Chapter 11: Made of Old Starlight

Night 124: A Round Blue Flame

On mission day 124, Noor's main task was an experiment with fire. Fire is one of the things that people on a space station fear most, so the experiment took place inside a strong, sealed box with thick windows, special fans and many layers of safety. Noor spent the first hour of her morning on a checklist. She checked the seals, the cameras, the gas supply and the computer that would control everything. Ade floated beside her and read each line aloud. Noor answered each line with one word, "checked", the way a pilot does before take-off.

At last the computer placed a tiny drop of fuel in the middle of the box and lit it. On Earth, a flame has the shape of a teardrop. Hot air is lighter than cool air, so it rises and pulls the flame upwards into a point. In weightlessness, hot air does not rise, because there is no "up" for it to rise to. So the flame on Noor's screen was a small, almost perfect ball. It was soft and blue, and it glowed quietly in the dark box, like a little planet made of light.

The ball of fire lasted only a few seconds. Then the fuel was used up and the blue light faded. The cameras had recorded everything, and scientists on the ground would study the pictures for months. They want to understand how flames behave without the rising air, because that helps them understand burning on Earth too, in engines and in furnaces. Noor and Ade repeated the test six times, with small changes each time, and by the afternoon her eyes were tired from watching the screen.

That evening, when the work was done, Noor read a message from Sami. He had seen a short film of the round flame that the crew shared with schools. "Beautiful," he wrote. "So is the Sun just a giant fire like that? Is it burning up there?" Noor smiled. It was a very good question, and many people believe the answer is yes. The true answer is stranger and more wonderful. It also leads to one of the most surprising facts in all of science: most of the material in Noor's body, in Sami's body and in your body was made inside stars.

So tonight's story is about how stars work, and about what they make. It follows on from the previous night of this book, when the ripples of the early universe grew into galaxies, pulled together by ordinary matter and by the mysterious dark matter. Tonight we look inside those galaxies, at the stars themselves, and we find that they are not only lamps. They are workshops.

Stars Are Not Fires

A fire, like the blue ball in Noor's box, is a chemical reaction. Fuel joins with oxygen from the air, and the atoms rearrange themselves into new molecules. This gives off heat and light. But in a fire, the atoms themselves do not change. A carbon atom in the fuel is still a carbon atom in the smoke. Only the way the atoms hold hands has changed.

If the Sun were a giant fire of this kind, it would have burned out long ago. It would have lasted only a few thousand years, not billions. And there is another problem: a fire needs oxygen from the air around it, and there is no air in space. So the Sun must shine in a different way. That way is called fusion.

Fusion happens in the centre of the Sun, in the part called the core. There, the weight of the whole Sun presses down, and the temperature is about 15 million degrees Celsius. At such heat and pressure, the centres of hydrogen atoms, their nuclei, move so fast that they crash into each other and stick together. Step by step, four hydrogen nuclei become one helium nucleus. The new helium weighs a tiny bit less than the four hydrogen nuclei that made it. That tiny missing piece of matter has turned into energy, and that energy is what makes the Sun shine.

Why does this need such enormous heat? The nuclei of hydrogen atoms all carry the same kind of electric charge, and things with the same charge push each other away, like two magnets turned the wrong way round. In the cool world around us, hydrogen nuclei never get close enough to join. Only when they move extremely fast, in extreme heat and under the weight of a whole star, do some of them come close enough to stick. Even in the Sun's core, any one hydrogen nucleus usually waits a very long time before it joins with another. The Sun shines steadily because it contains so many nuclei that, every second, a huge number of them succeed.

Here is the difference in one sentence. In a fire, atoms trade partners. In fusion, the centres of atoms join and become a different element. Fusion changes hydrogen into helium. It does not just rearrange things; it makes something new. That is why it is so powerful. Every second, the Sun turns about four million tonnes of its matter into energy, and it has done this for about 4.6 billion years. Even so, it has used only a small part of its hydrogen, because the Sun is enormous.

It may help to think of a bakery that works through the night. In the hot oven at the centre, simple dough becomes bread. In the core of a star, the simplest element, hydrogen, becomes the next one, helium. The light that comes out of the star is like the warmth that comes out of the bakery door. We feel the warmth, but the real work is happening deep inside, where we cannot see it.

The People Who Found How Stars Shine

For a long time, the energy of the Sun was a puzzle. In the nineteenth century, scientists tried to explain it with burning, or with the Sun slowly shrinking and heating up as it shrank. None of these ideas could give the Sun a long enough life. The rocks of the Earth and the slow work of living things seemed to need far more time than the Sun could last by any known process. The workers who studied rocks and the workers who studied the sky gave different answers, and nobody could make them agree.

The answer came from a new branch of science that studied the nucleus, the tiny centre of an atom. In the 1920s and 1930s, scientists learned that nuclei could join and release energy. In 1939, a scientist named Hans Bethe worked out, step by step, which reactions could take place in the core of a star like the Sun. He showed that the fusion of hydrogen into helium could keep the Sun shining for billions of years. The old puzzle was solved by careful calculation, done at a desk with paper and pencil.

Remember Cecilia Payne from an earlier night. In 1925, she found that the stars are made mostly of hydrogen and helium. Her discovery and Bethe's calculation fit together like two halves of one plate. The stars are made mostly of hydrogen, and the hydrogen is their fuel. The helium is partly what was made in the first minutes of the universe and partly what the stars have made since.

The next question was even bigger. If stars can make helium, can they make other elements too? In 1957, four scientists, Margaret Burbidge, Geoffrey Burbidge, William Fowler and Fred Hoyle, wrote a long paper that described how stars could build almost all the elements, one after another, in different stages of their lives. Fred Hoyle is the same person who had given the Big Bang its name on a radio programme years before, even though he did not agree with that idea. On this question, his work helped to show where most of the elements came from. Other scientists were working along similar lines at the same time, and over the following decades, many observations confirmed the main picture.

Consider how this knowledge was built. Nobody has ever visited the centre of a star. Everything was learned from light, from experiments with nuclei in laboratories on Earth and from calculation. Like so many discoveries in this book, it was the work of people at their desks and in their laboratories, checking each other's numbers, sometimes for many years.

How a Big Star Ends

A star like the Sun spends most of its life turning hydrogen into helium. When the hydrogen in its core runs low, the star changes. Its outer layers swell, and it becomes a large, cooler, reddish star called a red giant. In its core, helium nuclei now begin to join, making carbon and oxygen. At the end, a star like the Sun gently pushes its outer layers away into space, like a glowing shell of gas. What is left is a small, hot, very dense core called a white dwarf, which slowly cools for a very long time.

Stars much heavier than the Sun live faster and end more dramatically. Their great weight makes their cores hotter, so they can go further. After carbon and oxygen, they make neon, magnesium, silicon and finally iron. The star becomes layered, a little like an onion, with heavier elements nearer the centre. But iron is the end of this road. Joining iron nuclei does not give off energy; it takes energy away. So when the core of a big star fills with iron, its oven has nothing more to give.

The speed of a star's life depends on its weight. A star like the Sun has a total life of about 10 billion years, so it is now roughly in the middle of its working life. The heaviest stars use up their fuel in only a few million years, which is a very short time for a star. It is a little like two workers with different jobs: one works at a steady pace for a whole long career, while the other works at a furious speed and is finished very early. Both leave something behind, but the fast worker leaves behind the most dramatic results.

Without the energy pushing outwards, the core collapses under its own weight in a moment. The outer layers fall in, rebound and are thrown out into space in a huge explosion called a supernova. For a few weeks, one supernova can shine as brightly as a whole galaxy of stars. The explosion scatters the carbon, oxygen, silicon, iron and many other elements that the star made during its life. It also makes new elements in the violent heat of the explosion itself. What is left in the centre may be a neutron star, a ball of matter so dense that a spoonful would weigh as much as a mountain, or in the heaviest cases, a black hole.

Some of the heaviest elements, such as gold, need very special conditions. For many years, scientists thought that one possible place was the collision of two neutron stars. In 2017, astronomers observed exactly such a collision. Instruments that can sense tiny ripples in space itself picked up the signal of two neutron stars spiralling together, and telescopes around the world then saw the light of the event. In that light, they found signs of freshly made heavy elements, including elements like gold. Today, scientists think that heavy elements come from several kinds of events, and they are still working out how much each kind contributes.

The scattered elements do not stay lost. They mix into the clouds of gas and dust between the stars. Later, those clouds collapse into new stars and new planets. Each new generation of stars is born with a little more of the heavier elements than the one before. A galaxy is like a big, slow kitchen in which the same pots are used again and again, and each meal leaves a little more flavour behind for the next.

The Atoms in Your Body

Now let us come back to ourselves. Your body is made of many elements. By weight, the largest part is oxygen, most of it in the water inside you. There is carbon in every living cell. There is nitrogen in the proteins of your muscles. There is calcium in your bones and teeth. There is iron in your blood, carrying oxygen from your lungs to every part of you.

Almost none of these elements existed in the first minutes of the universe. As we saw on an earlier night, those minutes made mainly hydrogen and helium and a tiny amount of lithium. The carbon, nitrogen and oxygen in your body were made later, inside stars. The calcium and iron came from stars too, much of it from stars that exploded long before the Sun was born. In a very real sense, you are built from the leftovers of stars that lived and died before our solar system existed.

There is one important exception. The hydrogen in your body, in the water you drink and in the food you eat, was not made in stars. Hydrogen is the simplest element, and almost all of it comes from the first minutes of the universe. By number of atoms, hydrogen is the most common element in your body. So you carry inside you both kinds of history: the very early universe, in your hydrogen, and the long work of many stars, in almost everything else.

This is true of everything around Noor on the station as well. The aluminium of the walls, the oxygen in the air tanks, the silicon in the computer chips and the carbon in her food all came from stars. The metal of the box that held the blue flame was made, atom by atom, in stellar workshops billions of years ago. Even the light of that blue flame came from atoms that were once inside a star.

It is not easy to feel this with your whole heart. A cup of tea does not look like the remains of ancient stars. But the next time you hold something heavy, a tool at work, a pan in the kitchen, a stone from a path, you might remember that its atoms are older than the Earth. They have travelled through space, through clouds and through new stars before coming to rest in your hand.

A Message to Sami

At the end of the day, Noor floated into her sleeping cabin and zipped her sleeping bag up to her chest. She thought about the round blue flame and about the Sun, which had risen and set for the station sixteen times that day. She opened the small tablet and wrote to Sami.

"No, the Sun is not a fire," she wrote. "It is a kind of oven where hydrogen turns into helium. Big stars make other things too: carbon, oxygen, iron. When they die, they throw it all into space, and new stars and planets are made from it. The iron in your blood was made in a star. So when you are tired tonight, remember that you are made of very old and very tough material."

At about three in the morning, on a quiet road far below, Sami stopped for his break. He read the message twice, ate a sandwich and looked up through the windscreen at a few bright stars between the clouds. He wrote back: "Very tough material. I will tell that to my back when I lift the last boxes."

Perhaps you, too, are finishing a long day. Your hands may be tired, your back may ache, and tomorrow may already be waiting. But the material you are made of has been through the heart of stars. Tonight, it only needs a good rest.

Common Misunderstandings

Many people think that every atom in their body was made inside a star. It is a beautiful idea, and it is mostly true, but not completely. Most of the elements in your body, such as carbon, oxygen, calcium and iron, were indeed made in stars. But hydrogen, which is a part of every water molecule and of many molecules in your cells, was made in the first minutes of the universe. So it is more exact to say that you are made of material from the beginning of the universe and from the stars together.

Many people also think that the Sun and the stars burn like a fire. This is natural, because they are hot and bright, and the word "burn" is often used for them in everyday speech. But a fire is a chemical reaction that needs oxygen and changes only how atoms are joined. The Sun shines by fusion: the centres of hydrogen atoms join and become helium, and a little matter turns into a great deal of energy. That is why the Sun can shine for billions of years and why it does not need air.

Finally, some people think that gold and other heavy elements were made in the Big Bang, together with everything else. In fact, the first minutes of the universe made only the lightest elements: hydrogen, helium and a little lithium. The universe cooled too quickly to build anything heavier. Gold and the other heavy elements were made much later, in extreme events such as the collision of neutron stars and probably in some exploding stars. So the gold in a ring is much younger than the hydrogen in a glass of water. On the next night, we will follow some of those scattered elements as they gather into a new star and its planets: the Sun and the Earth.