about the stuff that makes us and where it all came from because understanding our own origins means understanding the lives of stars. And how their catastrophic deaths bring new life to the universe. Because every mountain, every rock on this planet, every living thing, every piece of you and me was forged in the furnaces of space. My creation story is the story of how we were made by the universe. It explains how every atom in our bodies was formed, not on Earth, but was created in the depths of space, through the epic lifecycle of the stars. And to understand that story, we will journey to the stars in all their stages of life. This is where stars are born, a nebula - a stellar nursery, where new stars burst into life. Those stars will burn for billions of years, until their voracious hunger for fuel forces them to blow up, to become giants hundreds of times the size of our sun. And when they die, stars go out with the biggest bang in the universe. But to understand how we came from the stars, we must begin our journey much closer to home. We have sent robot landers to our neighbouring planets and discovered that Mars is rich in iron, which has combined with oxygen to form its familiar rusty-red colour. And we know that Venus's thick atmosphere is full of sulphur. We've sent spacecraft to the edge of the solar system to discover that Neptune is rich in organic molecules, like methane. But what of the rest of the universe? It seems impossible that we could discover what the stars are made of, because they're so far away. Even the nearest star, Proxima Centauri, is ten thousand times more distant than Neptune, 4.2 light years from Earth. And the nearest galaxy, Andromeda, is another 2.5m light years away. Yet despite these vast distances, these alien worlds are constantly sending us signals, telling us exactly what they're made of. Our only contact with the distant stars is their light, that has journeyed across the universe to reach us, and encoded in that light is the key to understanding what the universe is made of. And it's all down to a particular property of the chemical elements. You see, when you heat the elements, when you burn them, then they give off light and each element gives off its own unique set of colours. This is strontium and it burns with a beautiful red colour. Sodium is yellow. Potassium is lilac. And copper is blue. Each element has its own characteristic colour. It's this property that tells us what the stars are made of. But it's a little more complicated than simply looking at the colour of the light that each star emits. You can see why, by looking at the light from our nearest star, the sun. This is a spectrum of the light taken from our sun and, you know, at first glance, it looks very familiar. It looks like a stretched-out rainbow, because that's exactly what a rainbow is. It's the spectrum of the light from the sun in the sky. But if you look a bit more closely, then you see that this spectrum is covered in black lines. These are called absorption lines. Each element within our sun not only emits light of a certain colour, it also absorbs light of the same colour. By looking for these black lines in the sun's light, we can simply read off a list of its constituent elements, like a bar code. For example, these two black lines in the yellow bit of the spectrum are sodium. You can see iron. Right down here you can see hydrogen. So, by looking at these lines in precise detail, you can work out exactly what elements are present in the sun and it turns out that that's about 70% hydrogen, 28% helium and 2% the rest. And you can do this, not only for the sun, but for any of the stars you can see in the sky and you can measure exactly what they're made of. That star there is Polaris, the Pole Star, and you can see that because all the other stars in the night sky appear to rotate around it. Now it's 430 light years away. But we know just by looking at the light that it has about the same heavy element abundance as our sun, but it's got markedly less carbon and a lot more nitrogen. And the same applies for other stars. Vega, the second brightest star in the northern sky, has only about a third of the metal content of our sun. Whereas, other stars are metal-heavy. Sirius, the dog star, contains three times as much iron as the sun. And Proxima Centauri is rich in magnesium. But although the quantities of the elements may vary, wherever we look across space, we only ever find the same 92 elements that we find on Earth. We are made of the same stuff as the stars and the galaxies. But where did all this matter come from? And how did it become the complex universe we see today? In order to understand where we came from, we have to understand events that happened in the first few seconds of the life of the universe. So when the universe began, it was unimaginably hot and dense. We, literally, don't have the scientific language to describe it, but it was, in a very real sense, beautiful. There was no structure, there was certainly no matter. It was exactly the same whichever way you look at it. We can get some idea of how the universe developed from this state of pure symmetry by looking at the behaviour of water in this remarkable landscape. These are the El Tatio geysers, high in the Chilean Andes. As the boiling water bubbles up through the ground to meet the freezing mountain air, water can be found in all three of its natural phases - vapour, liquid and ice. In its hottest state, water is, like the early universe, an undifferentiated cloud. But as it cools, it suddenly behaves very differently. You see, if you look at a cloud of steam, it looks the same from every direction, but as it cools down, as it lands on this plate of freezing cold glass, then it immediately crystallises out. It turns into solid water - ice. As the ice crystals form, the symmetry of the water vapour disappears from view and complex, beautiful structure emerges. In the same way, we think that the universe, as it cooled, went through a series of these events, where structure emerged. One of the most important was about a billionth of a second after the Big Bang. In that moment, an important part of the symmetry of the universe was broken. Known as electroweak symmetry breaking, this was the moment when subatomic particles acquired mass - substance - for the first time. Amongst them, were the quarks. As the universe continued to cool, those quarks joined together to form larger, more complex structures, called protons and neutrons. Way before the universe was a minute old, the quarks had been locked away inside the protons and the neutrons and they were the building blocks of all atomic nuclei, the building blocks of the elements. These same protons and neutrons are with us to this day. They form the hearts, the nuclei, of all atoms. Just a few seconds after the beginning of the universe, the fundamental building blocks of everything had been created. It sounds ridiculous, the fact that everything you need to make up me and everything on planet Earth and, in fact, every star and every galaxy in the sky was there, after the first minutes in the life of the universe. It's almost unbelievable, but we have extremely strong experimental evidence to suggest that that is the way that it is. But from that point on, it was just, in a sense, a process of assembling those bits into more and more complex things. That is an incredibly fascinating story in itself. It allows the simplest of ingredients to create the infinite variety of the universe. But although this bubble metaphor makes creating new elements seem simple, it is, in reality, incredibly difficult to achieve. So difficult that there's only one place in nature that it happens. It's in stars like our sun that the elements are assembled. They're the only places in the universe hot enough and dense enough to fuse atoms together. Even then, only a fraction of the star reaches the extreme temperatures necessary. The sun is 6,000 Celsius at its surface, not nearly hot enough to power fusion. But deep below, where the temperature reaches 15m degrees, the sun fuses hydrogen into helium at a furious rate. Every second, it burns 600 m tons of hydrogen. As it does so, it releases the huge amounts of heat and light that brings our planet to life. It is this process of converting one element into another that allows us to exist. For all its power, the sun only converts hydrogen, the simplest element, into helium, the next simplest. But there are over 90 other elements present in our universe, so where did they all come from? If the heavier elements are not being made in stars like the sun, then there must be somewhere else in the universe where they are assembled. It's important to know because it's the elements beyond helium that give our world its complexity, and when it comes to planet Earth and human beings, there's one element that is particularly important - carbon. Life is completely dependent on carbon. I mean, I'm made of about a billion billion billion carbon atoms, as is every human being out there, every living thing on the planet. Imagine how many carbon atoms that is. So where does all that carbon come from? Well, it comes from the only place in the universe where elements are made - stars. But in order for us to live, a star must die. Stars in the prime of their lives, like our sun, are only hot enough to make helium. Forming the heavier elements requires much higher temperatures. Temperatures that can only be reached at the end of a star's life. Looking out into space, you might think that the cosmos is a constant, unchanging place. That the stars will always be there. But in fact, the stars are only a temporary feature in the sky, and though they may burn brightly for many millions or billions of years, they can only live for as long as they have a supply of hydrogen to burn. When a star runs out of hydrogen, it begins to die, but it doesn't go quietly. Rather than cooling, the star becomes much hotter, until there's a sudden flash. Then the star starts to expand. Over tens of thousands of years, it balloons to many hundreds of times its previous size. But in this bloated state, the star is unable to maintain its surface temperature.
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