15 Million Degrees

by Lucie Green

  Lucie Green

  * * *

  15 MILLION DEGREES

  A Journey to the Centre of the Sun

  Contents

  Introduction

  1. Light: Don’t Believe Your Eyes

  2. Star Power

  3. Suns and Daughters

  4. The Secret Life of a Photon

  5. Sunspots

  6. The Spinning Sun: The Day the Sun Fought Back

  7. The Dynamic Sun

  8. Eclipses and Rainbows

  9. Bon Voyage

  10. Space Age

  11. The Flare Necessities

  12. Coronal Mass Ejections

  13. Living in the Atmosphere of the Sun

  14. What Comes Next?

  Conclusion: Our Special Sun

  Illustrations

  Appendix: How to Safely Observe the Sun

  Bibliography

  Glossary

  Acknowledgements

  Follow Penguin

  For Valerie and Alan Green, my parents and role models

  The author and publishers wish to point out that looking directly at the Sun is dangerous and can cause permanent damage to the eyes. The Sun can be viewed directly only when filters specially designed to protect the eyes are used, or if a small pinhole projector or other indirect viewing method is employed. See the Appendix on page 271.

  Introduction

  This is a book about one star. Just one out of the hundreds of billions of stars that there are in our Galaxy, the Milky Way, which in turn is just one galaxy out of the hundreds of billions there are sprawled across the known Universe. When you think about it, with numbers like these, what are the chances that our star should be special? Well, after having studied our Sun for almost twenty years, it’s clear to me that it is very remarkable indeed. And I want to show you why.

  At the heart of the Sun a gigantic nuclear furnace provides a continual source of energy that we would love to be able to emulate here on Earth. The Sun does it naturally and will do it for around nine billion years in total. The temperature in its core is over 15 million degrees, and the material there is under immense pressure. It’s in these extreme conditions that sunlight is created. I remember being startled to find out that sunlight, born as gamma radiation, takes 170,000 years to slowly trickle to the Sun’s surface. There it emerges as visible light, finally pouring towards the Earth, where just over eight minutes later we are able to see it. But light isn’t the only thing the Sun sends our way. And this is where things get even more intriguing.

  The Sun is often a violent star and produces the most powerful explosions and eruptions in the Solar System. In this book I want to reveal to you all the sides of the Sun’s character, from the serene but spotty surface we can see when we look at the Sun in visible light, to the explosive and unpredictable atmosphere that needs to be viewed through ultraviolet radiation, X-rays and gamma rays. Both the explosions and the eruptions are beautiful to watch, but, given that we are 150 million kilometres from the Sun, it is a challenge for us to see and understand them – for that, we had to go into space.

  The story of some of the early space pioneers is told in this book. We’ll discover that the opportunity they had to put telescopes in space was given by redundant Second World War technology, beginning in the US in the 1940s and in the UK the decade after.

  The first images of the Sun from space were taken with simple pinhole cameras. Today, the space kit we have at our fingertips includes NASA’s Solar Dynamics Observatory. Telescopes on this satellite take ten highly detailed images of the Sun’s atmosphere every twelve seconds. And that’s just the data being gathered by one of the three instrument suites on board the satellite. This phenomenal spacecraft adds another 1.5 terabytes of data to our archives every single day. More data is generated in one week, and for one star, than the Hubble Space Telescope generates in a year for the rest of the Universe.

  The space age gave us the luxury of being able to view the Sun at any wavelength in the electromagnetic spectrum and at any time, and the view above the Earth’s atmosphere has been stunning. But it comes at a price. Launching 1 kilogram (equivalent to one bag of sugar) into space costs over £10,000 – giving the largest space observatories almost a billion-pound price tag. I have worked with the SOHO satellite, which was launched in 1995 and came in at a cost of £700 million. The Solar Dynamics Observatory satellite, launched in 2010, cost £550 million. But this is money that is both spent well and spent right here on Earth employing highly skilled designers and engineers across academia and industry. Our studies of the Sun are important for science but they’re also important for the space sector as a whole. In the UK this industry has an annual turnover of over £10 billion and directly employs over 34,000 people. Globally, the space economy is worth over £200 billion.

  My career in science has been dominated by the space age and I want to show you just how important this period has been in transforming our understanding of the Sun. It has literally expanded our horizons and even shown that the atmosphere of the Sun reaches out to a staggering 18 billion kilometres beyond us – that’s 121 times further out than the Earth’s orbit. We are living in the Sun’s atmosphere! And as this atmosphere changes because of the Sun’s explosive side, stormy ‘space weather’ is felt on Earth. We’ll discover how this produces a threat to modern society through its impact on our electricity distribution, satellite technology and communications. But don’t worry: predicting space weather is common now and we always keep a watchful eye on the Sun for our own safety and for the sake of the space economy.

  Throughout the pages of this book we’ll see just how much lies behind a seemingly straightforward desire to understand the Sun – we have quite a journey ahead! We’ll cover thousands of years of naked-eye observations (do not try this yourself though), hundreds of years of telescopic observations (visit a specialist supplier) and decades of observations from space (images freely available on the internet). We’ll see how understanding the Sun needs the application of atomic physics, thermodynamics, electricity, magnetism, gravity and light. We’ll learn how energy is transported and changed into different forms and how this discovery was made by a ship’s doctor. We’ll find out that the discovery of the Sun’s magnetic field needed an incredible synthesis of science and engineering, but that the payoff was the birth of solar physics.

  You’ll probably end up thinking that I am completely obsessed by the Sun. And you’d be right. I would say that our Sun is the most important star there has ever been or ever will be, since it is the star that gave the energy required for life on Earth to emerge and thrive. As I am writing, we are searching for life elsewhere in the Universe, both within our Solar System and beyond. We are scanning the skies for radio signals from advanced civilizations, sending out our own messages and combing the surface of Mars for microbes. And we plan to expand the locations where we are looking for life to include the moons of planets too. But, as of 2015, life on Earth, orbiting around our Sun, is all we know.

  I have let other stars feature in the pages though. Their stories help us realize the full potential of the Sun in terms of the power it might one day unleash. But we’ll also see that the Sun has been a stepping-stone to understanding other stars across the Universe. The extra
ordinary detail that we can see has provided a wealth of information. All other stars are so distant that they are seen as specks of light (apart from rare exceptions such as Betelgeuse). So it was by studying the Sun that we were able to understand that, in their most basic sense, stars are spheres of plasma that shine because of nuclear processes happening in their cores. They sound elegantly simple. And in 1926 this sentiment was put down on paper when a British mathematician, physicist and astronomer, Sir Arthur Eddington, wrote: ‘it is reasonable to hope that in a not too distant future we shall be competent to understand so simple a thing as a star.’

  Eddington passed away in 1944 at the age of sixty-three, just a few years before the start of the space age. He knew nothing of the Sun’s million-degree atmosphere, or that we can probe the interior of the Sun using sound waves that are trapped inside it and reveal themselves by creating patches of rising and falling gas at the solar surface. If Eddington were alive today there is no question that the level and complexity of our understanding would impress him. In this book you will see just how far we have come in our quest to comprehend what he inspiringly described as something as simple as a star.

  1. Light: Don’t Believe Your Eyes

  Some of you will be reading this book outside in the sun. Perhaps on the beach during a holiday or making the most of a sunny weekend in the garden. Or maybe you have sneaked out of work to catch up on your solar physics under the guise of a lunchtime trip to the shops. Whatever the reason, you are able to make out the curves and angles of the lines creating each letter in each word because of the sunlight falling on the pages of this book. Where the page is white, much of the sunlight is being reflected back into your eyes. Where there is ink, very little of the sunlight bounces back, and so dark shapes appear, producing the letters of the words that you are reading now.

  The sunlight that you are using to read is continuously flooding from the Sun onto your page and it has travelled 150 million kilometres* to get here. Producing light is what defines our Sun as a star. All the planets, moons, asteroids and everything else in our Solar System are like the pages of this book: they are not luminous but are seen by reflected sunlight. This light that shines continuously from the Sun is an important phenomenon not only of our Solar System but also of the entire Universe. Our experience of the cosmos is mediated through the light generated by stars.

  As a species we have grown up with sunlight. Every human that has ever lived has relied on it and our bodies have evolved and adapted to use it. Our evolutionary history is intimately linked to the Sun: the development of the eye that gives us sight, the synthesis of vitamin D and the pigmentation in our skin that helps protect us from sunburn and skin damage – our relationship with sunlight is essential. Not only does sunlight allow us to see, but it made our rocky planet habitable for millions of species of animals and plants. It grows our food, it drives our weather and it can even be utilized to generate electricity to power modern society. Sunlight is amazingly versatile, but above all there is something about sunlight that means it is able to keep us alive. We literally could not live without it. Sunlight’s ubiquitous and central role in our lives raises the question: what is it?

  THE NATURE OF LIGHT

  While light is so common it can easily be taken for granted, by merely casting a shadow you know it is a … thing. We can block light to make a shadow, which means there is an area where light is missing. Light must be some kind of object or phenomenon, which means we have the capacity to understand it. And for generation after generation, we have been trying to work out what light is.

  Despite its being an ancient question, we have really been equipped to tackle the puzzle of ‘What is light?’ for just the last 150 years or so. Before that, we could only investigate what light does. And there’s a big difference between seeing how sunlight behaves and knowing why it does so, but one can lead to the other once the necessary scientific foundations are in place. One person in particular illuminated an aspect of what sunlight can do in 1666, when a seemingly simple experiment involving nothing more than a wedge of glass showed us why we have colour vision. The experiment was the brainchild of the legendary natural philosopher and mathematician Sir Isaac Newton.

  The telescope had been invented only a few decades before Newton was born. And the primitive instruments had already been turned skyward by people such as Galileo so that their circular glass lenses could gather the light from the distant celestial object and bring it to a focus to form an image – just as the lenses in your eyes are doing now with the light reflected off the page of this book. Investigating what light is began when Newton wondered what would happen if light were passed through a piece of glass that wasn’t circular in shape.

  Newton darkened his room and placed a triangular wedge of glass, a prism, in a shaft of sunlight coming through a gap in the window shutters. He had expected to see a circle of colours on the wall behind, just as in a telescope. But, instead, a tiny rainbow was splashed across the opposite wall, stretching out not in a circle but in a line. In that moment, Newton discovered a whole new aspect to the behaviour of sunlight: that it can be refracted and split into its components. More importantly, he deduced that the rainbow colours are an inherent property of light itself – it wasn’t the glass somehow colouring the light. Sunlight was in fact composed of a sequence of colours that when combined together looked white. He also proved it by reversing the process: a second prism could turn the rainbow back into a beam of white light again.

  Sometimes after a storm we get a glimpse of what Newton observed, on a grandiose scale. When the Sun emerges from behind the clouds and its light encounters water droplets in the air, they act as prisms and the colours that make up sunlight are suddenly unravelled to produce a rainbow. Perhaps you have by your side, as you read, a glass of water. If you do, you can see this effect for yourself. Try placing the glass on a white surface in a beam of sunlight and you can glimpse the colour produced as the sunlight passes into and out of the water.

  What corroborates Newton’s theory is that whether you use a prism or a glass of water or raindrops in the air, the colours of the rainbow always appear spread out in the same order: red, orange, yellow, green, blue, violet. The pages of this book only appear white because they reflect all these colours equally. We have a colourful world around us because different objects reflect differing amounts of these colours.

  Newton’s experiment, although illuminating, had not answered the question: what is light? Newton’s guess was that light was a substance, which was made up of beams of tiny particles with the mass of the light particle varying with colour. As the light passed from the air into the glass prism and back out again, the paths of particles of different masses were bent by different amounts – refracting, or bending, the sunlight and teasing out the sequence of colours.

  But Newton wasn’t the only person making new observations about the behaviour of light.

  Both the English polymath Robert Hooke and the Dutch natural philosopher and mathematician Christiaan Huygens were investigating the nature of light, and neither subscribed to Newton’s particle theory. They had cause to doubt him as well: it had been shown that two beams of light could cross right through each other without any effect whatsoever. This is not what we would expect if light were made of particles, which could collide with each other. But it is what we would expect if light were made of waves – an idea Newton dismissed completely.

  Hooke was the first Curator of Experiments at the Royal Society in London and he made contributions across many areas of science. He was also not afraid to court controversy. And when Newton published his theory of light
just a few years after Hooke had published his wave theory, their scientific dispute fuelled a major rivalry between them. Newton used the successful strategy of simply outliving Hooke; he withheld the details of his complete theory of light until after Hooke’s death in 1703.

  In that same year Newton became president of the Royal Society. The organization continues to this very day, something I am very grateful for as I receive funding through it as a Royal Society University Research Fellow. I know the Royal Society and its building in London very well, and I’ve noticed that there isn’t a single painting, portrait or sculpture of Hooke on display. And that’s because none are known to exist, which is very odd for someone so significant within the organization. The (unsubstantiated) rumour I have heard is that when Newton became president, he disposed of everything to do with the late Robert Hooke.

  Newton also outlived Huygens, who died in 1695. Huygens had developed the wave idea into a much more detailed theory, which lived on posthumously. By considering sunlight to be a wave moving through an unknown medium, similar to water waves rippling across the surface of a pond, Huygens could explain why sunlight behaved the way it does, matching theory to observation. Refraction (bending) is a consequence of the front surface of the wave reaching a transparent material at an oblique angle, with the part of the wave front that touches the transparent material slowing down and the rest of the wave carrying on at the same speed as before. This explains why the wave front swings around within the transparent material and changes direction. According to Huygens’ theory, waves could cross each other because there was no matter to collide.

  The next task in developing the wave theory of light was to work out what kind of waves they were. What was the substance that enabled light to be propagated? What exactly was ‘waving’? The answer to these questions came from an unexpected direction – from the study of electricity.