First Light: Switching on Stars at the Dawn of Time

A NOTE ON THE AUTHOR

Emma Chapman is a lecturer and Royal Society research fellow based at the University of Nottingham and one of the world’s leading researchers in search of the first stars to exist in our Universe. Emma is the recipient of multiple commendations and prizes, including the Royal Society Dorothy Hodgkin Research Fellowship, one of the most prestigious science fellowships in the UK. She was presented with the Royal Society Athena Medal in 2018 and highly commended in the L’Oréal-UNESCO Women in Science award in 2017. In 2014, she won the Institute of Physics Jocelyn Bell Burnell Prize.

@DrEOChapman

Also available in the Bloomsbury Sigma series:

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Spirals in Time by Helen Scales

A is for Arsenic by Kathryn Harkup

Suspicious Minds by Rob Brotherton

Herding Hemingway’s Cats by Kat Arney

The Tyrannosaur Chronicles by David Hone

Soccermatics by David Sumpter

Goldilocks and the Water Bears by Louisa Preston

Science and the City by Laurie Winkless

Built on Bones by Brenna Hassett

The Planet Factory by Elizabeth Tasker

Catching Stardust by Natalie Starkey

Nodding Off by Alice Gregory

Turned On by Kate Devlin

Borrowed Time by Sue Armstrong

The Vinyl Frontier by Jonathan Scott

Clearing the Air by Tim Smedley

Superheavy by Kit Chapman

The Contact Paradox by Keith Cooper

Life Changing by Helen Pilcher

Kindred by Rebecca Wragg-Sykes

Our Only Home by His Holiness The Dalai Lama

First Light by Emma Chapman

Ouch! by Margee Kerr & Linda Rodriguez McRobbie

Models of the Mind by Grace Lindsay

The Brilliant Abyss by Helen Scales

Overloaded by Ginny Smith

Handmade by Anna Ploszajski

Beasts Before Us by Elsa Panciroli

Our Biggest Experiment by Alice Bell

Worlds in Shadow by Patrick Nunn

Aesop’s Animals by Jo Wimpenny

Fire and Ice by Natalie Starkey

Sticky by Laurie Winkless

Racing Green by Kit Chapman

To Lyra, Cassie and Olive.

Never stop asking questions

Contents

Introduction

Chapter 1: Over the Rainbow

Chapter 2: Where is Population III?

Chapter 3: The Small Bang

Chapter 4: A Lucky Cloud of Gas

Chapter 5: The Dark Ages

Chapter 6: Fragmenting Stars

Chapter 7: Stellar Archaeology

Chapter 8: Galactic Cannibalism

Chapter 9: The Cosmic Dusk

Chapter 10: The Epoch of Reionisation

Chapter 11: Unknown Unknowns

References

Acknowledgements

Index

Plates

Introduction

Teach me your mood, O patient stars!

Who climb each night the ancient sky,

Leaving on space no shade, no scars,

No trace of age, no fear to die.

Ralph Waldo Emerson

In an age where we have particle colliders and space telescopes, it’s hard to imagine a time when we could solve the biggest questions of the cosmos by just looking up. Look up at the night sky, and what do you see? You might be lucky to live in an area of low light pollution, so perhaps you can see the Milky Way splashed across the sky. Or perhaps there is a full Moon. Overall, though, the key characteristic of the night sky is that it is dark. Why? The littlest of words with the biggest of consequences.

For centuries, natural philosophers, physicists, astronomers and even poets wondered why the sky is dark. Their belief was that the Universe was infinitely old and infinitely large, as they had no evidence to the contrary. Olbers’ paradox (named after the German astronomer Heinrich Wilhelm Olbers) states that if the Universe is infinitely old and unmoving then every direction you look in should land on a star. The problem captured the imagination of many and even Edgar Allan Poe weighed in, in his 1848 prose poem ‘Eureka’: ‘Were the succession of stars endless, then the background of the sky would present us a uniform luminosity, like that displayed by the Galaxy – since there could be absolutely no point, in all that background, at which would not exist a star.’

The sky should be as bright as the Sun, everywhere. Well, that’s not right, because if that were the case we’d have no need for street lighting. And so we wondered, for generations, what made the sky dark. To break a paradox, you break the assumptions, and in this case both assumptions of infinite age and lack of movement are wrong. Our Universe started with a Big Bang, beginning the expansion of space-time. In one fell swoop, the Big Bang requires the Universe to begin (i.e. not be of infinitely old age), and expand (i.e. move). We can solve the paradox because, even though it has been 14 billion years since the Big Bang, insufficient stars have been born for every line of sight to land on a star. Even in the deepest images from the Hubble Space Telescope we see that the galaxies account for only a small fraction of the picture, and each galaxy hosts billions of stars. Breaking the assumption of an infinitely old Universe has profound consequences. The Universe began. There were not just stars, but first stars, and second and third stars for that matter. What we experience now is only one stage of a much bigger cosmological lifetime (Figure 1). It’s a lifetime that we can be pretty smug about understanding. We have observations of young and old stars, galaxies that are ancient and galaxies that are newly formed. We live in a time with unprecedented access to the Universe and its history, and our ability to fill in those gaps in knowledge has increased at a lightning-fast pace. Astronomy has broadened from a necessity in ancient civilisations, through a curious hobby for the rich, to an existential science broadcast to the masses. It has become so much a daily part of our lives that, while once we celebrated the discovery of new stars and new galaxies, now we barely lift an eyebrow if we discover a new planet. We understand our Universe’s lifetime, even right back to the Universe’s birth story: the Big Bang. We have lots of data … but it isn’t enough. Despite the exponential increase in technology and progress, there is a period in our Universe that, until recently, we had no observations of at all. From 380,000 years after the Big Bang to about 1 billion years after it, the Universe has remained in the Dark Ages. The first stars were born less than 200 million years after the Big Bang. Formerly, the Universe had been dark and empty until a simple star roared to life. A chain of internal nuclear reactions sparked into operation, producing light and heat into the barren surrounding space. Another star lit up, then another, until they speckled the sky with the dawn of first light. It was too early for planets, so there was no one around to witness that initial burst of star formation or mourn their almost immediate deaths. As the second population of stars established itself, that first generation was quickly forgotten, yet it was their intervention above all that prepared the Universe for the immense variety of structure and life that we see today. The absence of observations from the era of the first stars is alarming your local astrophysicist for two reasons.

Figure 1 We are missing data from about 380,000 years after the Big Bang to 1 billion years after the Big Bang.

1. Incomplete data = incorrect conclusions

First, a lack of data on this scale really matters. Missing data in any situation is a cause for concern, as it is data that we use to inform our decisions and progress our understanding. A lack of data can lead to uncertainty and misunderstandings or failings. Let us consider how aliens in our neighbourhood galaxy, Andromeda, might seek to understand the human life cycle. If our research aliens are like researchers on this planet, they are overworked and rely on the whim of funding agencies to give them the money and time to study. Our aliens don’t have the resources to spend 30 years gathering data by scanning humans around the Earth, and instead will do what all scientists do when studying sizeable populations: take a sample. For example, they might take a photograph in a nursery or nursing home, or on a beach in Australia or the casinos of Las Vegas. If our aliens are really feeling the pinch of the latest research budget cuts, they might observe one location, perhaps choosing the queue of Space Mountain in Walt Disney World at random. They will take their data and walk away unaware that they have missed an extensive section of society in that sample, namely pregnant women and humans under the age of seven. Unfortunately for our aliens, with incomplete data often come incorrect conclusions. Without these sections of society, it is hard to understand the human life cycle and easy to go down the wrong path. Perhaps during a cursory literature review they read the story of babies being delivered by a stork and shouted ‘Eureka!’, as this fitted their data adequately. Whoops. Incomplete data? Incorrect conclusions.

In human terms, the missing cosmological data is equivalent to missing everything from the moment of conception to the first day of school, perhaps apart from a single ultrasound. It may be a small fraction of time compared to the total lifetime, but when you consider how formative these early years are for humans, it is no wonder that astrophysicists quake at this much missing data when it comes to the history of our Universe. What incorrect conclusions are we coming to about the stars around us or how the Universe is behaving now, because of this lack of data?

Your local astrophysicist is alarmed at this lack of data for a second reason too.

2. The era of the first stars is unique

The first stars are not a first edition Harry Potter, with the same story but printed on older pages. The first star is a species unto itself, a missing link that may now be extinct altogether. But surely one star is like another star? We have one really close by after all, and it would save a lot of effort if we could just study that one. Look up into the night sky and you will see only a few thousand of the several hundred billion stars in the Milky Way. To our Universe the Sun is an unremarkable star. Even so, generalising all those stars from looking at the one closest to us would be like our aliens arbitrarily observing only one human and generalising that therefore the entire human population was called Elvis, measured 1.8m and enjoyed peanut butter sandwiches. It is only in the last 250 years that we have understood that not all stars are (in fact most stars are not) the same as our Sun.

Stars are made mostly of hydrogen and helium but we can divide the stars we see into three populations, based on the amount of metals within them. In astronomy when we discuss metals we don’t exclusively mean gold, silver, platinum and so on. When we look at the abundances of elements in the Universe, hydrogen and helium dominate. Because of this, and as astrophysicists are used to rounding up astronomical distances and huge timescales, we rounded up the periodic table too. Because I am an astrophysicist, in this book I will refer to all chemical elements other than hydrogen or helium as metals. For clarity I have reproduced the astrophysicist’s periodic table overlaid on the chemist’s periodic table in Figure 2.

Figure 2 The chemist’s periodic table overlaid by the astrophysicist’s periodic table.

Back to the populations. When the Universe started out, it was predominantly filled with hydrogen and helium. Anything heavier than that had to be created in the hot furnaces of stars or in the energetic explosions that ended their lives. With each generation of stars, the gas became more dense with metals, and the more metals would be present inside the next-generation stars that formed from that gas. The most recent generation, Population I stars, are young stars that have lots of metals inside them. They are luminous, hot and live in a galaxy’s disk. Population II stars are older and have fewer metals. They reside in the centre of the galaxy or outer halo. It doesn’t take much imagination to carry on down this road and ask: what about the oldest stars? The stars with no metals at all, the stars that started it all. Where are they? The first stars produced the first metals, seeding the Universe and enabling galaxies to form. They were metal-free to start with and we call them Population III stars.¹

Population III stars were ancient beasts of mammoth proportion, up to several hundred times the mass of our Sun. They lived fast, with lifetimes of only a few million years compared to the 10-billion-year lifetimes of less massive stars such as our Sun. The same diversity of lifetimes in anthropology would be equivalent to finding an early humanoid species that aged and died only three days after birth. And yet in such short lifetimes, those stars are the ones most responsible for changing the Universe. As they roared to life, they illuminated the Universe, irradiating it and seeding it with metals that could then form stars, planets and us.

* * *

Midway through writing this book I posted a picture on Twitter that showed me working on a chapter with a frown on my face, while holding my five-week-old daughter on my shoulder. That was at around 9.00 a.m. At 4.00 p.m. I started to feel a bit funny and kept getting single-digit sums wrong while helping out with homework. At 5.00 p.m. I was struggling to breathe in the emergency department of my nearest hospital. I had developed sepsis, a life-threatening condition where an infection overwhelms the immune system, leading to multiple-organ failure, and in 20 per cent of cases in England, death. Why am I telling you this? Well, partly because I hope I’ve guilt-tripped the more empathetic of you browsing in the bookshop into buying the book: I nearly died writing it. Open your wallet! The actual reason I’m telling you this, however, is because one of the contributing factors to my body breaking down was my lack of exposure to starlight, or since we are on a first-name basis with our nearest star, sunlight. It turned out, after months of recovery and medical investigations, that I was deficient in vitamin D, a vitamin our bodies make with the help of UVB sunlight. We associate exposure to sunlight with the risk of sunburn, skin cancer and sunstroke, all important concerns, but a complete lack of sunlight can also damage human life. Severe vitamin D deficiency is a major contributing factor to debilitating skeletal conditions and is associated with suppression of the immune system. Now, I am a scientist so I realise that as a sample of one I cannot quantify the contribution vitamin D deficiency had in my case. Allow me to commandeer a personal dramatic story for a larger message, though. The human species depends on the Sun, not just to enable and maintain our physical habitat, but in terms of the survival of our biological habitat, our bodies, too. Our bodies are made from the metals forged in the very first stars, as well as the generations that followed. We are a machine made from the dying expulsions of stars, and we need sunlight to keep that machine running.

Interpreting the surrounding Universe is important for many reasons, and the era of the first stars has the potential to shed light not only on the early years of our Universe, but on the contemporary mysteries astro­physicists grapple with today. To understand a troubled teenager, we need to consider what happened when they were a toddler. Maybe the growing pains of galaxies 500,000 years after the Big Bang can explain why there are so many dwarf galaxies orbiting the Milky Way now.

* * *

Science is about asking questions and so I expect you have finished the Introduction with far more questions than you started with. Hopefully, by the end of the book we will have many of those questions answered... though it being science we will also probably have added more too, in the fractal way of scientific queries. We may still be stumbling in the dark, but we have established some important facts with which to start our search for first light. We know that there were first stars because the Universe began with a Big Bang, and therefore there is a first of everything. We know a lot about the history of the Universe since the Big Bang, but the era of the first stars constitutes a billion- year-long blank space in that timeline. It’s not just for completeness that we want to fill in that gap – the astrophysics is exotic and worth investigation. The first stars were a species unto themselves: hot, massive and fleeting. They produced the metals (elements heavier than helium) that seed the Universe today and make up our own planet and other bodies. Learning about the baby steps of the Universe could also help us understand modern impenetrable mysteries, like how the black hole at the centre of our Galaxy got so very large. This book charts what we know about the era of the first stars, the early black holes and the first galaxies. It covers the race to discover those Population III stars, either by doing stellar archaeology or by searching for their dying signals from long ago. We will decipher clues from the Big Bang and follow the fossils of the oldest galaxies that exist in our Universe. We will need a myriad of telescopes, from the space-bound infrared to the more down-to-earth radio contraptions sitting in deserts and fields around the globe. But most of all, we will need our curiosity as we poke, prod and prise open the lid on a time that, until now, has been left in the dark. What we will see, the surprises we will unearth, well … sit back, relax and get ready to watch the Cosmic Dawn.

Notes

1 The counter-intuitive numerical order of these populations is an artifact of their historical discovery and grouping.

CHAPTER ONE

Over the Rainbow

In January 1925, US Navy navigator Alvin Peterson climbed back into the belly of an airship and realised that his cheeks, chin and fingers were frostbitten.¹ Peterson hadn’t noticed this before because he had been too busy standing on top of the 200m (650ft) -long airship, steadily cranking a film camera and making the first motion picture of a solar eclipse. The USS Los Angeles was then the largest balloon airship ever made, built in Germany as part of the First World War reparations package. For this special voyage, scientists had commandeered it. Far below the USS Los Angeles, small groups of representatives of the Edison Company stood on the rooftops of apartment buildings in Manhattan.² Wearing long coats and fedoras, their faces stony with concentration, they struck a rather sinister picture. In each group, one person observed the eclipse and noted the degree to which the Moon covered the Sun, and whether totality, full coverage, was achieved. As a secondary check, another employee would look at the ground and determine whether they stood in the shadow of the Moon or not. In the streets below were amateur astronomers and schoolchildren filling out questionnaires detailing what they could see.³ Enormous resources had gone into viewing this eclipse, from the chartering of army vehicles to the drafting of the general populace as citizen scientists. This broad engagement was engineered to improve models of the Moon’s motion, and to have as many eyes as possible on the part of the Sun that was only revealed during an eclipse. As the Moon moved in front of the solar disc, one sole figure stood on top of an air balloon and was the closest human to the Sun. Alone and in darkness, Alvin Peterson was the best able to see the outermost layer of the Sun, the corona, flash into view. The corona, ‘crown’ in Latin, is usually hidden because the central disc of the Sun outshines it by far. Earlier eclipses had indicated that the corona contained a new chemical element called coronium,⁴ and the 1925 eclipse provided an opportunity to study it further. However, if you check a twenty-first-century pocket periodic table, you will note that there is no coronium included. So what happened?

Eclipses and their revelations

To experience a total solar eclipse, the Moon must be in the right place in between the Earth and Sun according to the observer. The Moon’s orbital plane (the surface it sweeps out as it travels around the Earth) is at an incline of around 5 degrees compared to the ecliptic, the name we give to the Earth’s orbital plane around the Sun. If the two were aligned then total solar eclipses would occur once a month for any one observer on Earth. As it is, the Moon only crosses the Earth-Sun orbital plane while in between the two twice a year. A further complication comes from where in its orbit the Moon is, as its orbit is elliptical and not circular. Try sticking out your thumb and looking at it with one eye closed. As you move your thumb further away from you it blocks out a smaller area of the background. If the Moon is in the elongated part of its orbit as it moves in front of the Sun, the Moon will appear too small, leaving us with a ring of fire instead of totality: an annular eclipse.

Figure 3 Eclipse mechanics. The ellipticity and tilt of the Moon’s orbit conspire such that total solar eclipses are rare as observed from any one place on Earth.

If the Moon is crossing the ecliptic, and in between the Sun and Earth, and is the right angular size to block out the Sun, then a total solar eclipse will occur and the resulting shadow will saunter across a long, thin section of the Earth as it rotates. Taking all of this into account, total solar eclipses occur somewhere on Earth around every 18 months. Any one location on Earth will experience a total solar eclipse about every 375 years.⁵ Lunar eclipses, where the Moon passes into the Earth’s shadow so that the Earth is in between the Sun and Moon, occur for a much greater proportion of people each time. This is because the Earth’s shadow on the Moon is broader than the Moon’s shadow on Earth, so when a lunar eclipse happens, everyone on the night-time side of Earth can see it.

In the past, eclipses were greeted with apprehension, their rarity lending itself to an unnatural, or mystical, explanation. They may be rare but the dance of the Earth and Moon has long been well known to some, allowing the prediction of eclipses for centuries. In 1504, when moored in Jamaica, the Italian explorer Christopher Columbus was facing a mutiny on his ship after the local population had stopped its coerced gifts of food and supplies.⁶ Columbus told local leaders that God was angry at them for their reluctance and predicted (using his astronomical tables) that the Moon would soon rise with wrath. When a lunar eclipse occurred as predicted, the donations resumed. One of the most fascinating and committed attempts to predict eclipses was the Antikythera mechanism built more than 2,000 years ago in Ancient Greece.⁷ The Antikythera device comprised more than 30 gears and is believed to have been the first analogue computer. The gears worked together to model movements of the Moon and Earth and predict when the two would align to create an eclipse. The machine wasn’t perfect, as