La tête dans les étoiles - C'est pas sorcier [Intégrale]

Come on, Jamy, hurry up, the night is falling. I have an appointment with Antares. Who's that? A star. A star? No, a star in the sky. Today we're doing astronomy. Come on, Marcel. Oh, that's Antares. When you want to observe the stars, the ideal is to find a small corner of nature, very quiet, especially away from the light of the cities, because obviously it is in the darkness that you see the stars best. And tonight, Jamy, it's not bad because the sky is clear, there are no clouds and there are not too many moons. It's always the same story, when the moon is too bright at that moment, we are dazzled and we no longer see the stars. Hi everyone. Hi. Can I sit here, I don't bother you? Yes, yes, no problem. Well, the Transat, that's the essential tool for amateur astronomers. It avoids ending the night with a torticoli. In fact, Jamy, you know, there are still so many stars in the sky, night and day. Absolutely. But during the day, we don't see them because of the sun. It's exactly like here. When there is light during the day, well, my stars shine. Only we don't see their twinkling because it is masked by the sun, well, by the spotlight. However, as soon as it gets dark, Marcel, it's night, well, there we see the stars shine. Your stars are really beautiful, Jamy. The astronomer's sworn enemy is artificial light. For them, it is a real pollution. It makes all the stars in the sky disappear. And today, if you want to observe the stars, you have to go at least a hundred kilometers from a big city. The other astronomer's enemy is the atmosphere. As the air around us is in perpetual motion, the images of the stars are blurred. And to be as little as possible disturbed by the Earth's atmosphere, observatories are generally located in high mountains. The observatory of Silao, Chile, is located at an altitude of 2,400 meters. But the highest is that of Hawaii, perched at 4,200 meters. To see the stars even better, the best solution is to send telescopes into space, above the atmosphere. Since 1991, Hubble has been orbiting the Earth at an altitude of 610 kilometers. The images it sends us are ten times clearer than those of a terrestrial telescope. Look, here is a gigantic cloud of gas, the Nebula of the Eagle seen from Earth. And now, this is what Hubble shows us. This is the Nebula of the Cat's Eye, seen by a terrestrial telescope. And now, with the Hubble Space Telescope. And thanks to this telescope, we have discovered thousands of galaxies, so far unknown. I don't know if you noticed, but the more we observe the sky, the more we see stars. And this is normal, because our eye gradually gets used to the darkness. And when I see all these stars, I tell myself that we are really very small. In fact, do you know where we are in the universe? Well, we live on Earth, a planet that revolves around a star, the Sun. So, in total, nine planets revolve around the Sun. This is what we call the solar system. The Sun does not live alone in the universe. On the contrary, it is within a galaxy that brings together a hundred billion stars. So, our galaxy looks like a kind of spiral, with four arms. And we, well, the Sun, we are on the periphery of the galaxy, somewhere over there. So, you can't imagine that all this is immobile. On the contrary, the stars revolve around the heart of the galaxy, us too, so much so that we have the impression that the galaxy revolves around itself. And we must not believe either that there is only one galaxy in the universe. In fact, there are billions of them. There is even one that we can see with the naked eye, well, with a telescope, it's even better. It's the galaxy of Andromeda. Did you find it, François? It's in the middle of the field. Can I see? Is that the galaxy of Andromeda? The spot is slightly blurry. Yes. And there are a lot of stars inside? Hundreds of billions. Okay. So, we say that Andromeda is the closest galaxy to us, but anyway, it is still 2 million light years away. That means that if we could move at the speed of light, that is to say at 300,000 km per second, we would still take 2 million years to get there. So, with your truck, Jamy, you're not done. Yes, well, in the meantime, we may be content to look at it with a telescope. Come on guys, show us the beautiful Andromeda, and in color this time. So, these hundreds of billions of galaxies are not isolated? In the universe, they form groups, what we call clusters of several galaxies that are arranged next to each other. And all these galaxies do not date from yesterday. Thanks to their calculations, astronomers think that the age of the universe is between 15 and 20 billion years. 15 billion years ago, we had a universe in an extremely dense state, a very, very hot universe, a kind of energy ball that physics has trouble describing, and this energy ball would suddenly relax. It was the original explosion, what we call the Big Bang. It was at this moment, in the first moments of the universe, that matter was formed, the matter that we know today, that is to say that the first particles were formed that gave birth to the nucleus of atoms and then to atoms and molecules. And a billion years later, the first galaxies and stars were formed. Since the Big Bang, the universe has been expanding. It is expanding. The galaxies are moving away from each other. They are always escaping further and faster. And we don't know if it will stop one day. Our galaxy, we can see it too, but only in part, of course, since we are inside. And what we generally see is the Milky Way. You know, it's this big white strip, finally slightly luminous, that crosses the sky and forms a kind of arch. Here, with binoculars, we can see much better. Let's see. So, with binoculars, yes, it becomes much clearer. We see a multitude of stars, because we must not forget, Jamy, that in our galaxy, there are still 100 billion stars, and even more, it seems. Wait, I counted them. Well, good luck! So why do we see this white strip, the famous Milky Way? Well, it's because of our position in the galaxy. Seen from above, we saw that this galaxy looked like a kind of spiral. And we are somewhere over there, in the suburbs. Seen from a profile, now. Well, we can see that this galaxy has a certain thickness. It looks like a flying saucer with a whirlwind in the center, which corresponds to a concentration of stars in this place. Therefore, seen from Earth, this tiny little blue dot, when we look towards the center of the galaxy, well, we discover this thickness. The stars then form a white strip, the famous Milky Way. So, of course, we see stars outside this white strip. However, these are stars that are closer to us. And there are also stars to the right, to the left, behind, in front. There are everywhere. And our Milky Way, we can still see it entirely, thanks to satellites that observe it under all its seams. As in the universe, everything is in motion, there are sometimes collisions. Our neighbor Andromeda approaches us at 90 km per second. And it is possible that one day it will swallow our galaxy. But rest assured, the shock is not for now. Because Andromeda is still very far from us. Well, let's go back to observe some stars. Hey sir, what are you looking at? I am on a distant galaxy that is very beautiful. It is true that it is beautiful. And there is no age to admire the stars. Well, now, how to spot all these stars? Well, the first thing is to know a few constellations. You know, these are groups of stars to which we gave names, like the Great Bear or Cassiopeia, which draw some kind of figures in the sky. But the problem is that there are still 88 constellations and the landscape up there does not stop changing. So at first, what you have to do is use a map of the sky and learn how to use it. Tell me, Julien, could you perhaps teach me? So how do you use a map of the sky? Well, you have to adjust the time we are at on the date. So what time is it now? It is one o'clock. So it is one o'clock. You have to be careful that it is in universal time. That means that in summer you have to leave at two o'clock and in winter you have to leave at one o'clock. So we start at midnight and then we match with the 25th of October. So here we have the map of the sky. Now we have to orient our map, don't we? To orient it, we have to direct it to the north. So how do you know the north? Well, it is the pole. Where is the pole? Over there. So the pole is over there and our map is well oriented. Here we have the landscape of the sky. Is that right? Yes, yes. All right, so wait, I will try to find myself. That means that over there we must have the constellation of the bull. Is that right? Yes. And do you know where it is? It is in the zodiacal band. In the zodiacal band? What is that, Jamy? The zodiacal band is a band on which the constellations are all on the same plane. At the same level, if you prefer, we are talking about the ecliptic plane. It is on this plane that the Earth revolves around the Sun. Come and see, Jamy, over there is the constellation of the Gemini. It is also on the zodiacal band. In total, there are 13 constellations of the zodiac, but we can never see them all at the same time. And do you know why? Of course, we can't see all the constellations of the zodiacal band at the same time simply because from Earth's point of view, some are on the other side of the Sun. And even though we are oriented in their direction, well, the Sun's light masks them. Therefore, when the Earth is in this position relative to the Sun, in autumn, in the month of October, well, we can see the Gemini and the Taurus. However, we don't see the Sagittarius at all. Fortunately, the Earth revolves around the Sun in a year on the plane of the ecliptic, so that, over the months, we discover all the constellations. Thus, in the month of May, for example, we can see the Sagittarius. However, we don't see the Gemini or the Taurus. But be careful, there are constellations elsewhere than on the Zodiac. There are constellations above, below, on the sides. There are constellations everywhere. Well, there are constellations that we can see all year long. This is the case, for example, of the Great Bear. It has a lot of names. It is also called the Great Carriage or others call it the Great Pan. In any case, it is there every night. It is beautiful, isn't it? Well, the sky is beautiful. Jamy, let's take a picture. I'm taking a break. Come on, stars, a little smile! We can see the Great Bear all year long because it is close to the axis of the North Pole. But we can only see it if we live in the Northern Hemisphere. Those who live in the Southern Hemisphere see another constellation all year long, the Southern Cross. And since we don't see the same stars in the North and in the South, we have built observatories all over the world. Here, this is the Magellan cloud which is only visible from the Southern Hemisphere. Look at this, Jamy! My picture is beautiful! Well, your picture is completely blurry, Fred. What do you mean, blurry? Well, look! Yes, you're right, it's blurry. It looks like the stars have moved. No, the stars don't move. We move. Indeed, the Earth rotates on itself in 24 hours, therefore, during the night. We move. We don't have the same landscape, the same stars above our heads. In any case, there is a star that is always in the same place in the sky. It is the Polar Star. How do you find it, Jamy? It's not complicated. First, you have to locate the Great Bear, the pan, and then you take the two stars that are opposite the handle of this pan and you extend their distance by 5 times. By going up like this? By going up and you will find the Polar Star. Ah yes, OK, it's there. Well, for those who didn't understand let's start again. From the stars on the edge of the pan, you count up to 5 and you find the Polar Star. That's it. I see the Polar Star So, why doesn't it move with this star? Well, simply because it is in the direction of the North, right in the extension of the axis of rotation of the Earth. It's a little bit as if the Earth was turning around this star. Therefore, when we look at this star, well, the Earth can turn. As we turn around this star, well, it doesn't move. But to observe the other stars for several minutes as the Earth turns, you have to constantly move your telescope. Fortunately, most are equipped with an engine that makes them turn at the same speed as the Earth, but in the opposite direction. As a result, the star stays fixed in the lens all night and astronomers can get very clear images. In France, there are two large observatories. The highest is located in the Pyrenees at more than 2,800 metres in altitude. It is the observatory of the Pic du Midi. The other large French observatory is the Observatory of Haute-Provence. It was built about 50 years ago and it's no coincidence because the sky is one of the purest in France. It is perfectly clear 230 nights a year, which is rather a good thing for astronomers who come here. Yes, Fred, astronomers come from all over Europe to work in this observatory. They have to book their night at least 6 months in advance. Even though the telescopes are not ultra-modern, we still make discoveries. Here, this one is over a century old and is still in use. We have here telescopes that used to be among the largest in the world but have become obviously small telescopes. There are many research subjects that need small telescopes. And I think that the best proof was given to us in 1995 with the 1.93-metre telescope. We discovered the first planet revolving around another sun. And yes, we detected the presence of this new planet near a star in the Pegasus constellation but we couldn't observe it directly because it was too far away. They don't track the planets but the galaxies. Look, here is the moon from last night. Well, now we close the dome because it's not night yet. We'll have to wait a little longer to observe the stars. You know, Jamy, I think you can put your telescope away. I found one much bigger. We can even go inside to visit it. Wait, we'll lower it. Be careful, a telescope is not a big magnifying glass or an astronomical telescope because it doesn't work with lenses but thanks to a mirror which collects the light of the stars and concentrates it in a point called the foyer. And the bigger the mirror, the more it allows to observe objects that are dimly lit so far away. And this mirror, Jamy, it measures 2 metres in diameter. Wow! It's moving! Hey! How's it going? I'm inside! Phew! Thanks, Jamy. I'll be back soon. So now, how do you point a star? Well, it's not very complicated because the position of each star is written in a document called the astronomical ephemerides. Let me explain. In the same way that on Earth to locate a point we use longitude and latitude, in the sky to locate a star we use what we call right ascent and declination. Then, we just have to enter the coordinates of the star in this machine. So, what are we looking at? Well, listen, I'd like to see where the stars are born. A place where the stars are born? That's a good idea. So, we're going to look at the Orion Nebula. Let's go. Hey, Fred, drive slowly because this telescope still weighs 70 tonnes and it's fragile. Come on, little stars, show yourself. Oh, my. There, there they are. So, do I have it? That's good. Very well pointed. Did you see that, Jamy? Not bad for a first. So, stars are born in nebulae. What is a nebula? It's very simple. It's a huge cloud of gas and dust. So, there's a lot of gas but, in particular, hydrogen which is quite hot because it's at 10,000 degrees. And this Orion Nebula which is located in our galaxy is actually a real berth of stars because we estimate that, currently, there are about a hundred stars forming inside. In fact, Jamy, do you know how a star How are stars formed? Well, the gas in the cloud where the star will be born is made up of small elements, atoms of hydrogen and helium. These atoms will attract each other because of gravity. The cloud contracts. The atoms are more and more close to each other. The pressure rises and there, it's like with a bicycle pump when the pressure rises, it heats up. When the temperature reaches 10 million degrees, the star heats up. How? Well, four atoms of hydrogen will fuse, transform into an atom of helium. And there, an extraordinary phenomenon happens. There is a loss of mass. In other words, when we weigh four atoms of hydrogen, they are heavier than an atom of helium. So, this mass didn't fly away. It transformed into energy, into radiation. And as millions of tons of hydrogen transform into helium, well, the star will inflate. And throughout the life of the star, well, these two forces will oppose each other and balance. On one side, gravity which maintains the collapse of the star. On the other side, fusion and radiation which maintain the inflation of the star. The closest star, our Sun, is 109 times bigger than our Earth. Like all stars, the Sun was born in a huge cloud of gas and dust, a nebula. Every second, it loses 4 million tons of matter which are transformed into energy. And yet, it is still far from having exhausted all its fuel. On its surface, there is an intense activity. Gigantic jets of gas escape from the Sun. This is what we call So our Sun is exactly 4 billion, 600 million years old. And it still has about as much to live. So what amazes me is that you can calculate the age of stars. How do you do it? It's not very simple. If you want, what you have to understand is that at its birth, the life expectancy of a star is linked to its mass. Yes. A small star like the Sun has a life expectancy of 10 billion years. There are stars that are 10, 20, 30 times bigger than the Sun. And these have, on the other hand, a very, very short So the bigger ones are not the ones that live the longest. Exactly. So how do you calculate the mass of a star? Well, what we have, what the observation gives us, is the luminosity of the star which is linked to its mass and then its temperature. And depending on the temperature, that is, the color of the star and its luminosity, we can know approximately at what point in its life it is located. Speaking of which, Jamy, you know that with my telescope I can also show you stars that are dying. Do you want to see some? Follow me! We're going to point the telescope at Capella in the constellation of Cochet. It's one of the brightest stars in our galaxy and we can say that it is dying because it has turned into a red giant, that is, a star about ten times bigger than the Sun. Let's go for Capella! Well aimed, Fred! Here is Capella! And to say that this superb star is dying! A red giant is what the Sun in its old days. So what exactly is going to happen? Well, when all the hydrogen has turned into helium, the expansion stops. It is the gravitation that takes over and the heart of the star collapses on itself. The pressure also increases the temperature. It rises so much that this time it is the helium that will fuse. Three helium, two helium, will fuse into a carbon. And there, the energy produced is even much bigger so that the star will inflate. It will grow, grow, grow so much that its periphery will cool down. That's why we see it red and no longer white. Oh dear, you never worry me! And us, what are we going to become if the Sun turns into a red giant? Well, when the Sun turns into a red giant, Mercury will be englobed, Venus, Earth and Mars. That said, before that happens, we still have good days ahead of us. And it's not over, Jamy, in your red giant, when all the helium has turned into carbon, the heart of the star collapses and the star gradually becomes a white dwarf. I think I have one at the end of my telescope! Not quite, Fred. This is a planetary nebula and it is the star at the centre that will become a white dwarf. Let's recap. Throughout its life, a star the size of the Sun turns its hydrogen into helium. Then, in its old days, after several billions of years, the star swells and becomes a red giant. Then, the red giant turns into a planetary nebula and then into a white dwarf. And when the white dwarf cools down, it becomes an invisible black dwarf. At that moment, there will be almost no trace of our Sun. Hey, Jamy! it's true for a star like the Sun, but if you take a bigger, bigger star, do you know what happens? It explodes! That's what we call a supernova. Come and see. That's the Crab Nebula. We can observe it in the Northern Hemisphere. it's the remains of a star that exploded on 4 July 1054 and its matter continues to spread in the Universe. It's been nearly a thousand years! Do you realize that? Yes, Fred, but I'm even better. These are the remains of a supernova that exploded more than 10,000 years ago. So how do the stars that are much bigger than the Sun end up? Well, the fusion doesn't stop with carbon. It continues. Lots of elements appear and then fuse again until we are made of iron. Then, the fusion stops and the heart of the star collapses again on itself. The iron, which was in a gaseous state, then becomes solid, but extremely solid, extremely dense. Because of gravity, the other gaseous layers of the star will collapse on this heart. But it's so dense that the layers will bounce and then there's the explosion. That's what we call a supernova. The supernova explosion is so violent that it releases as much energy as 10 billion suns In the middle of the cataclysm, a tiny star remains very small and very dense. It's a neutron star which starts spinning very quickly on itself. During the explosion of the supernova, the residues are not lost for everyone. They provide matter for the birth of new stars. We can close the doors of the dome. The sun rises and there are no more stars to observe. Ah yes, there's one left. Really? And which one? The Sun. Oh, I had forgotten this one. You can observe it without me. I'm going to bed. Don't worry, I've got an eye on it.