Chapter 3: The Universe Was Bigger Than Anyone Thought

Night 19: Lettuce Under Pink Light

On her nineteenth day, Noor spent the morning as a gardener. In one corner of the station there is a small growth chamber, about the size of a kitchen cupboard, where plants are grown under coloured lights. The light is a mix of red and blue, which makes the chamber glow pink. The red and blue are the colours that leaves use most for growing, so there is no need to waste power on the others. Inside, a row of young lettuce plants was pushing up from small cloth pillows filled with a clay-like growing material and fertiliser.

Her task was to check each plant, take photographs, measure the leaves and add a careful amount of water through a thin tube. In weightlessness, water does not soak downwards the way it does in soil on Earth. It can form a ball and sit on top, or creep along the side of the pillow, so it must be given slowly. The roots, too, have a strange problem. On Earth, roots grow down because they sense the pull of gravity. Here, there is no "down" for them to feel, so the plants rely mostly on the direction of the light. The leaves reach towards the lamps, and the roots mostly grow the other way, into the damp growing material.

Noor liked this work. It felt close to ordinary life, like tending a small garden on a balcony. The scientists on the ground would study her photographs and measurements to learn how plants could one day feed crews on long journeys, and in a few weeks the crew would be allowed to eat some of the leaves. Ade had already announced that he would bring the salt.

In the afternoon, Tomás asked her to help with a different job: packing used equipment and rubbish into a cargo ship that would later leave the station and burn up harmlessly in the upper air. It was the kind of work that every workplace has, the kind nobody photographs. Each bag had to be labelled, weighed on a special device and strapped into its place, so that nothing would shift when the ship's engines fired. By the time they finished, the cargo ship's small cabin looked like a well-packed moving van, and Noor's arms were tired from pushing heavy bags that weighed nothing.

That evening, after the last checklist was done, Noor went to the window. The station passed into darkness over an ocean, where there were no city lights at all. As her eyes adjusted, she saw a pale band of light stretching across the sky, uneven and soft, like a trail of mist. It was the Milky Way. She had seen it on Earth only a few times, far from towns, but here it was clear and wide. She remembered that only about a hundred years ago, many astronomers believed that this band, and the stars around it, made up the whole universe.

When she wrote to Sami that night, she began with a question of her own. "Do you know how big the universe was in 1900? Not the real universe. The universe people believed in."

A Universe of One Galaxy

The pale band of the Milky Way is our own galaxy, seen from the inside. A galaxy is a huge family of stars, gas and dust, held together by gravity. Our galaxy is shaped roughly like a flat disc with a bulge in the middle. The Sun is one star in that disc, far out from the centre, roughly halfway to the edge. When we look along the disc, we see so many distant stars crowded together that they blur into a soft band. When we look away from the disc, we see fewer stars and more darkness.

A simple picture can help. Imagine standing in the middle of a large, flat forest. If you look straight ahead, in any direction along the ground, the trees overlap so thickly that you see a wall of trunks. If you look straight up, you see only a few branches and then open sky. We live inside the disc of our galaxy like a person inside that forest. Looking along the disc, we see a crowded wall of stars, the pale band. Looking up out of the disc, we see open space.

Today we know that the Milky Way is about 100,000 light-years across and contains hundreds of billions of stars. That is already a size almost impossible to picture. Light, the fastest messenger, needs about 100,000 years just to cross it from one side to the other. Yet at the start of the twentieth century, many astronomers thought that this was everything. In their picture, the universe was one great island of stars floating in an endless dark sea, with nothing beyond it.

There was one puzzle in that picture. With telescopes, astronomers had found many faint, fuzzy patches of light scattered across the sky. They called them nebulae, from an old word for clouds or mist. Some of these nebulae were clearly glowing clouds of gas among the stars. But others were different. Many had a beautiful spiral shape, like water swirling down a drain, and they seemed to be made of a soft light that no telescope of the time could split into separate stars. These were called spiral nebulae.

What were they? Some astronomers thought they were small clouds inside our own galaxy, perhaps new star systems being born. Others thought they might be whole galaxies of their own, so far away that their billions of stars blurred into a mist, just as the Milky Way does for us. The idea that there could be other galaxies, other "island universes", was not new. Thinkers had suggested it more than a century earlier. But nobody could prove it either way, because nobody could measure how far away the spiral nebulae were.

This is the central difficulty of astronomy. When you look at a light in the sky, you cannot easily tell whether it is a dim light close by or a bright light far away. A candle on the other side of a room and a lighthouse on a distant coast can look equally bright. To measure the size of the universe, astronomers needed a way to know how bright a light really is. They needed what is sometimes called a standard candle: a kind of light whose true brightness is known, so that its distance can be worked out from how bright it looks.

The Woman Who Measured the Stars

The key came from a woman named Henrietta Leavitt. She worked at an observatory in the early 1900s as one of a team of women employed to study photographic plates. These were sheets of glass coated with a light-sensitive layer, which had been placed at the end of a telescope to record the stars. Night after night, the telescope took pictures. Day after day, the women examined them with magnifying glasses, measured the brightness of the stars, compared plates taken on different nights and wrote everything down in careful notes.

It was slow, patient work, and it was not well paid. The women were often called computers, because their job was to compute, in the days before machines did it. Many of them were very skilled astronomers, though at the time they were rarely given that title. Leavitt's task was to find stars whose brightness changed over time. She studied plates of two small, cloud-like galaxies that can be seen from the southern half of the Earth. In them she found more than a thousand such changing stars.

Among them was a special kind of star that grows brighter and dimmer in a steady rhythm, like a slow heartbeat, over a period of about a day to several weeks. These stars are called Cepheids. Leavitt noticed something remarkable about them. The brighter Cepheids took longer to complete their rhythm. The dimmer ones beat faster. And because all the stars in one of those small galaxies are at about the same distance from us, the difference in brightness had to be real, not just an effect of some stars being closer.

She published this result in 1912. It meant that the rhythm of a Cepheid reveals its true brightness. Measure how long the star takes to pulse, and you know how bright it really is. Compare that with how bright it looks, and you can work out how far away it is. A Cepheid was exactly the standard candle that astronomers had been missing. Once a few nearby Cepheids had been measured by other methods, Leavitt's rule could be used like a measuring tape across the sky.

Henrietta Leavitt did not live to see how important her discovery became. She died in 1921, after many years of quiet work, a few years before her rule was used to answer one of the greatest questions in astronomy. Her story is a good reminder that great results often grow out of careful, unglamorous work done by people whose names are not always remembered.

An Argument Without an Answer

By 1920, the question of the spiral nebulae had become so important that two leading astronomers presented their opposite views at a scientific meeting. This event is often remembered as the Great Debate. One astronomer argued that our galaxy was enormous, much larger than people had thought, and that the spiral nebulae were small objects inside it or near it. The other argued that our galaxy was smaller, and that the spiral nebulae were distant galaxies in their own right.

What is interesting about the debate is that each side was partly right and partly wrong. The first astronomer had used Cepheids and other clues to show, correctly, that our galaxy was very large and that the Sun was not at its centre. That was an important step. The second astronomer was right that the spiral nebulae were separate galaxies, but he thought our own galaxy was smaller than it really is. Neither had enough evidence to settle the question.

The debate was not a contest with a winner announced at the end. Each astronomer presented his evidence, and afterwards both wrote longer papers explaining their reasons. Other astronomers read them, argued about them in letters and at meetings and went back to their telescopes. In the end, the question was not settled by who spoke more convincingly. It was settled, a few years later, by a measurement. That is how it should be in science, although it is not always so quick.

This happens often in science. Two careful people look at incomplete evidence and draw different conclusions. Neither is foolish. They are both doing their best with what they have. The argument itself is useful, because it makes clear what evidence is needed. In this case, everyone agreed on what would settle it: a reliable measurement of the distance to at least one spiral nebula. If someone could find a Cepheid in one of them, Leavitt's rule would give the answer.

The difficulty was practical. A Cepheid in a distant spiral nebula would be extremely faint, much too faint for most telescopes of the time to pick out as a single star. To see it, an astronomer would need one of the largest telescopes in the world, a great deal of patience and many cold nights of work.

One Star in Andromeda

That astronomer was Edwin Hubble. In 1923 and 1924, working night after night at one of the largest telescopes of the time, on a mountain far from city lights, he photographed the largest and brightest of the spiral nebulae, the one in the constellation Andromeda. Comparing his photographic plates, he found a star whose brightness rose and fell in a steady rhythm. It was a Cepheid. Soon he found others.

A small detail of this story has been kept. When Hubble first saw the star on one of his glass plates, he marked it as a possible new star, the kind that flares up once and then fades. When he compared it with older plates of the same patch of sky and saw that its light rose and fell in a repeating rhythm, he crossed out his first mark and wrote a short note saying that it was a variable star, a star that changes its brightness. That plate, with his corrected note on it, is still kept. It is a nice reminder that even great discoveries begin with a person correcting their own first guess.

Using Leavitt's rule, Hubble worked out how far away these stars were. The answer was far greater than the size of our own galaxy, even in the largest estimates of the time. The Andromeda nebula was not a small cloud inside the Milky Way. It was another galaxy, far outside ours, made of an enormous number of stars of its own. His result was announced around the start of 1925.

Hubble's first estimate of the distance was less than half of today's value, because of errors in the measuring tape that were corrected later. Today, the Andromeda galaxy is known to be about 2.5 million light-years away. But even his first number was enough. In a short time, the universe had grown from one galaxy to countless galaxies, scattered through space in every direction. The Milky Way was no longer the universe. It was one island among a great many.

It is hard to overstate how large this change was. Within the working life of one generation, the known universe became so much larger that the old picture seemed tiny. And this was only the beginning. Hubble and others soon began to look at the light from these galaxies more closely, and they found something even stranger, which is the story of the next night of this book.

Before Sleep

Noor stayed at the window until the station crossed back into daylight. Then she wrote to Sami. "In 1900, many astronomers thought our galaxy was the whole universe. Then a woman who measured stars on glass plates found a way to measure distance, and a man at a big telescope used it to show that the Andromeda nebula is a separate galaxy. Suddenly there were countless galaxies. Today's homework: if you get a dark night, look for a pale band across the sky. That is our galaxy, from the inside."

Sami replied the next morning, before he went to sleep after his shift. He wrote that he had once seen the Milky Way, years ago, on a delivery route through the countryside, when he had stopped at the side of a road with no lights. He had thought it was a thin cloud. "Now I know it was a few hundred billion stars," he wrote. "I will look again."

Noor read his message at breakfast, floating beside Ade, who was eating dried fruit from a packet. She showed it to him. Ade said that he liked the idea of a truck driver at the side of a dark road, looking at a few hundred billion stars and thinking they were a cloud. "Most of the people who ever lived thought the same," he said. "Your brother is in good company."

If you live in a town or city, you may rarely see the Milky Way, because street lights and lit windows brighten the sky. That is not something to feel sad about. It is simply a reason, if you ever travel far from lights on a clear night, to look up for a while. The pale band will be there, as it has been every night of human history, waiting for anyone who has a moment to notice it.

Common Misunderstandings

Many people think that the Milky Way is the same thing as the universe, or that the two words mean almost the same. This was, as we have seen, the belief of many astronomers only a century ago. But today we know that the Milky Way is one galaxy among a vast number of galaxies, far too many to count one by one. The Milky Way is our home galaxy, the one we live inside. The universe is everything: all the galaxies, all the space between them and everything else that exists.

Some people believe that other galaxies can only be seen with large telescopes. Most of them, it is true, are much too faint for the eye. But a few can be seen without any instrument at all. The Andromeda galaxy, on a clear, dark night far from lights, appears as a small faint smudge, a little oval of soft light. It does not look like the spiral photographs in books, because the eye can see only its bright centre. Still, if you find it, you are seeing with your own eyes light that left another galaxy about 2.5 million years ago. From the southern half of the Earth, the two small galaxies that Henrietta Leavitt studied can also be seen as two pale clouds in a dark sky.

A third misunderstanding concerns the word nebula. Today, astronomers use it for clouds of gas and dust in space, such as the glowing places where new stars are born. In the past, the same word was used for any faint fuzzy patch in a telescope, including the spiral nebulae that turned out to be galaxies. That is why old books speak of the Andromeda nebula, while modern books speak of the Andromeda galaxy. They are the same object. Only our understanding has changed. Changes like this, where the name stays but the knowledge grows, are part of the story of science, and on the next night we will see how a careful look at the colours of galaxies changed our picture of the universe once again.