his book, The Eternal Constitution of Stars. Yeah, it's almost well, it's no, it's more than 90 years old, meanwhile, but still a very illustrative book to read. And his frustration was that we couldn't do experiments on the interior of stars. We cannot grab a part of the sun and do laboratory experiments, as experimental physicists can do.
We have to come up with some more original way of doing the testing now. And so he wondered what appliance can ever pierce through the outer layers of a star and test the conditions within? And now we know at least one answer how to do that. And that's the topic of today. It's something that we call astro seismology. So differently. This is the study of star quakes, as I tend to call it, because it's a bit more easy to understand if you compare with earthquakes.
And in fact, the the mathematical treatments is the same. So what the geologists here of the Earth do, well, they love earthquakes, particularly quakes that they involve themselves in that are modest in in amplitude. Yeah, they love that. Why? Because earthquakes create waves that travel through the earth, that bounce back at the and a silicon core of the earth. And then the waves travel back and their seismographs measure the travel time of these waves.
And the travel time tells you where the cause of our planet's and how big it is and what its chemical composition could be. So waves are a very powerful tool for physicists in general. Now, I cannot drill a hole here until I reach the core of the earth. That's frustrating, right, for geophysicists. Well, I cannot look inside. The star is the same, but stars. Earthquakes and quakes create waves so we can do the same physics. And this has become possible because we can observe these star quakes.
Now, how does that work? Well, so first of all, this work means that we know the stars, their oscillations, and we are clever people. So we could sort of do a reasoning using these oscillations and then understand the stellar interior. So this is what Eddington couldn't do because he didn't have the data to do that. And so here in this drawing, I've given a cartoon like picture of how that works. So you have this star just like you have the earth.
There are quakes going on. These quakes create waves. And here are four types of waves the red, yellow, green and purple one. And these quakes move in the interior of the star and they bounce back up. And this happens inside the star. It happens inside the sun right now. We don't notice that as human beings here, because the soul of quakes are very, very tiny. And that's in general the case for most of the stars.
Now, imagine that you follow such a wave, and here is a many patterns of the green and the yellow one, because these quakes have almost the same periodicity, the same frequency. Yes. So imagine the stellar surface goes, you know, in a complicated way up and down. And it does that with the certain periods or frequency, like that's one over the period.
And then you follow these waves as they probe the physical properties in this yellow layer, and the green one does the same, except it goes a little bit deeper. So some of the waves manage to go a bit deeper than the others. And so what are what do we want to do? We want to have the Green Wave study and the yellow one, and each has their own frequency.
If we subtract these two frequencies from each other, then we get the information of the area in the star, which is green on this cartoon and not yellow. That tiny little layer is felt by the green wave, but not by the yellow. With as so, its properties are a bit different and that property difference is connected or is determined by the physics inside that layer. Okay, so if we can do that for a whole lot of these waves, then we can build up the physics layer by layer inside this stuff.
That's how it works in practice. There's a whole mathematical scheme behind this. I will not bother you by that. But the picture that is in this cartoon sort of gives you a handle on how we do these things. Now these waves propagate inside the star. They don't come towards us. The sun for the moment is has quakes. Hundreds of solar quakes. You don't notice this when you look at the sun and you can try, but please protect your eyes.
Yeah, because they have. These are such low amplitude that you cannot see. But these quakes create sound waves that propagate inside the sun. Yeah. And so that's a bit similar then. I'm creating sound waves all the time now in this auditorium. So I have this beautiful cavity here. Sounds is propagating and you can hear me. Yeah. If this auditorium would have different air inside the inside here.
Let's say there was helium gas here. It would sound I would sound weirdly, the frequency of the sound would change. So the frequency of an oscillation mode that creates a wave also changes according to the chemical composition of the medium where the sound waves travel. Okay. So we can actually make an analogy with sound waves that you're familiar with. And I tend to do the analogy with respect to music because people like music. And so let's do a break and the interludes.
Oh, and are there any musicians in the audience? Don't worry. I will help make you play yourself to be shy. Okay. Musicians know very well how sound waves work without doing the mathematics of it necessarily, but you know, more intuitively. Yeah. And so, in fact, the stellar oscillations create sound. The travel inside the star. But we can't hear them. Why not? The sound waves are in the concert hall.
As so my concert halls are stars and the sounds don't reach me because there's no medium between me and the stars. So the sound cannot propagate outside the setup. Okay, so I'm going to have to cheat a little bit. But what you can all do is, well, my students always have to work. You're my students now. So I will do an exam. It's not going to be difficult. And I sometimes I do it with real musicians, but now I have a limited amount of time and I'm another.
Another musician myself with it. Really? Let's do a thought experiment. Right. So I'm showing a musical instrument. Now, let's say a contrabass, and it can produce sound waves. And you can listen to it. Now, what would happen if I have another musician and I lets him or her play on this instrument, a tiny little violin? And the question would be, well, the sounds of this instrument and of this instrument is different.
Right. And so the frequency of this instrument or the tones, if you like, to make more artistic vocabulary. Yeah. The question is, which of the two will play the highest tones, the big instrument or the small instrument? And I will do a yes and no. So who votes? Highest thoughts? This instrument fool. Who thinks that's a good answer? Hailstones and this instrument were things. That's the correct answer. You are all seismologists. You notice a smaller instrument gives higher frequencies.
That's something you are familiar with, right? So if I could only listen to the sounds inside the star, then I would know the size of the star. And that was one thing. We really want to improve our knowledge with that. So let's try to do an experiment. The Sun is a musical instrument for me. It's a three dimensional guitar, you could see. Right? And a one dimensional guitar is played by musicians in this way.
You can play it in the fundamental mode, as it's called, or with the first overtone or the second overtone. You could also call them harmonics. And so any tune that's being played has an amplitude. That's the height with which this goes up and down. And it has a periods up and down and up and up. Or a frequency. Yeah. We tend to work in frequency rather than in period. And so when I speak of frequencies, you can think of musical tones, so to speak.
And the strength of the sounds is the amplitude. And so some of my notes have an audio point, and others have two notes and then three notes and so on. Yeah. So now we turn that into a three dimensional string, let's say, and then you get a stop. Yeah. And so a star could come. Can be seen as such a motion. Yeah. This is one isolation load, and different layers go up and down. Yeah, this is the. The Stellar Oscillation or the quake. And we can't look in the interior, but we would like to.
Yeah. And so that's not possible for a star. We have to somehow find a way to listen to the star's tones through the frequencies without being able to inject ourselves into the star. Okay, so let's try to do that. And I have first have to confess something. I want to place you inside the sun and let you enjoy the symphony of the sun. We unfortunately cannot hear it in our daily life, but I can make a sound file and let you listen to the quakes of the sun.
The problem is that your ears are not well-suited. You are not in the audible range of the solar earthquakes. So I made a factor 100,000 to bring it to your audible range. But I do that with the global symphony, a symphony of the sun. Okay, so we're not going to listen to the solar quakes as if you're inside the sun and then you can enjoy the symphony of the sun. In my language, that means the frequencies of the solar oscillations. Okay. Are you ready? Just enjoy. Is this a nice symphony?
How lucky we are that we don't have to listen to that all the time. Okay. That, for me is beautiful. Not particularly the sound. But what have you heard now? You have heard this symphony of the sun. And this is measurements from the Soho satellites. But don't forget, I have multiplied by 100 thousands. And you see here the strength of the sounds versus frequency expressed in microwaves. And all these lines that go up are the musical tones that you hear.
And the higher the line, the stronger the tone. And so the strongest one for the sun occurs more or less at 3000 micro hertz. If you turn that into periods, that's about 5 minutes. So the solar surface experiences a quake, a dominant quake that goes up and down by a period of 5 minutes. And there are many quakes going on at the same time. So we can construct this type of diagram and it tells us a lot about the physics of the sun, among other things, its size.
Think of the violin versus contra bass by just measuring these quakes. Now, we can't listen to them, but what we can measure is that the quakes make the surface parts go up and down. And this changes slightly the temperature at at the surface. And this we can measure in light intensity. So that's the way we observe the quakes, even though we can't listen to. Okay. And so now we have a quiz number two. Now I'm going to want to know how the quiz sounds when the sun is about to die.
And, you know, as it will die, it will shrink its core in its inner parts to make it harder for nuclear fusion of helium. And at the same time, it will expand its outer layers. So we're going to make a big star. So we're going to make a big instrument. So will the frequencies go up or down? Who votes for down the lower frequencies. So I'm not going to make you enjoy the sound waves of a red giant that has been observed by the Kepler satellites.
And your prediction is that the frequencies should be lower? Yeah, that's far lower. And kits like this in Belgian disco since we did this press release. So sometimes I have to ask questions. Why do you want to ask to support fundamental science? Well, it has some practical use. Okay. So this is a red giant. And so here you have strength of the music, so to speak, as a function of frequency. And now it has beautiful quake's sound waves created.
And it takes it about, let's say, 66, 67 micro hertz, much lower than the sun, which was at 3000. So this must be a much bigger star. And we scaled and then we know how big the star is while it's a dot on the sky. Yeah, that's impressive, because the way we can do the sizes of stars in this way is with a relative precision of only a few percent, while it's, you know, billions of kilometres away. Yeah. Again, we can't hear this, but we can see the fluctuations on this telescope.
Okay. Is this a bigger star in the sun or a smaller one? Smaller one. This is typical of the previous one was a cosmic buzzing, cosmic pickle. This is a tiny star compared to the sun and it has very high tones because of that. But it's very special in the sense that it is a super star. It was measured in La Palma with the William Herschel telescope. So you can also do this science from the ground because it has big quakes. And you see, that's why the noise level here is larger. Yeah.
Compared to the previous two that were measurements from space. Oh, less noisy. And so here we are at about 7 million or 7000 micro hertz, which is higher than the sun. Okay, so this is a very tiny, small star. Yeah. And it's very faint. So we can. We can this stellar sizing we can do. So we can predict in the in the solar life. How the sun as a red giant. How big it will become. Oh. And it might be a little bit less bleak by the time it reaches the hydrogen exhaustion than we anticipate.
How does this research field work in practice? Not by making sound files. That's fun for a public talk because it avoids me to have to show you mathematics. And mathematics is really fun and beautiful, but it's not for every person in the audience. So in practice, we measure the quakes by the sort of light intensity metre that we put into space. We can do this from the ground for very big quakes.
Yeah, but we can't do it for the majority of stars because the tiny little variations that you want to measure are screwed up by the Earth's atmosphere. Atmosphere is unstable, and the very people say that stars, twinkle, stars twinkle is the atmosphere of the earth that makes us think that the stars are right. And so that destroys our beautiful seismic signal on this stuff. So we need space missions. And these are two missions that we're operational.
You see now, more than ten years ago, a European and an American one. And there are unmanned space missions. Sometimes I get the question. You go up there. Why don't we don't want me up there. I would screw up the measurements, probably. And so these are scientific missions. But onboard is an ultra precise. Seismograph, as I tend to call it, that can measure the tiny little brightness variations as the quakes score. And we can measure that with the precision that we express as parts per million.
So imagine that we give the equilibrium of the sun when it doesn't have quakes, a value of a million in water in a unit, energy unit. The quakes make it deviate from that equilibrium and the deviation is one out of a million. So instead of a million, it would be 1,000,001. And we can measure that or 900 words.
I can't even pronounce that number. Okay. So tiny little fluctuations and we can measure them uninterruptedly because the space mission is not bothered by a day and night rhythm from the earth. We are always blocked because now there are plenty of stars, but you can't see them during the day and you can't measure them. Okay. So that's why we made such big progress in these fields. And so what are we doing? Practice.
So we know we have all these quakes going on at the surface, and that creates waves that propagate in the interior. We measure their brightness. They create the variations as a function of time. And this is one star observed by the Kepler satellites. And this is sort of my seismograph, so to speak. So you see all these fluctuations. It's not strong here, stronger here. That's what we call beating of stellar oscillations that strengthen each other here and weaken each other there.
And all these quakes we can measure and these were the diagrams that you just heard for three different stars, strength as a function of frequency. And so for this star, for instance, it peaks its quakes are strongest at 75 micro hertz. So, you know, that must be a bigger star than the sun, right? Because the sun peaks at 3000 micro. So this one was must be bigger. And that's also what we have for this star is there is more it's more massive than the sun.
And that means it's bigger now. So again, that's why I showed this movie before we measured the quakes at the surface of the star and the signal that we get in such what we call a light curve. We can then say something about what is happening inside. So that's the appliance of Eddington. Yeah. We can go inside the star by measuring quakes and doing and an interpretation of the seismology of the star. And here you see four stars observed by Kepler. You see their light curves only during 20 days.
In reality, we are 1500 days. We have four years uninterrupted data. And this white curve, you see the fluctuations go up and down faster than this blue curve. So shorter periods is higher frequency. That's a smaller star. Yeah, that's typical, you would say. And so intermediate cases here. So the sizes are not the scale, but you get the picture right. We observe such a curve. We did use the frequencies of this of the quakes.
And then we know how big it is. So that's it's as simple as that, so to speak. Yeah. And we can do that nowadays for thousands of stars, thanks to this space mission. I have, again, something special. And I change this because everywhere I give this talk, it's coffee with milk. But I'm in your case as a courtesy to the nice tea milk session I had with the students. Why? What do I mean by that? Well, what do you do when you drink tea here and you want to drink tea with milk?
You pour milk in your cup and you're not going to sit and wait because then your tea gets cold. By the time that everything is mixed, doesn't mix very efficiently, you take a spoon and you stir. Right. In my language, you add angular momentum to the mixture of the tea with milk. So your spoon makes the tea and the nuke rotates and that means the material is better mixed. And then you can drink it while it's warm, right?
So rotation gives mixing and we don't know how stars rotate in their interior or we didn't know how that worked until a few years ago. We thought we knew from theory, from theoretical models. And then came the experiments with the seismology. And it turns out we didn't know how it worked. Yeah. So why are Star Plates so helpful here? Well, imagine that I have this violin player.
Yeah. And I let the person play a nice symphony, and I. Check it a little bit behind the scenes and I make the podium turn around. Then the symphony screwed for you as an audience. It's horrible. It's a nice experiment. Sometimes I do that, but then I need the real theatre now. And that's still to help you think of the fact that rotation. Shifts the frequencies of all the sound waves, and it screws up the same thing.
Yeah. Now imagine that we have a quake travelling through a star and the star rotates. Then it shifts the frequencies and we measure frequencies with these satellites so we can compare a star that doesn't rotate its frequencies with the star that seemingly looks the same from the exterior. Yeah, but that does not rotate versus rotate strongly in the interior. And the frequencies will be shifted. And we can unravel that.
That's why I make this analogy. So we have mammoth thanks to very high precision measurements from Kepler. And this is put in an animation stellar sound. But you know, the sun here, it rotates. We know how fast it rotates at the surface and we know that for other stars, too, we can measure surface rotation.
But I want to know how the material is mixed inside the star, because if you have rotation in the area where the nuclear fusion is going on, then you add hydrogen to the core region and it can take part in the energy production and the star can live longer. So the more mixing, the longer the star lives. And that gives me a handle on predicting the ages of the stars.
So in this animation done by my former students, Paul Beck, he unravelled from data from the Kepler mission that the Red Giants have a core that rotates faster than the envelope. And that's also what theory predicts, because remember I told you, let's assume that the hydrogen is gone in the core of the sun. What will it do? It will start shrinking its core to make it hotter in an attempt to burn helium, and the outer layers will expand.
And then if you shrink the core, it will start spinning faster and the outer layers are expanding, so they go slower. So we can expect that a faster core then envelope rotation. But the models were two orders of magnitude wrong. So what the quakes tell us is that the cause of red giant stars are a factor 100 slower than what our theoretical models predict. And that's fascinating, because that means that we can learn something, and it means that we thought we knew things well, but we don't.
And this is what I would still call it, an unsolved problem. So that's one of the major achievements that we have in this field. And it's couples to the question I had on my first, like, how old are the stars? Well, we need to know the amount of matter that can take part in nuclear fusion because that sets the age of the star. Yeah. And if we want to know how much matter is in the nuclear reactor, we need to understand the rotation inside the star.
Typically, classically, without quakes, we would measure the rotation of the sun at the surface. You can do that. That's an experiment we do when school children come and visit us. Why? Because you have seen in the animation the sun has sunspots and they rotate in your line of sight. Again, don't try to do this without protective glasses, but you can follow that and reduce the surface rotation of the sun.
So for the non astronomers in the audience, what would be the rotation periods of the sun at the surface? You know, that's. Who knows that? Isn't that weird? That's our host star. That's our mother star. And we don't even know how it rotates at the surface. So we make students derive that. But as there are many astronomers here, it's about 26 days. Yeah. That's an experiment you can't do on a day. So we have to have the Terminators come frequently.
But that doesn't say anything about how fast or slow the sun rotates near its nuclear reactor. We don't know that. And so we were assuming well, maybe to say, yeah, why not? If we have to make an assumption, let's say it's the same. We still don't know that today. We know it's for a giant stars that are very far away in the galaxy. But we do not know it for our own sun. Why not? Because the quakes of the sun do not go deep enough.
Remember I had this cartoon with this red and yellow and this green wave. And there was also a purple one that goes straight to the centre of the star. And I love purple waves because they probe the in the region of the sun. And that's where the life is directed. The sun may have such waves, but we have yet to detect them. They are blocked for the sun. More massive stars do have such purple waves and ridges also. And so we can use them to deduce the rotation and the mixing of the core.
Right. So it's all a matter of getting to this in the region of a star. And this white area here is a is a cartoon to to imagine the parts of the star that take part in the nuclear fusion. And surrounding it is mixing going on and you see indicate that's here and then the radiation comes out the matter in this next area is something that determines how long the star can live. And this is very different for different types of stars.
So we have been able now to measure somehow contributions of rotation to mixing tea with milk. Efficient mixing. Fast. Yeah. And that's what's going on in stars. And we need more. We found that there is more matter that can take part in the nuclear fusion than we had anticipated.
So very massive stars that I haven't discussed very much so far will not only burn hydrogen and transform it into helium, then they contract and then they start burning helium and turn it into carbon and they build up the whole chemical table, so to speak. And the way they do that really depends on how the material is efficiently mixed. So we found recently that more helium is produced in the very stable, earliest phases of stars.
And that's means that the chemistry that the universe is undergoing and the Milky Way in general may be different than we had anticipated before. So this is quite a recent fun. No. And that is thing we want to know is how far away are the stars. This is relevant for those of you who were in yesterday. We had a fascinating lecture on extraterrestrial life. And so quite often we get the question, tell us where to fly to another planet because it's not going very well here.
Well, that's. I even had once received the question from somebody. You telling me the planets and I pay you money. Yeah. That's really the type of questions you get sometimes. So then I gave a whole lecture on how stars live and how their planets live along with the stuff. And so one of the things that then comes into play is how far away are stars? Again, this seems a simple question. Why is that so easy? Because you see them as point sources in the sky.
Now, this requires that we are able to determine the intrinsic energy output of the star. Think of a lamp. You know, if you have a lamp and I put them at two metres from you, then if you experience a certain what we call luminosity strength of the light. But if I put it a kilometre further away, even if it shines intrinsically in the same way, you won't see it anymore because the distance to the star is really an important factor here, and stellar distances are not so easy to determine.
So what do we need to do for that? Well, we can know the ages of stars now from seismology. And so if we know the radius and the temperature at the surface, then we know the intrinsic energy output at the stellar surface, and then we can derive the distance. And I call that a seismic distance because we use star quakes to derive that. And so then you can give a diagram.
So people who study galaxies or our own Milky Way for the moment, the age of the star from the quakes as a function of the distance. And this is very far out in the galaxy. This is a result obtained from the coral satellites. So the European satellite that flew about ten years ago where each of these blue dots is a red giant star in our galaxy where the seismology has given us the H and we still have quite some uncertainty.
You see that and the distance and the uncertainties are quite large, but you have to realise that before the coronation this diagram was empty, so to speak. Yeah. So this is really a major challenge, particularly if you think of how far these stars are. And we astronomers tend to express that in a unit called Parsec. Yeah, but I've written down for you. What that means is 31 and then 15 zeros if you express in kilometres. Yeah. That's very, very far away. And yet we can determine their ages.
That's amazing. Thanks to the star eclipse. So where do we want to go with this domain? Well, it's really an interesting time, as we also heard yesterday, because we are really at the good time to start hunting for extra terrestrial life. What does that have to do with stars? A lot. Because planets are in orbits around host stars, as I call them. The Sun is our host star. And we can determine our ages of stars and sizes across their life.
So the question then is will stars when they have planets? Will they have an ideal circumstance for life or not? Does the life stay there for a long time or not? When when the star starts expanding as a giant and, you know, burn the planet in general. So there are all sorts of interesting questions that couple planets around stars to their own. And we want to understand that better. And so ideally, you want to study exoplanets and measure their quakes, because then we have a good handle on this.
And this is what the Kepler mission and the current mission used as a way to discover exoplanets. So this talk is not about exoplanets, but stars. But this is a star and in front of it passes are planets. And as we'll see moving on, how do we find these exoplanets efficiently nowadays? We can't see them. As I said in the beginning, because they don't they don't produce nuclear fusion. They can't do that. And so their brightness is mainly reflected light from their host star.
But when a planet passes in front of its parent star, its host star, you see a tiny little dip in its brightness because you cover a part of the surface without being able of resolving the surface. You do see when you have a very high precision instrument that measures brightness variations, you do see a, you know, oh, there is an object passing in front of the star and in my line of sight. And so this dip here happens to be of older parts per million.
So, in fact, people who study star quakes and who hunt for planets around other stars and the sun meet the same data. And that's why these two science cases go hand in hand. Of course I'm on this part. Yeah, others are on this part. But we work together to make this science case optimal because we have many planets. We heard that yesterday. We have thousands of planets discovered meanwhile, but we don't know how large they are. And we don't know what their ages.
If we. Cannot have that information from their staff. And so that's where ostracism ology can help, and that's what we are doing in practice. So the Kepler mission mainly has given us many, many transits, detections of exoplanets. And here is one of their first ones. So we number them that's boring naming. But then so what do you do when you have a transit? Well, if you have the radius of the star, then you can deduce from the duration of the eclipse.
The radius of the planets. Yeah. And we can do that from the transits. Then you need the radius of the star with a very high precision and quakes give you an extremely high precision compared to any other. So that's one thing. But then we also want to know, is this a planet where it would be nice to live or not? Is it a gaseous planet or a rocky planet? You could say, yeah, because on Jupiter we would not want to go and live Slap Pleasant there.
So when we think of life as we know it on Earth, you want to know as a first rough estimate the density of the planets. And so how do you get a density if you have the size? You also need to know the mass, because then we can compute the average density by dividing the mass by the volume of the planet. And so for the mass, we must rely on data from the ground. I love this this synergy between space and ground based data, because this is really very powerful in this time.
And so how do we do stellar masses? Well, you have a star. You have planets going around it, and they are in each other's gravitational field. So the whole star is wobbling because there is a planet circling it. Right. And we also make the sun wobble. But the mass of the earth versus the sun is so very different that this wobble is very tiny. So you need a very accurate velocity metre. But we can do that in ground based observatories nowadays.
And so here you see the wobble of the star Kepler that has a planet can be because of the planet that's going around. And so from Johannes Kepler, we then know how to compute the ratio of the masses. And so in that way, we can get the mass of the planets divided by the volume from seismology and from this transits duration. And then we get a density. And for this planet, it is 8.8 grams per cubic centimetre. Yeah. Would that be a Jupiter type planets or would that be an earth like planets?
What's the average density of our earth? It's about five. Yes. Again, a typical question that's a non astrophysicist audience would like you. I never thought about that. So here are some numbers. So from this, we know that this is a rocky planet. And that's why it made this press release, because it was one of the first ones that the Kepler mission could detect. Okay. Now, where are we heading for? I have two projects in the near future that are very interesting for the science case.
And one is the the first mission. That's another mission. And it's going to do an all sky survey with the aim to find planets around stars. But it's going to find big gaseous planes in nearby orbits around hosts. So not planets. Interesting to search for habitable life. But the other mission that we are very fond of here in Europe is the so-called Plato mission.
Plato is a mission that is designed to do is to assess mythology of stars and hunt for planets like the Earth in low orbits, like one year orbits, like where we are. And to see if we can study these planets with yet other missions that are not yet approved, but certainly under development. And we hope that they will then also make it then we are past 2030 because Plato will do that for nearby stars, much nearby than Kepler.
And then we can hope to estimate even the the habitability of the planets from infrared spectroscopy that's further down the line. Then I'm retired, so somebody else will. We'll take that up. I'm sure it will happen. And to round off and give you an idea, we know many, many exoplanets around very interesting stars. The closest one is our closest neighbour.
And I've written down here the distance because these rich persons who ask me to give up planets when they see this, then somehow they come to reality. And that is important for me to realise because it is telling us how precious we should be for our own. Yeah. So the best cases of these extrasolar planets found so far. I couldn't even pronounce this number in English. Sorry. That's only my third language. And it's difficult to do big numbers in languages that are not their mother tongue.
Mars is about this distance. Just to give you an idea, we can fly to Mars. We can do that. We can bring you there. So the closest planets with current rocket technology and without underestimating our engineers, it would take us 100,000 years. Not very practical. Okay, so why am I ending like this? Because I find it really important that we realise that, you know, it's not going to be like in this cartoon, you know, if we screw up here, then we really have a problem.
So even if there are copies of the Earth, we're not going to go there immediately. And we should really take care of our planet because it's not going to be like this. And with that, I'd like to end with much gratitude so that I can explain fundamental science to a broad audience, which is one of my favourite activities. Thanks.