Chapter 9: The Dark Ages and the First Stars

Night 97: Pictures of the Night Side

On day 97, the ground had asked the crew for night photographs of the Earth. Scientists on the ground use such pictures to study many things: the glow of towns and roads, the spread of lights over the years, storms and the shapes of coastlines in the dark. The task sounds simple. In practice, it is one of the more difficult jobs on the station, because the station moves so fast.

At about 28,000 kilometres per hour, the ground below slides past quickly. A photograph of the night side needs a long exposure, because there is so little light, but a long exposure blurs when the camera and the ground move against each other. So Noor used a camera mount that slowly turned to follow the ground, and she checked each picture on the screen before taking the next. Ade floated beside her, reading the times when the station would pass over each area that the scientists had asked for.

Between the planned pictures, they saw things that no schedule could plan. Far to one side, a storm was flashing with lightning, one cloud lighting up after another, like lamps being switched on and off in a large dark house. Near the edge of the Earth, a green curtain of aurora rippled slowly, with a faint touch of red at its top. Above it all, along the curve of the planet, lay a thin, pale band of glowing air, and above that, the black sky and the stars.

Noor knew what she was seeing. The aurora appears when tiny particles streaming out from the Sun are guided by the Earth's magnetism towards the regions near the poles, where they make the thin upper air glow, a little like the gas in a glowing sign. The pale band along the edge of the planet was the air itself, giving back, very faintly, some of the energy that it had collected from sunlight during the day.

When the station crossed back into sunlight, Noor packed away the camera and looked through the pictures. Most of them were good. A few were blurred, and she marked those so that the scientists would know. Then she sat for a moment with the last picture on the screen: a dark Earth, a few scattered lights and, above, complete darkness. It reminded her of something she had read about the universe after the oldest light, a time when there was not yet a single star.

That evening, she asked Sami a question instead of answering one. She asked him what the darkest place on his night route was. He wrote back that it was a long road between fields, with no towns and no street lights, where he sometimes switched off the radio and drove in silence. Noor told him that the universe once had a darkness like that, but much longer, lasting millions of years. And that the first lights to appear in it were the first stars.

After the Oldest Light

On an earlier night, we saw the fog clear at about 380,000 years after the beginning. Atoms formed, and the oldest light was released. At first, that light was warm and bright, the glow of something at a temperature of a few thousand degrees. If you could have been there, the whole sky would have shone with a soft orange light. But the universe kept expanding, and the light kept stretching. Over some millions of years, its waves became longer than the eye can see. The sky, to human eyes, went dark.

There were no stars yet to replace that light. The universe was filled with gas, almost all of it hydrogen and helium, the simple elements of the first minutes. It was spread fairly evenly through space, though not perfectly evenly, because of the tiny ripples that we saw in the oldest light. And there was something else, which we will meet properly on the next night: a large amount of invisible matter, called dark matter, which does not shine but does pull with its gravity.

How long did the dark ages last? They began when the oldest light faded from view and ended when the first stars began to shine. Since the first stars probably formed within the first few hundred million years, the dark ages probably lasted for a hundred million years or more. Nobody knows the exact length. It was, in any case, a very long night, far longer than all of human history many times over.

Astronomers call this long period the cosmic dark ages. It was not dark because it was empty. It was dark because nothing in it was shining yet. Imagine a great theatre after the evening's last show. The audience has gone, the lights are off, and the stage is quiet. But the building is not empty. The seats, the curtains and the stage are all still there, waiting for the next performance.

During the dark ages, gravity was slowly at work. In the places where the universe was very slightly denser, gravity pulled a little more strongly. Those places drew in more matter, which made them denser still, which made their pull stronger. This is the patient work of gravity: small differences grow larger over time. The invisible dark matter gathered first into clumps, and the ordinary gas fell into those clumps, like water running into hollows in the ground.

As the gas collected and became denser, it also became hotter at its centre, in the same way that air becomes warm in a bicycle pump when you squeeze it. For a long time, the clumps of gas grew without becoming stars. Then, in the densest places, the gas at the centre became so hot and so tightly packed that a new kind of process began. Hydrogen nuclei started to join into helium, giving off energy, in a process called fusion, which we will explore on a later night. When that happened, a star was born. The first light after the dark ages had been switched on.

The First Stars

When did the first stars appear? Astronomers think that they probably formed within the first few hundred million years after the beginning. The exact time is not known, and it may have been different in different places, since the densest clumps would have formed stars first. Compared with the full age of the universe, about 13.8 billion years, a few hundred million years is quite early. In the story of one long working day that we will tell on the last night, it is somewhere within the first half hour after midnight.

The first stars were probably very different from the Sun. They were made almost entirely of hydrogen and helium, because there were no heavier elements yet. Heavier elements help gas clouds to cool down and break into smaller pieces. Without them, the gas clouds of the young universe probably stayed warmer and collapsed into much larger lumps. So many astronomers think that the first stars were often very large, perhaps many times heavier than the Sun.

Large stars live fast. A star much heavier than the Sun burns through its fuel far more quickly than a small one, because its centre is so much hotter and denser. The Sun has been shining for about 4.6 billion years and is only about halfway through its life. The largest of the first stars may have lived for only a few million years. Then they ended, probably in great explosions, and those explosions scattered the first heavier elements into space. That is the beginning of a story that we will follow on later nights.

It is important to be honest here. Much of what we have said about the first stars comes from careful calculations and computer models, not from direct observation. Astronomers are fairly confident that the first stars were made of hydrogen and helium and that many were large. But how large they were, how many there were and exactly how they formed are still being studied. Different models give somewhat different answers. Nobody has yet seen one of the very first stars clearly and been certain of what it was.

There is also a beautiful effect of the first stars that astronomers can study. Large, hot stars give off a great deal of strong light, much of it ultraviolet, the kind of light that burns skin on a sunny day. This strong light was able to knock electrons off the hydrogen atoms in the gas around the stars. Slowly, around each early star and each early galaxy, bubbles of gas with free electrons grew. The bubbles spread and joined together. By the time the universe was about a billion years old, most of the gas between the galaxies had been changed in this way. The light of the early stars had cleared the last of the darkness.

The People Who Looked Back

How do astronomers look for the first stars and galaxies? The answer goes back to the very first nights of this book. Light takes time to travel. The further away something is, the longer its light has travelled to reach us, so the further back in time we see it. To see the earliest galaxies, astronomers must look at the most distant ones. Those are extremely faint, so they need very large and very sensitive telescopes.

In 1990, a telescope was launched into orbit around the Earth, above the air that blurs the view from the ground. It is called the Hubble Space Telescope, after the astronomer Edwin Hubble, whom we met on earlier nights. In the mid-1990s, its team did something that seemed almost wasteful at first. They pointed the telescope for many days at a tiny patch of sky that looked completely empty, a patch far smaller than the full Moon. When they added all the pictures together, the empty patch was full of galaxies, thousands of them, many so far away that their light had travelled for billions of years.

But even that telescope had a limit. As we saw on the night of the moving galaxies, light from very distant galaxies is stretched by the expansion of space. Light that left the first galaxies as blue or ultraviolet light arrives today as infrared light, with waves too long for our eyes. Infrared light is the kind that we feel as warmth, for example from a stove. To see the earliest galaxies well, astronomers needed a telescope built especially for infrared light.

That telescope is the James Webb Space Telescope. It was launched in December 2021, after many years of planning and building by a large number of engineers, technicians and scientists. Its large mirror was too big to fit inside a rocket, so it was made of many pieces that unfolded in space, like a flower opening. Because the telescope looks for faint warmth, it must be kept extremely cold, so it carries a large shield that blocks the heat of the Sun. And it was sent much farther from the Earth than the Moon, to a quiet place where the Sun, the Earth and the Moon all stay on the same side, behind the shield.

The journey itself was a long piece of careful work. The telescope took about a month to reach its place, and during that time it unfolded step by step: the shield, the mirror pieces and many smaller parts. Each step had been tested on the ground, and each one had to work in space, where nobody could go to repair it. Many of the people who had built it watched those weeks very closely, after years of their working lives spent on this one machine.

Since it began its work, the Webb telescope has found galaxies whose light left them when the universe was only a few hundred million years old. Some of these early galaxies are brighter and more developed than many astronomers had expected. That has started lively discussions about how quickly the first galaxies grew. These discussions are still going on. It is a good example of how a new instrument does not only answer old questions. It also asks new ones.

Still Looking for the Very First

Even the Webb telescope has not yet clearly seen the very first stars on their own. The earliest galaxies that it sees already contain many stars, and some of those stars may already contain heavier elements made by stars before them. The very first stars, made only of hydrogen and helium, are probably too faint and too far away to be seen one by one. Astronomers hope to find signs of them in other ways, for example in the light of the earliest galaxies or in the bright flashes of their explosions.

There are other ways of searching too. The hydrogen gas of the dark ages gave off a very faint radio signal of its own. Several groups of scientists are building radio instruments to try to detect it. This is very hard work, because the signal is so weak and the radio noise from the Earth and from our own galaxy is so strong. It is a little like the engineers of the oldest light, trying to hear a quiet voice in a room full of hiss. Nobody has yet measured this signal in a way that everyone agrees on.

Astronomers also study old stars close to home, in our own galaxy. Some stars in the Milky Way are very old and contain only tiny amounts of heavier elements. They are not the very first stars, but they may be their children or grandchildren, formed from gas that had been enriched by only one or a few earlier generations of stars. By studying the small amounts of heavier elements in these old stars, astronomers can learn something about the stars that came before them. It is a little like learning about great-grandparents by looking at the family resemblance in their descendants.

So the search for the first stars is happening in many places at once: with telescopes in space that look far away, with radio instruments that listen for the dark ages and with telescopes on the ground that study the oldest stars nearby. Each method gives part of the picture. None of them yet gives the whole picture. This is how much of science works, through many small, careful pieces of work, done by many people over many years, slowly fitting together.

Before Sleep

Before sleep, Noor looked out of the window once more. The station was over the night side again, and this time she switched off the light in the small round room, so that her eyes could adjust. After a few minutes, more stars appeared, then more, until the black sky was full of them. From the station, they did not twinkle. They shone steadily, each one a sun, some near and some far.

Ade drifted in quietly and looked out beside her for a while. He said that every time he saw so many stars, he found it hard to believe that there had once been none at all. Noor said that she found it hard to believe that anyone had worked it out. Then they both went back to their cabins, because the next working day would start early.

She wrote to Sami: "Next time you drive your dark road, think of this. For a hundred million years or more, the whole universe was darker than that road. Then, one by one, the first stars switched on, like the first lights of a town at dusk." Sami replied that on his dark road, the first light he usually sees is the petrol station where he buys coffee, and that he would now think of it as a star.

After a long day of work, perhaps you also walk or drive home in the dark. When the first light of your street or your home appears ahead of you, it is a small, ordinary comfort. The universe had its own version of that moment, long ago, when the first stars began to shine in the darkness. Everything that came afterwards, the galaxies, the Sun and the Earth, grew from that beginning.

Common Misunderstandings

Many people think that the cosmic dark ages were dark because the universe was empty. It was not empty at all. It was full of hydrogen and helium gas, full of invisible dark matter and still full of the oldest light, which by then had stretched beyond what eyes could see. The dark ages were dark because nothing was shining yet. They were also a busy time in a quiet way, because gravity was slowly pulling matter into the clumps where the first stars would form.

Many people also imagine that the first stars were like the Sun. They were probably very different. They were made almost only of hydrogen and helium, without the heavier elements that the Sun contains. Many of them were probably much larger and hotter than the Sun, and they lived much shorter lives. The Sun is a much later star. As we will see on a later night, it formed from gas that had already been enriched by many generations of stars before it.

Finally, some people believe that if they find a very dark place and look up, they can see the first stars with their own eyes. Unfortunately, they cannot. The first stars ended long ago, and their light, if it reaches us at all, is far too faint and too stretched to see without the largest telescopes, and even those have not yet clearly seen them. The stars you see on a dark night are much nearer and much younger, almost all of them inside our own galaxy. But every one of them is part of the same long story. On the next night, we will see how the first clumps of stars grew into galaxies, and how something invisible helped them grow.