Wonders of the Solar System Text Only

Wonders of the Solar System Text Only Andrew Cohen Professor Brian Cox The Sunday Times BestsellerIn Wonders of the Solar System – the book of the acclaimed BBC TV series – Professor Brian Cox will take us on a journey of discovery where alien worlds from your imagination become places we can see, feel and visit.The Wonders of the Solar System – from the giant ice fountains of Enceladus to the liquid methane seas of Titan and from storms twice the size of the Earth to the tortured moon of Io with its giant super-volcanoes – is the Solar System as you have never seen it before.In this series, Professor Brian Cox will introduce us to the planets and moons beyond our world, finding the biggest, most bizarre, most powerful natural phenomena. Using some of the most spectacular and extreme locations on Earth, Brian will show us Wonders never thought possible.Employing his trademark clear, authoritative, yet down-to-earth approach, Brian will explore how these previously unseen phenomena have dramatically expanded our horizons with new discoveries about the planets, their moons and howthey came to be the way they are. TEXT ONLY EBOOK EDITION For Gia, George and Mo, who kept our family together whilst I was away filming Wonders. Brian Cox For Anna, Benjamin, Martha and Theo, the true wonders of my solar system. Andrew Cohen Contents Cover (#uc733e5b6-8c2f-5b3b-9d41-47812f8c214f) Title Page (#u447fc2fb-09ae-57dc-bf89-9d31b75867b5) CHAPTER 1 - THE WONDER CHAPTER 2 - EMPIRE OF THE SUN CHAPTER 3 - ORDER OUT OF CHAOS CHAPTER 4 - THE THIN BLUE LINE CHAPTER 5 - DEAD OR ALIVE CHAPTER 6 - ALIENS Index Acknowledgements Copyright About the Publisher (#litres_trial_promo) CHAPTER 1 On a frosty winter’s afternoon the day after New Year 1959, a tiny metal sphere named First Cosmic Ship ascended, gently at first but with increasing ferocity, into a January sky above the Baikonur Cosmodrome a hundred miles east of the Aral Sea. Just a few minutes into her flight the little spacecraft separated from the third stage of her rocket and became the first man-made object to escape the gravitational grasp of planet Earth. On 4 January she passed by the Moon and entered a 450-day long orbit around the Sun somewhere between Earth and Mars, where she remains today. First Cosmic Ship, subsequently renamed Luna 1, is the oldest artificial planet and mankind’s first explorer of the Solar System. Half a century later, we have launched an armada of robot emissaries and twenty-one human explorers beyond Earth’s gravitational grip and onwards to our neighbouring worlds. Five of our little craft have even escaped the gravitational embrace of the Sun on journeys that will ultimately take them to the stars. No longer confined to Earth, we now roam freely throughout the Empire of the Sun. Our spacecraft have returned travellers’ tales and gifts beyond value; an explorer’s bounty that easily rivals the treasures, both material and intellectual, from the voyages of Magellan, Drake, Cook and their fellow navigators of Earth’s oceans. As with all exploration, the journeys have been costly and difficult, but the rewards are priceless. Our first half-century of space exploration – less than a human lifetime – has revealed that our Solar System is truly a place of wonders. There are worlds beset with violence and dappled with oases of calm; worlds of fire and ice, searing heat and intense cold; planets with winds beyond the harshest of terrestrial hurricanes and moons with great sub-surface oceans of water. In one corner of the Sun’s empire there are planets where lead would flow molten across the surface, in another there are potential habitats for life beyond Earth. There are fountains of ice, volcanic plumes of sulphurous gases rising high into skies bathed in radiation and giant gas worlds ringed with pristine frozen water. A billion tiny worlds of rock and ice orbit our middle-aged yellowing Sun, stretching a quarter of the way to our nearest stellar neighbour, Proxima Centauri. What an empire of riches, and what a subject for a television series. There are worlds of searing heat and intense cold; planets with winds beyond the harshest of terrestrial hurricanes and moons with great sub-surface oceans of water. When we first began discussing making Wonders of the Solar System, we quickly realised that there is much more to the exploration of space than the spectacular imagery and surprising facts and figures returned by our robot spacefarers. Each mission has contributed new pieces to a subtle and complex jigsaw, and as the voyages have multiplied, a greatly expanded picture of the majestic arena within which we live our lives has been revealed. Mission by mission, piece by piece, we have learnt that our environment does not stop at the top of our atmosphere. The subtle and complex gravitational interactions of the planets with our Sun and the billions of lumps of rock and ice in orbit around it have directly influenced the evolution of the Earth over the 4.5 billion years since its formation, and that influence continues today. Our moon, unusually large for a satellite in relation to its parent planet, is thought to stabilise our seasons and therefore may have played an important role in allowing complex life on Earth to develop; we probably need our moon. It is thought that comets from the far reaches of the outer Solar System delivered much of the water in Earth’s oceans in a violent bombardment only half a billion years after our home planet formed. This event, known as the Late Heavy Bombardment, is thought to have been the result of an intense gravitational dance between the giant planets of our Solar System, Jupiter, Saturn and Neptune. Without this chance, violent intervention, there may have been little or no water on our planet; we probably need the comets, Jupiter, Saturn and Neptune. More worryingly for us today, we have no reason to assume that this often-violent interaction with the other inhabitants of the Solar System has ceased; colossal lumps of rock and ice will visit us again from the distant vaults of the outer Solar System, and if undetected and unprepared for, we may not survive. It is thought that this very real threat is diminished by the gravitational influence of Jupiter on passing comets and asteroids, deflecting many of them out of our way; we probably need Jupiter. This picture of the Solar System as a complex, interwoven and interacting environment stretching way beyond the top of our atmosphere is one of the central stories running throughout the series. In order to understand our place in space we must lift our eyes upwards from Earth’s horizon and gaze outwards across a vast sphere extending perhaps for a light year beyond the Kuiper belt of comets and onwards to the edge of the Oort cloud of ice-worlds surrounding the Sun. Reaching for worlds beyond our grasp is an essential driver of progress and necessary sustenance for the human spirit. Curiosity is the rocket fuel that powers our civilization. Our exploration of the Solar System has also given us very valuable insights into some of the most pressing and urgent problems we face today. Understanding the Earth’s complex weather and climate system, and how it responds to changes such as the increase of greenhouse gases in the atmosphere, is perhaps the greatest challenge for early twenty-first century science. This is a planetary science challenge: Earth is one example of a planet with an atmosphere of a particular chemical composition, orbiting the Sun at a particular distance. For a scientist, having only one example of such a vast and interdependent system is less than ideal, and makes the task of understanding its subtle and complex behaviour tremendously difficult. Fortunately, we do have more than one example. Our Solar System is a cosmic laboratory, a diverse collection of hundreds of worlds, large and small, hot and cold. Some orbit closer to our star than we do, most are vastly further away. Some have atmospheres rich in greenhouse gases, far denser than our own, others have lost all but faint traces of their atmospheres to the vacuum of space. Our two planetary neighbours, Venus and Mars, are salient examples of what can happen to worlds very similar to Earth if conditions are slightly different. Venus experienced a runaway greenhouse effect that raised its surface temperature to over 400 degrees Celsius and atmospheric pressure to ninety times that of Earth. Because the laws of physics that control the evolution of planetary atmospheres are the same on Earth and Venus, our understanding of the Greenhouse Effect on Earth can be transferred to and tested on Venus. This provides valuable additional information that can be used to tune and improve those models. The discovery that this benign, blue planet which shimmers brightly and with such beauty in the twilight skies of Earth was transformed long ago into a hellish world of searing temperatures, acid rain and crushing pressure has had a genuine and profound psychological effect because it demonstrates in stark terms that runaway greenhouse effects can happen to planets not too dissimilar from our own. Similar salutary insights have accrued from our studies of Mars. In the early days of Mars exploration, observations of the evolution of dust storms on the red planet provided support for the nuclear winter hypothesis on Earth. Storms that began in small local areas were observed to throw large amounts of dust into the Martian atmosphere. Over a period of a few weeks, the dust encircled the entire planet, exactly matching the models of the evolution of the dust and smoke clouds that would be created by a large-scale nuclear exchange. When a planet is shrouded in dust, the warmth of the Sun is reflected back into space and temperatures quickly fall, leading to a so-called nuclear winter that could last many decades. On Earth, this could lead to the extinction of many species, including perhaps our own. The observation of a mini-nuclear winter periodically playing itself out on Mars was a major factor in the acceptance of the theory, and this in turn profoundly influenced the thinking of major players at the end of the cold war. As former Russian president Mikhail Gorbachev said in 2000, ‘Models made by Russian and American scientists showed that a nuclear war would result in a nuclear winter that would be extremely destructive to all life on Earth; the knowledge of that was a great stimulus to us, to people of honor and morality, to act in that situation’. Wonders is also a story of human ingenuity and engineering excellence. Russia’s First Cosmic Ship began its voyage beyond Earth just fifty-five years after the first powered flight by Orville and Wilbur Wright in December 1903. Wright Flyer 1 was constructed from spruce and muslin, and powered by a twelve-horsepower petrol engine assembled in a bicycle repair shop. By 1969, less than one human lifetime away, Armstrong and Aldrin set foot on another world, launched by a Saturn V rocket whose giant first-stage engines generated around 180 million horsepower between them. The most powerful and evocative flying machine ever built, the Moon rocket stood 111 metres (364 feet) high, just thirty centimetres (twelve inches) short of the dome of Wren’s magisterial St Paul’s Cathedral. Fully fuelled for a lunar voyage, it weighed 3,000 tonnes. Sixty-six years before Apollo 11’s half-a-million mile round trip to the Moon, Wright Flyer 1 reached an altitude of three metres (ten feet) on its maiden voyage. This rate of technological advancement, culminating with our first journeys into the deep Solar System, is surely unparalleled in human history, and the benefits are practically incalculable. Most importantly of all, Wonders is a celebration of the spirit of exploration. This is desperately relevant, an idea so important that celebration is perhaps too weak a word. It is a plea for the spirit of the navigators of the seas and the pioneers of aviation and spaceflight to be restored and cherished; a case made to the viewer and reader that reaching for worlds beyond our grasp is an essential driver of progress and necessary sustenance for the human spirit. Curiosity is the rocket fuel that powers our civilization. If we deny this innate and powerful urge, perhaps because earthly concerns seem more worthy or pressing, then the borders of our intellectual and physical domain will shrink with our ambitions. We are part of a much wider ecosystem, and our prosperity and even long-term survival are contingent on our understanding of it. ‘Many years ago the great British explorer George Mallory, who was to die on Mount Everest, was asked why did he want to climb it. He said, “Because it is there.” Well, space is there, and we’re going to climb it, and the Moon and the planets are there, and new hopes for knowledge and peace are there. And, therefore, as we set sail we ask God’s blessing on the most hazardous and dangerous and greatest adventure on which man has ever embarked.’ — John F. Kennedy, Rice University 1962 In 1962, John F. Kennedy made one of the great political speeches at Rice University in Houston, Texas. In the speech, he argued the case for America’s costly and wildly ambitious conquest of the Moon. Imagine the bravado, the sheer power and confidence of vision in committing to a journey across a quarter of a million miles of space, landing on another world and returning safely to Earth. It might have been perceived as hubris at the time, but it worked. America achieved this most audacious feat of human ingenuity within nine years of launching their first manned sub-orbital flight. Next time you glance up at our shimmering satellite, give a thought to the human beings just like you who decided to go there and plant their flag for all mankind. At the turn of the twenty-first century, the Solar System is our civilization’s frontier. Our first steps into the unexplored lands above our heads have been wildly successful, revealing a treasure chest of new worlds and giving us priceless insights into our planet’s unique beauty and fragility. As a species, we are constantly balanced on a knife-edge, prone to parochial disputes and unable to harness our powerful curiosity and boundless ingenuity. The exploration of the Solar System has brought out the best in us, and this is a precious gift. It rips away our worst instincts and forcibly thrusts us into a face-to-face encounter with the best. We are compelled to understand that we are one species amongst millions, living on one planet around one star amongst billions, inside one galaxy amongst trillions. The beauty of our planet is made manifest, enhanced immeasurably by the juxtaposition with other worlds. Ultimately, the value of young, curious and wonderful humanity as we take our first steps outwards from our home world is brought into such startling relief that all who share in the wonder must surely be filled with optimism and a powerful desire to continue this most valuable of journeys. CHAPTER 2 AN ORDINARY STAR At the heart of our complex and fascinating Solar System sits its powerhouse. For us it is everything, and yet it is just one ordinary star amongst 200 billion stars within our galaxy. It is a large wonder that greets us every morning; a star that controls each and every world that it holds in its thrall – the Sun. The Sun reigns over a vast empire of worlds and without it we would be nothing; life on Earth would not exist. Although we live in the wonderous empire of the Sun, it is a place we can never hope to visit. However, thanks to the continual advances in technology and space exploration, and through observation from here on Earth, each spectacular detail we see leads us closer to understanding the enigma that is the Sun. In the north of India, on the banks of the river Ganges, lies the holy city of Varanasi. It is one of the oldest continuously inhabited cities in the world, and for Hindus it is one of the holiest sites in all of India. Varanasi, or Banares, to give the city its old name, is a city suffused with the colours, sounds and smells of a more ancient India. Mark Twain famously wrote: ‘Banares is older than history, older than tradition, older even than legend, and looks twice as old as all of them put together’. Each year a million pilgrims visit Varanasi to bathe in the holy river and pray in the hundreds of temples that cover the city. Part of what makes the city so special is the orientation of its sacred river as it flows past; it’s the only place where the Ganges turns around to the north, making it the one spot on the river where you can bathe while watching the Sun rise on the eastern shore. And sunrise over Varanasi is certainly one of Earth’s great sights. The humid, tropical air adds a soporific quality to the light, which in turn lends a fairy-tale quality to the brightly coloured buildings and palaces that line the holy river. It is a misty, pastel-shaded, dream-like experience, as though the city is materialising not from the dawn but from the past. But on 22 July 2009, at precisely 6.24am, a different type of pilgrim was to be found waiting beside the Ganges to witness one of the true wonders of the Solar System. At this time, across a small strip of the Earth’s surface, the longest total eclipse of the Sun since June 1991 was about to be visible to a lucky few. For three and a half minutes the Moon would cover the face of the Sun and plunge this ancient city into darkness. SOLAR ECLIPSES A total solar eclipse is possibly the most visual and visceral example of the structure and rhythm of our solar system. It is a very human experience, and one that lays bare the mechanics of this system. At the centre is the Sun, reigning over an empire of worlds that move like clockwork. Everything within its realm obeys the laws of celestial mechanics discovered by Sir Isaac Newton in the late seventeenth century. These laws allow us to predict exactly where every world will be for centuries to come. And wherever you happen to be, if there’s a moon between you and the Sun, there will be a solar eclipse at some point in time. Eclipses occur all over the Solar System; Jupiter, Saturn, Uranus and Neptune all have moons and so eclipses around these planets are a frequent occurrence. On Saturn, the moon Titan passes between the Sun and the ringed planet every fifteen years, while on the planetoid Pluto, eclipses with its large moon, Charon, occur in bursts every 12o years. But the king of eclipses is the gas giant Jupiter; with four large moons orbiting the planet, it’s common to see the shadow of moons such as Io, Ganymede and Europa moving across the Jovian cloud tops. Occasionally the eclipses can be even more spectacular. In spring 2004, the Hubble telescope took a rare picture (opposite top) in which you can see the shadows of three moons on Jupiter’s surface; three eclipses occurring simultaneously. Although this kind of event happens only once every few decades, the timing of Jupiter’s eclipses is as predictable as every other celestial event. For hundreds of years we’ve been able to look up at the night sky and know exactly what will happen when. Historically, this precise understanding of the motion of the Solar System provided the foundation upon which a much deeper understanding of the structure and workings of our universe rests. A wonderful example is the extraordinary calculation performed by the little-known Dutch astronomer Ole Romer in the 1670s. Romer was one of many astronomers who attempted to solve a puzzle that seemed to make no sense. The eclipses of the Galilean moons, Io, Europa, Ganymede and Callisto, by Jupiter were accurately predicted once their orbits had been plotted and understood. But it was soon observed that the moons vanished and reappeared behind Jupiter’s disc about twenty minutes later than expected when Jupiter was on the far side of the Sun (the accurate modern figure is seventeen minutes). When the predictions of a scientific theory disagree with evidence the theory must be modified or even rejected, unless an explanation can be found. Newton’s beautiful clockwork Solar System was on trail. Romer was the first to realise that this delay was not a glitch in the clockwork Solar System. Instead, it was caused because light takes time to travel from Jupiter to Earth. The eclipses of the Galilean moons happen just as Newton predicts, but we don’t see the eclipses on earth until slightly later than predicted when Jupiter moves further away from the Earth, simply because it takes more time for light to travel a greater distance. From this beautifully simple observation of the eclipses of Jupiter, fellow Dutch astronomer, Christian Huygens, was able to make the first calculation of the speed of light. The speed of light, we now know, is a fundamental property of our universe. It is one of the universal numbers that is unchanging and fixed throughout the cosmos. Ultimately, an understanding of its true significance had to wait until Einstein’s theory of space and time, Special Relativity, in 1905, but the long and winding road of discovery can be traced back to Romer and his eclipses. Closer to home, eclipses become even more familiar. In 2004, the Mars Exploration Rover Opportunity looked up from the surface of Mars and took possibly the most beautiful picture of any extraterrestrial eclipse (opposite bottom). In this remarkable image you can see Mars’ moon Phobos as it makes its way across the Sun – an image of a partial solar eclipse from the surface of another world. Eclipses on Mars are not only possible but commonplace, with hundreds occurring each year, but one event we will never see on Mars is a total solar eclipse. Here on Earth, though, humans have the best seat in the Solar System from which to enjoy the spectacle of a total eclipse of the Sun – all thanks to a wonderful quirk of fate. For a perfect total solar eclipse to occur, a moon must appear to be exactly the same size in the sky as the Sun. On every other planet in the Solar System the moons are the wrong size and the wrong distance from the Sun to create the perfect perspective of a total solar eclipse. However, here on Earth the heavens have arranged themselves in perfect order. The Sun is 400 times the diameter of the Moon and, by sheer coincidence, it’s also 400 times further away from the Earth. So when our moon passes in front of the Sun it can completely obscure it. With over 150 moons in the Solar System you might expect to find other total solar eclipses, but none produce such perfect eclipses as the Earth’s moon. It won’t last forever, though; The clockwork of the Solar System is such that the raising of the tides on Earth caused by the Moon has consequences. As the Earth spins beneath tidal bulges raised by the Moon, its rate of rotation is gradually, almost imperceptibly, reduced by friction, and this has the effect of causing the Moon to gradually drift further and further away from Earth. This complex dance, in precise accord with Newton’s laws, is also responsible for the fact that we only see one side of the Moon from the Earth – a phenomenon called spin-orbit locking. The drift is tiny, only around 4 centimetres (1.6 inches) per year, but over the vast expanses of geological time it all adds up. Around 65 million years ago the Moon was much closer to Earth and the dinosaurs would not have been able to see the perfect eclipses we see today. The Moon would have been closer to Earth and would therefore have completely blotted out the Sun with room to spare. In the future, as the Moon moves away from the Earth, the unique alignment will slowly begin to degrade; while drifting away from our planet the Moon will become smaller in the sky and eventually too small to cover the Sun. This accidental arrangement of the Solar System means that we are now living in exactly the right place and at exactly the right time to enjoy the most precious of astronomical events. IN THE REALM OF THE SUN Our closest star is the strangest, most alien place in the Solar System. It’s a place we can never hope to visit, but through space exploration and a few chance discoveries our generation is getting to know the Sun in exquisite new detail. For us it’s everything, and yet it’s just one ordinary star among 200 billion starry wonders that make up our galaxy. To explore the realm of our sun requires a journey of over thirteen billion kilometres; a journey that takes us from temperatures reaching fifteen million degrees Celsius, in the heart of our star, to the frozen edge of the Solar System where the Sun’s warmth has long disappeared. On 14 November 2003, three American scientists discovered a dwarf planet at the remotest frontier of the Solar System. Sedna is a planetoid three times more distant from the Sun than Neptune. Around 1,600 kilometres (1,000 miles) in diameter, Sedna is barely touched by the Sun’s warmth; its surface temperature never rises above minus -240 degrees Celsius. For most of its orbit Sedna is further from our star than any other known dwarf planet. On its slow journey around the Sun, one complete orbit – Sedna’s year – takes 12,000 Earth years. From its frozen surface at least thirteen billion kilometres from Earth, a view of the Sun rising on Sedna would give a very different perspective on our solar system and a clear depiction of how far the Sun’s realm stretches. Sunrise on Sedna is no more than the rising of a star in the night sky: from this frozen place, our blazing sun is just another star. To travel from the outer reaches of Sedna’s orbit to one of the first true planets of the Solar System we would need to cover over ten billion kilometres. Uranus was the first planet to be discovered with the use of a telescope, in 1781, by Sir William Herschel, and like all the giant planets (except Neptune) it is visible with the naked eye. Even so, sunrise on Uranus is barely perceptible; the Sun hangs in the sky 300 times smaller than it appears on Earth. Only when we have travelled the two and a half billion kilometres past Jupiter and Saturn do we arrive at the first world with a more familiar view of the Sun. Over 200 million kilometres out, sunset on Mars is a strangely familiar sight. On 19 May 2005, the Mars Exploration Rover Spirit captured this eerie view as the Sun sank below the rim of the Gusev crater. The panoramic mosaic image was taken by the rover at 6.07pm, on the rover’s 489th day of residency on the red planet. This sunset shot is not only beautiful, but it also tells us something fundamental about the Martian sky. Repeated observations have revealed that twilight on Mars is a rather long affair, lasting for up to two hours before sunrise and two hours after sunset. The reason for this long slow progression to and from darkness is the fine dust that is whipped up off the surface of Mars and lifted to incredibly high altitudes. At this height the Sun’s rays are scattered by the dust from the sunlit side of Mars around to the dark side, producing the long, leisurely and beautiful journey between day and night. Here on Earth, some of the longest and most spectacular sunrises and sunsets are produced by a similar mechanism, when tiny dust grains are catapulted high into the atmosphere by powerful volcanoes, scattering light into extra-colourful moments on our planet. Moving past Earth, which is 150 million kilometres out, we head to the heart of the Solar System. Mercury is the closest planet to the Sun, just forty-six million kilometres (twenty-nine million miles) away. It spins so slowly that sunrise to sunrise lasts for 176 Earth days. Beyond it there is nothing but the naked Sun, a colossal fiery sphere of tortured matter burning with a core temperature of about fifteen million degrees Celsius. The sheer scale of the Sun is difficult to conceive; at 40,000 kilometres (865,000 miles) across it is over 100 times the diameter of Earth, which means you could fill it with over a million Earths. Its mass is 2 x 10 kilograms – 330,000 times that of our planet. If you add up the masses of all the planets, dwarf planets, moons and asteroids, you would find they contribute less than half a per cent of the total mass of the Solar System. The Sun is dominant – the rest is an afterthought. Throughout human history, this majestic wonder has been a constant source of comfort, awe and worship, but our understanding of the Sun has developed slowly. For centuries, the finest minds in science struggled to understand how it created such a seemingly endless source of heat and energy. As recently as the nineteenth century science had little knowledge of what the Sun was made of, where it had come from, or the secret of its phenomenal power. THE ENERGY OF THE SUN In 1833, John Herschel, the most famous astronomer of his generation, travelled to the Cape of Good Hope in South Africa on an ambitious astronomical adventure to map the stars of the southern skies. This voyage was the end of an extraordinary odyssey for the Herschel family; he completed the work his father, William Herschel, had begun in the northern skies 50 years earlier. In 1838, Herschel attempted to answer one of the most fundamental questions we can ask about the Sun – how much energy does it produce? It may seem an incredibly ambitious calculation, but Herschel knew that to measure this ‘solar constant’ he would need nothing more than a thermometer, a tin of water, an umbrella and the predictable blue skies of Cape Town. When you want to measure the Sun’s radiation across billions of miles of space you need to start small. So Herschel began by asking how much energy the Sun delivers onto a small part of the Earth’s surface – in this case, onto a tin full of water. Herschel waited until December to conduct this experiment, when the Sun would be directly overhead, then placed his tin under the shade of the umbrella in the midday Sun. Once the water had heated up to ambient temperature he removed the shade to allow the Sun to shine directly onto the water. In direct sunlight, the water temperature begins to rise and by timing how long it takes the Sun to raise the water temperature by one degree Celsius, Herschel could calculate exactly how much energy the Sun delivered into the can of water. The calculation was simple because Herschel already knew something called the specific heat capacity of water – in modern units it is the amount of energy required to raise the temperature of 1 kilogramme of water by one Kelvin. Kelvin is a temperature scale usually favoured in science: 1 Kelvin = 1 degree Celsius, and -273 K = 0 degrees Celsius. (For the record, the specific heat capacity of water is 4187 Joules per kilogramme per Kelvin.) From this calculation it’s a small step to scale the number up and work out how much energy is delivered to a square metre of the surface of the Earth in one second. It turns out that on a clear day, when the Sun is vertically overhead, that number is about a kilowatt. That equates to ten 100-watt bulbs being powered by the Sun’s energy for every metre squared of the Earth’s surface. With this number Herschel could now take a leap of imagination and calculate the entire energy output of the Sun. He knew that the Earth is 150 million kilometres (93 million miles) away from the Sun, so he created an imaginary giant sphere around the Sun with a radius of 150 million kilometres. By adding up each of those kilowatts for every square metre of this entirely imaginary sphere, he was able to estimate the total energy output of the Sun per second. It’s a number that begins to reveal the sheer magnitude of our star. Every second the Sun produces 400 million million million million watts of power – that is a million times the power consumption of the United States every year – radiated in one second. It’s an ungraspable power, but a power that we have calculated using the very simplest of experiments and some water, a thermometer, a tin and an umbrella. Every second the Sun produces 400 million million million million watts of power – that is a million times the power consumption of the United States every year – radiated in one second. A STAR IS BORN It’s a wonder of the Sun that it has managed to keep up this phenomenal rate of energy production for millennia. Stars like the Sun are incredibly long-lived and stable – our best estimate for the age of the Universe is 13.7 billion years, and the Sun has been around for nearly five billion years of that, making it about a third of the age of the Universe itself. So what possible power source could allow the Sun to shine with such intensity day after day for five billion years? The best way to find the answer is to go back to the beginning, to a time when this corner of the galaxy was without light, and the Sun had yet to begin. The picture above shows the Milky Way. The dark areas with an absence of stars are called molecular clouds; clouds of molecular hydrogen and dust that are lying between us and the stars of the Milky Way galaxy. Taken by the Very Large Telescope (VLT) at Paranal Observatory, in Chile, this image is of Barnard 68, a molecular cloud well within our galaxy at a distance of about 410 light years. Take a close look, because you are looking at a future star, a cloud of dust and gas that in the next 100,000 years or so will collapse and begin its journey to becoming a new light in the heavens. Barnard 68, like all molecular clouds, contains the raw material from which stars are made, vast stellar nurseries that are among the coldest, most isolated places in the galaxy. This particular cloud is around half a light year across, or twenty million million kilometres, and has a mass that is about twice that of our sun. Most importantly, it is incredibly cold; in the heart of this cloud the temperature is no more than 4 Kelvin, that’s -269 degrees Celsius. That matters because temperature is a measure of how fast things are moving, so in these clouds the clumps of hydrogen and dust are moving very slowly. The stability of a cloud like Barnard 68 is in a fine balance. On one hand, the clumps of hygrogen and dust are moving around, which leads to an outward pressure that acts to expand the cloud. Counteracting this is the force of gravity – an attractive force between all the particles in the cloud that tries to collapse it inwards. In order for the cloud to become a star, gravity must gain the upper hand long enough to cause a dramatic collapse of the cloud. This can only happen if the particles are moving very slowly, i.e., if the temperature is low. Over millennia gravity’s weak influence dominates and the molecular clouds begin to collapse, forcing the hydrogen and dust together in ever-denser clumps. We have a name for clumps of gas and dust collapsing under their own gravity: stars. As the clouds collapse further and further they begin to heat up and eventually in their cores they become hot enough for the hydrogen to begin to fuse into helium. The stars ignite, the clouds are no longer black and the life cycle of a new star has begun. Five billion years ago a star was born that would come to be known as the Sun. Its birth reveals the secret of our star’s extraordinary resources of energy, because the Sun, like every other star, was set alight by the most powerful known force in the Universe. THE FORCES BEHIND THE SUN Nuclear fusion is the process by which all the chemical elements in the Universe, other than hydrogen, were produced. There are just three fundamental building blocks of matter required to make up everything we can see – from the most distant stars to the smallest piece of dust in our Solar System. Two kinds, the Up and Down quarks, make up the protons and neutrons in the atomic nuclei, and a third, the electrons, orbit around the nuclei to make atoms. These particles make up literally everything, including the book you are reading, the hand holding the book and the eyes reading the print. We live in a universe that is simple at heart! The Universe today is, of course, far from simple. It is a complex, beautiful and diverse place with stars, planets and humans. Nuclear fusion is one of the primary processes that built that complexity. The Universe began 13.7 billion years ago in the Big Bang. In the first instant it was unimaginably hot and dense, but it expanded and cooled very quickly. After just one second it was cold enough for the Up and Down quarks to stick together into protons and neutrons. The hydrogen nucleus is the simplest in nature, consisting of a single proton. Helium is the next simplest, built of two protons and one or two neutrons. Then comes lithium, beryllium, boron, carbon, nitrogen, oxygen and so on, each with one more proton and accompanying neutrons. This process of sticking more and more protons and neutrons together to form the chemical elements is known as nuclear fusion. The process of fusion is not easy. Protons carry positive electric charge, which means that they feel a powerful repulsive force when they get close to one another. The force that drives them apart is one of the four fundamental forces of nature: electromagnetism. If the protons can get close enough, another force – called the strong nuclear force – takes over. The strong force is aptly named (it is the strongest in the Universe) and can easily overcome the weaker electromagnetic repulsion. We don’t notice the strong force in everyday life because its effects are felt over a very short range and it stays trapped and hidden within the atomic nucleus. The way to get protons close enough for fusion to occur is to heat them up to very high temperatures. As I’ve explained before, this is because temperature is a measure of how fast things are moving around; if the protons approach each other at high speed they can overcome the electromagnetic repulsion and get close enough for the strong force to take over and bind them together. For the first few moments in the life of the Universe, all of space was filled with particles that were hot enough to smash together and fuse, but this only lasted a few brief minutes. Around ten minutes after the Big Bang, the Universe had cooled down enough for fusion to cease. At that time, our Cosmos was approximately 75 per cent hydrogen and 25 per cent helium, with very small traces of lithium. Fusion did not reappear in the Universe until the first stars were born, a few hundred million years later. The high temperatures inside stars like our Sun mean that the hydrogen nuclei in their cores are moving fast enough for the electromagnetic repulsion to be overcome and the strong nuclear force to take over, initiating nuclear fusion. The process is quite complex and involved, and very, very slow. First, two protons must approach each other to within a thousand million millionth’s of a metre (written as 10 m). Then something very rare must happen – a proton must change into a neutron. This happens through the action of the third of the four forces of nature: the Weak Nuclear Force. The Weak Force is, as its name suggests, unlikely to act: an average proton will live for billions of years in the Sun’s core before fusion begins. When this first step towards fusion finally occurs, a closely bound proton and neutron are formed. This nucleus is known as Deuterium. In the process, an anti-matter electron (known as a positron) and a sub-atomic particle called a neutrino are released. There is also an important extra ingredient, which is the key to understanding why stars shine. If you add up the mass of the Deuterium, the electron and the little neutrino, you find that it is slightly less than the mass of the original two protons. Mass is lost in the fusion process and turned into energy. This is an application of Einstein’s most famous equation: E=mc . This energy emerges from the Sun as sunshine – it is the primary power source for all life on Earth. The fusion process then proceeds much more quickly because the action of the Weak Nuclear Force is no longer required. The positron bumps into an electron and disappears in another flash of energy. A proton fuses with the Deuterium nucleus to make a form of helium known as helium 3 (two protons and one neutron), and then two helium 3 nuclei fuse together to form helium 4 – the end product of fusion in the Sun – releasing two protons. At each stage mass is converted to energy, keeping the Sun hot and shining brightly. At the end of their life, stars run out of hydrogen fuel in their cores and more complex fusion reactions occur. Heavier elements are produced – oxygen, carbon, nitrogen – the elements of life. Every element in the Universe today was fused together from the primordial hydrogen and helium left over from the Big Bang. THE POWER OF SUNLIGHT Once photons leave the Sun, the journey to Earth is a relatively short one. Light, like all forms of electromagnetic waves, travels at the same speed – almost 300 thousand kilometres a second, and so a photon leaving the surface of the Sun will reach the Earth in about eight minutes. Having travelled almost 150 million kilometres across space, each and every photon has a remarkable ability to shape and transform our planet. On the border of the Brazilian state of Paran? and the Argentine province of Misiones is the Igua?u river. Stretching for over one thousand kilometres, the Igua?u eventually flows into the Parana, one of the great rivers of the world. It’s these river systems that eventually drain all the rainfall from the southern Amazonian basin into the Atlantic. Billions of gallons of water flow through this river system each day and all of it, every molecule in the river, every molecule in every raindrop in every cloud, has been transported from the Pacific over the Andes and into the continental interior here by the energy carried in single photons from our sun. The Sun is the power that lifts all the water on the Blue Planet, shaping and carving our landscape and creating some of the most breathtaking sights on Earth. The Igua?u Falls are one of the most spectacular natural wonders on our planet. Almost three kilometres (two miles) long, comprising over 275 individual falls and reaching heights of over 76 metres (250 feet), a quarter of a million gallons of water flow through the Falls every second. The spectacular energy of these waterfalls is a wonderful example of how this planet is hardwired to the seemingly constant and unfailing power of the Sun. For centuries it was assumed that the Sun, like all the heavens, was perfect and unchanging, but gradually we’ve come to realise that the Sun is far more dynamic then just a perfect beautiful orb in the sky. Even tiny fluctuations in its brightness can have huge effects here on Earth. SUNSPOTS: THE SEASONS OF THE SUN As long ago as 28 BC, Chinese astronomers in the Central Asian deserts had observed dark spots on the surface of the Sun. When the wind blew enough sand into the air to filter the Sun’s glare they could see these strange spots and recorded them in the Chinese history book, The Book of Han. Over the next 1,500 years many other people recorded these strange dark spots on the surface of the Sun, but it wasn’t until the invention of the telescope that Galileo was able to correctly explain the phenomena of sunspots. The picture on the opposite page was taken by the SOHO probe – the Solar and Helical Observatory that was launched in December 1995. SOHO is giving us unprecedented detail on the life of our Sun and delivering the most beautiful and intricate images of our star that we’ve ever seen. In the picture above (also taken by the SOHO probe) you can see a beautiful example of the birth, life and death of a sunspot. It may look small compared to the size of the Sun, but the sunspot you are looking at is in fact bigger than the Earth. Sunspots are transient events on the surface of the Sun that are caused by intense magnetic activity that inhibits the flow of heat from deep within the Sun up to the surface. These spots appear dark because they are dramatically cooler than the surrounding area – often 2,000 degrees Celsius cooler. In the eighteenth century it was thought they might even be cool enough to allow humans to land on the surface of a sunspot, but at a toasty 3,000–4,500 degrees Celsius even these cool spots on the Sun would melt a spaceship instantaneously. Sunspots expand and contract as they move across the surface of the Sun, and they can be as large as 80,000 kilometres (50,000 miles) in diameter, making larger ones visible from Earth without a telescope. Advanced technology and space observation now allows us to track their numbers as they ebb and flow across the face of the Sun. Since sunspots are cooler areas than on the rest of the Sun, we might have expected to find that the power of the Sun dimmed when the sunspot activity was at its height. In fact, we’ve found the opposite to be true: the greater the number of sunspots, the more powerful our star becomes. This variation is not just random, as we have studied the Sun in greater and greater detail we have begun to observe patterns emerging; patterns that seem to have direct links to our climate back here on Earth. We’ve discovered that the Sun has seasons. THE SUN AND EARTH: SHARING A RHYTHM? For decades scientists have sought to understand how these subtle changes in the Sun’s power might be affecting the Earth. It’s a puzzle that led one man to look away from the Sun and focus instead on the rivers around the Igua?u Falls. Argentinean astrophysicist Pablo Mauas has spent the last decade analysing data that details every aspect of this river system – from water levels to flow rates – from 1904 all the way through the twentieth century. Unlike many of the world’s great rivers, the Parana is so large that it can be navigated by very big ships – and where there are ships there are records. These records enabled Pablo to uncover an extraordinary history and to reveal that, just like sunspots, the river too has a rhythm. Pablo and his team found that the stream flow of the river fluctuated dramatically three times during the last century, but the records gave no indication as to what was behind these fluctuations. The amount of water in the river Parana seems to follow a pattern, and Pablo had a hunch that this rhythm might be connected to the rhythms of the Sun. To try to link events on the Sun, 150 million kilometres away, to the flow of the great Parana, Pablo looked first at the most obvious rhythm of our star. We’ve known for over 150 years (since the German astronomer Heinrich Schwabe collated the data that stemmed back to Galileo’s earliest observations of sunspots) that the Sun follows a cycle that is repeated approximately every eleven years. This cycle reflects a rhythmic variation in the number of sunspots, which gives us a very clear indication of the amount of radiation given off by the Sun: the greater the number of sunspots, the greater the energy that is reaching the Earth. But when Pablo Mauas looked for a link between the Paranas rhythm and the eleven-year cycle, at first he found nothing. So instead he turned to calculations that described the Sun’s underlying brightness during the last century. We already know that climate change and events such as El Ni?o can boost the flow of the river, but when Pablo removed both of these effects from the data there appeared to be a strong relationship between the solar data and the stream flow. Superimpose the solar data on the water levels in the river (see below) and you see that when solar activity rises, the volume of water in the river goes up. There is a beautiful correlation between the flow in these rivers and the solar output. Pablo has revealed an amazing link across 150 million kilometres of space that may one day help us to not only better understand the impact of the Sun on our climate, but also to predict the likelihood of floods in the heavily populated waterways of one of South America’s greatest rivers. Changes in the Sun seem to move weather systems elsewhere, too. In India, the monsoon appears to follow a similar pattern to the river Parana, boosting precipitation when solar activity is at its greatest, whereas in the Sahara desert the opposite seems to occur: more solar activity means less rain. The exact mechanisms by which our star may affect Earth’s weather remain, for now, a mystery. We know that the energy production rate of the Sun – the power released in the fusion reactions at the core – is very constant, indeed. It doesn’t change, as far as we can tell, and so the changes that we see must be to do with the way in which the energy exits the Sun. And while the amount of radiation that falls onto the surface of the Earth is only at the tenths of a per cent level, it really does reveal the intimacy and delicacy of the connection between the Sun and the Earth. HOW TO CATCH A SUNBEAM We are tied to our star in the most intimate of ways. All the planets in our solar system are bathed in varying levels of sunlight, but only on one do we know of a phenomenon that does more than just passively receive the warmth of the Sun. Here on Earth, we actually feed on starlight. The Sun is the source of energy for almost all life on Earth; every plant, algae and many species of bacteria rely on the process of photosynthesis to create their own food using the power of the Sun. This in turn creates the foundations for the complex web of life here on Earth; not only does the process of photosynthesis maintain the normal level of oxygen in the atmosphere, but it is also the basis on which almost all life depends for its source of energy. We are only just beginning to understand the complex mechanisms by which plants capture sunlight; some of this explanation may take us off into the quantum world, but at its most basic chemical level photosynthesis is a simple process. Inside every leaf are millions of organelles called chloroplasts, and it’s these little units that do something magical when they capture a photon that has taken the eight-minute, 150-million-kilometre (93,000-million-mile) journey from the Sun. The chloroplasts take in carbon dioxide and water, and by capturing the energy from a sunbeam they convert this into oxygen and complex sugars. It’s these complex sugars, or carbohydrates, that are the basis of all the food we eat – whether directly through the consumption of photosynthetic plants or indirectly through the consumption of animals that feed on them. The amount of energy trapped by photosynthesis is immense – around 100 terawatt-years, which is six times larger than the power consumption of human civilisation. This intimate link between our planet and the Sun is all around us. Yet although we are surrounded by vast swathes of wonderful green machines that are all feeding on the Sun, the leaves and plants that cover so much of the planet do not just rely on any sunlight. In fact, plants are fussy eaters and have evolved to use just a fraction of the sunlight that makes its way through Earth’s atmosphere. On the surface of Earth sunlight may appear white, but when you pass it through a prism, you can see that it is made up of all the colours of the rainbow. Different wavelengths of light have different colours – from the blues with the shortest wavelength to the reds with the longest – but it’s not just their colour that distinguishes them. The prism reveals the recipe of light that is specific to our star; we see the red, green and blue photons that make up the sunlight all around us and each of these photons has very specific characteristics. The red photons don’t carry much energy but there are lots of them, whereas the blue photons, although there are fewer, carry a little more energy. Plants have evolved to gain the maximum energy most efficiently out of the recipe of light our star throws at us, so they don’t just use any photons for photosynthesis but only the ones from the red and blue bit of the spectrum. This intricate relationship between the evolution of plants and our star has had a profound effect on one of the defining features of our planet. When a red or blue photon hits a plant it is absorbed and so those wavelengths of light can no longer bounce back into your eye. Whereas when a green photon hits a plant it is not absorbed but reflected, so this wavelength of light bounces off a leaf and into your eye to create a living world that is defined by one colour more than any other: green. So the verdancy of the forests and jungles that cover our planet is all down to how plants have adapted to the quality of our star’s light. SOLAR ECLIPSE IN VARANASI Nothing prepares you for a total solar eclipse, and nothing prepares you for Varanasi. The old Solar City is never quiet and deserted; it is a little slice of ancient India and feels more hectic and vibrant even than the twenty-first-century version. But on the morning of 22 July 2009, the banks of the holy river were packed with people. There was no room, not a square centimetre of space, amongst a million sandaled feet crammed onto the Ghats. Green, yellow, red and orange saris and bronzed torsos bared to the early morning summer Sun formed a continuous bridge between the stepped shore and the heavily silted Himalayan waters of the Ganges. The ritualistic instinct to wash in the holy river powered a continuous convective flow of bodies down the concrete steps of the Ghats to the water’s edge – a circulating and impenetrable wall of humanity, simultaneously frenzied and calm. As I stood with them I marvelled at the patience of the Indian people – something British film crews dripping with tripods and flight cases will never be able to emulate. With immense difficulty, we had found a place to stand in a miraculously under-populated Ghat. We subsequently discovered why it wasn’t crowded – it was the public toilet. However, we decided that the unrivalled view of the rising Sun compensated for the smell and we settled down to drink water and wait. The moment of first contact came at 5.28am, when the limb of the Moon touched the solar disc. The Sun hovered over the river, partially obscured by low cloud, which dimmed the light and made the first moments of the eclipse easier to see. There was little change in the mood of the crowd because, unless you had special cardboard solar sunglasses, there was no perceptible reduction in the Sun’s power. Over the next thirty minutes the Moon’s disc quickly slipped across the face of the Sun and I became aware of a strange and unexpected feeling. The Moon moved quickly, and quite unlike the countless other nights I had stared up at its face, it was obviously in orbit – an inhabitant of space rather than a bright disc in the terrestrial sky. I developed a kind of vertigo, because the reality of the Moon as a ball of rock spinning quickly through space transferred to my own situation. I realised that I too was standing on a ball of rock. By 6.20am, almost an hour after first contact, totality approached. Very, very quickly, the morning light seemed to ebb away, as if time was flowing backwards. But this dimming was not like a sunset because it was so fast. It was not a fading of light; more of a removal. The sound of a million voices dimmed, too, but the Sun still hung as a fainter disc, seemingly unobscured to unshielded eyes. Then at 6.24am, instantly and with Newtonian precision, the Moon slotted into place in front of our star like a perfect Rolexian cog. And quite spontaneously, an enormous cheer erupted from the Ghats. I then had longer than any TV presenter will have this century to speak to camera about the eclipse. We had worked on words back in London, of course, because we knew this event would be one of the centrepieces of the series, but when the moment came all I could think of was the surprising vertiginous feeling. The red-blue sky of dawn had quickly faded to black as a dark rock swept across a glowing sphere of plasma on its orbital path, leaving me and a million other souls exposed on our own rock to the void. I glimpsed Pascal’s terror at the silence of the infinite spaces, turned to camera and said what I felt: ‘If you ever needed convincing that we live in a solar system, that we are on a ball of rock orbiting around the Sun with other balls of rock, then look at that. That’s the Solar System coming down and grabbing you by the throat.’. THE INVISIBLE SUN From 150 million kilometres away the Sun in our sky looks like a perfect disc. It is in fact closer to a near-perfect sphere than any planet or moon in the Solar System; it measures half a million kilometres across, but the variation in its breadth from top to bottom and side to side is little more than ten kilometres. However, this near perfection belies the incredible complexity of the structure. Its constituents are simple enough – to a very good approximation, our Sun is composed of the two simplest elements in the Universe – hydrogen and helium. THE STRUCTURE OF THE SUN’S ATMOSPHERE Hydrogen makes up about three-quarters of the mass of the Sun, with helium making up about a quarter. Less than 2 per cent consists of heavier elements such as iron, oxygen, carbon and neon. This spinning ball of the simplest elements is almost 330,000 times as massive as Earth. It is neither gas, liquid or solid, but a fourth state of matter known as a plasma. A plasma is a gas in which a large proportion of the atoms have had their orbiting electrons removed. This happens because the temperature is high enough to literally strip the atomic nuclei of their electrons. Plasmas are the most common state of matter in the Universe, and in fact we encounter them every day on Earth – fluorescent light bulbs are filled with glowing plasma when they are illuminated. Because plasmas contain a high proportion of naked, positively charged atomic nuclei and free negatively charged electrons, they are electrically conductive and so hugely responsive to magnetic fields. This gives the Sun a whole host of strange characteristics that are not found on any other body in the Solar System. It rotates faster at its equator than at its poles, with one rotation taking twenty-five days at the equator and over thirty days at the poles. One hundred and fifty times denser than water and reaching temperatures of up to fifteen million degrees Celsius, the core of the Sun is a baffling and bewildering structure. It is where the Sun’s fusion reactions occur, generating 99 per cent of its energy output. Around 600 million tonnes of hydrogen are fused together every second, creating 596 million tonnes of helium. The missing four million tonnes is converted into energy – as much as ninety billion megatons of exploding TNT – and this energy is transported to the surface by the high-energy photons or gamma rays released in the fusion reactions. The life of a newly created photon in the core of the Sun is a not a simple one, though. Most are quickly absorbed within a few millimetres of their point of creation by the dense plasma of the core, then they are re-emitted in random directions. In this way the journey of a gamma ray from the core of the Sun to its surface is like a very hot, very long and very unpredictable game of pinball; one that results in the release of millions of lower-energy photons at the Sun’s surface. All the light that reaches us here on Earth is incredibly ancient; it is estimated that a single photon can take anywhere from 10,000 to 170,000 years to make the journey from the Sun’s core to surface before it can make the eight-minute journey into our eyes. By the time a photon reaches the surface, or photosphere, the Sun’s temperature has dropped from thirteen million degrees Celsius to about 6,000 degrees. It’s this massive change in temperature that causes the vast convection currents that swirl through the Sun, creating thermal columns that carry hot material to the surface and create its characteristic granular apeparance we see from Earth. This is only just the beginning of the story of our sun’s mighty physical presence. Beyond the surface of the Sun is the strange and invisible layer known as the solar atmosphere. Only visible to the naked eye on Earth during a total solar eclipse, the Sun’s atmosphere is made up of a thin collection of electrically charged particles, protons and electrons. Unsurprisingly, the atmosphere of the Sun cools as you get further away from the surface. At a distance of 500 kilometres (310 miles) is an area known as the Temperature Minimum, which has a temperature of around 4,400 degrees Celsius. This location, as the name implies, is the coolest area of our star and the first place in which we can find simple molecules like water and carbon dioxide surviving in close proximity to the Sun. Beyond this region something odd happens. As you move further away from the Sun into space, the atmosphere doesn’t get cooler, it gets dramatically hotter. This outer region of the Sun’s atmosphere is known as the corona. This mysterious layer of the Sun only becomes visible to the naked eye during a total solar eclipse but when it is revealed you are seeing a structure that is larger and hotter then the Sun itself. With an average temperature of a million degrees Celsius and some areas reaching colossal temperatures of up to twenty million degrees Celsius, this vast cloud is, in places, hotter than the core of the Sun. The mechanisms that drive the corona to these high temperatures are not yet fully understood, but this effect is certainly due to the complex magnetic interactions that occur between the surface and the corona. What is known is that each and every day, at the very top of the atmosphere, some of the most energetic coronal particles are escaping. The Sun leaks nearly seven billion tonnes of corona every hour into space; a vast superheated supersonic collection of smashed atoms that en masse are known as the solar wind. This is the beginning of an epic journey that will see the Sun’s breath reach the furthest parts of the Solar System, creating the final vast structure of our star – the heliosphere. One hundred and fifty times denser then water and reaching temperatures of over 13 million degrees Celsius, the core of the Sun is a baffling and bewildering structure. THE HELIOSPHERE The heliosphere is a gigantic magnetic bubble in space that contains our solar system, the solar wind and the entire solar magnetic field. This bubble extends far into the Solar System, possibly even forty to fifty times further from the Sun than the Earth, and is shaped by the solar winds coming from the Sun. DEFENCE AGAINST THE FORCE OF THE SUN A stronomy has a long history of discoveries by amateurs. From Clyde Tombaugh, the man who discovered Pluto, to David Levy, the co-discoverer of the Shoemaker- Levy comet, the freedom of the skies has always tempted non-professionals to bypass the experts and break new ground. Amateur British astronomer Richard C. Carrington is a worthy member of this list. In 1858, Carrington made the first observation of an event that would eventually become known as a solar flare. This massive explosion in the Sun’s atmosphere releases a huge amount of energy and Carrington noticed that this event was followed by a geomagnetic storm, a massive disruption in the Earth’s magnetic field, the day after the eruption. Carrington was the first to suspect the two events may be connected. Beyond the weather in our swirling atmosphere, the solar wind creates another more tenuous atmosphere and weather system that surrounds our planet. We rarely notice this ethereal weather high above us, because by the time the solar wind reaches Earth it’s pretty diluted. If you went into space close to the Earth and held up your hand, you wouldn’t feel a thing. In fact, there are about five protons and five electrons for every sugar cube’s worth of space, but they’re travelling very fast, carrying a lot of energy – enough to severely damage our planet’s atmosphere, were it not for a defence system generated deep within the Earth’s core. On a beautiful sunny winter’s day in the Arctic, it’s hard to imagine that our star could be a threat. Yet, high above us deadly solar particles stream our way at speeds topping a million kilometres an hour and bombard the Earth. On a beautiful sunny winter’s day in the Arctic, it’s hard to imagine that our star could be a threat. Yet, high above us deadly solar particles stream our way at speeds topping a million kilometres an hour and bombard the Earth. Down here on the surface we’re protected from that intense solar wind by a natural shield that deflects most of it around us. To see that shield, you need nothing more than a compass. That’s because the Earth’s force field is magnetic, an invisible shell that surrounds the planet in a protective cocoon. The magnetic field emanates from deep within our planet’s spinning iron-rich core. It’s this gigantic force field, known as the magnetosphere, that deflects most of the lethal solar wind harmlessly away into space. However, the planet doesn’t escape completely; when the solar wind hits the Earth’s magnetic field, it distorts it. It stretches the field out on the night side of the planet and in some ways it’s like stretching a piece of elastic. More and more energy goes into the field and over time this energy builds up, stretching the tail until it can no longer hold on to it all. Eventually the energy is released, accelerating a stream of electrically charged particles down the magnetic field lines towards the poles. When these particles, energized by the solar wind, hit the Earth’s atmosphere, they create one of the most beautiful sights in nature: the aurora borealis, or Northern Lights. LIGHT FANTASTIC – THE AURORA BOREALIS The northern Norwegian city of Tromso is known as the gateway to the Arctic. At latitude seventy degrees North, deep inside the Arctic Circle, it has permanent sunlight from mid-May until the end of July, and permanent darkness from late November to mid-January. In late March the Arctic Ocean is a dark frosty blue, the white-crested waves matching the layers of snow and ice packed solid onto the wooden jetties and the well-weathered decks of the fishing boats. It was an utterly magical place to begin filming on 22 March 2009. We had gone to see the aurora borealis. Tromso is perfectly positioned on the auroral arc – the thin circle around the North Pole along which the elusive light show usually appears. March and September are the best months to see it, due to the alignment of the Earth’s magnetic field relative to the Sun, and we were told that, given clear skies, we would have a strong chance of glimpsing the Northern Lights. The Northern Lights reveal in exquisite beauty our planet’s connection with the rest of the Solar System. The Earth’s environment does not end at the edge of our atmosphere; it stretches at least to the Sun. Our guide told me of a Sami legend about the aurora. (The Sami are the people of the North, whose domain stretches from Tromso in the west, across northern Sweden and Finland and into Russia.) The legend has it that the aurorae are the spirits of women who died before they had children. Trapped between the frozen land and heaven, they are condemned to dance forever in the dark Arctic skies. As dusk fell, we rode snowmobiles out into the dense forests by the Fjord to get away from the city lights and settled down in the Sami camp with hot reindeer stew to wait. Just after midnight, the aurora came. I walked out into the frigid night air, enjoying the crunch of footsteps in fresh snow, and looked up. They came gently, a vague hint of green, but built quickly; sheets of colour drifted slowly then suddenly broke off and danced impossibly fast, a three-dimensional rain of light rising and falling between land and sky. They were mostly green, with hints of orange and red close to the horizon. They were like nothing I have ever seen, and as I turned to camera I realised that I didn’t care about the physics of what I was seeing. My reaction, composed whilst sitting at my desk in Manchester, was worthless in the face of Nature at its most magnificent. The Sami had it right – an aurora isn’t the light shaken out of atoms of nitrogen and oxygen as they are bombarded by high-energy particles from Earth’s ionosphere accelerated down magnetic field lines towards the poles, it is made of majestic, mournful, dancing spirits, trapped in the Arctic night. The Northern Lights reveal in exquisite beauty our planet’s connection with the rest of the Solar System. The Earth’s environment does not end at the edge of our atmosphere; it stretches at least to the Sun. We are bound to our star by the visible light that creates and nurtures life on Earth and the unseen, constant solar wind that only appears to us at night in special circumstances. Each and every planet in the Solar System shares this connection, and the same laws of physics apply. As the solar wind races out into the Solar System, wherever it meets a planet with a magnetosphere aurorae spring up. Jupiter’s magnetic field is the largest and most powerful in the Solar System, and the Hubble space telescope reveals that there are permanent aurorae over the Jovian poles. Jupiter’s moons, Io, Europa and Ganymede also have aurorae, created by Jupiter’s atmospheric wind interacting with the moons’ atmospheres. Saturn too puts on an impressive display, with aurorae at both its poles, but because Saturn’s magnetic field is uneven, the aurorae are smaller and more intense in the north. As the solar wind reaches the edge of the heliosphere it begins to run out of steam. Incredibly, there is a probe out there that will discover where these solar winds end. VOYAGERS’ GRAND TOUR In the autumn of 1977, a pair of identical 722-kilogramme (1,592-pound) spacecraft were launched from Cape Canaveral, Florida. Voyagers 1 and 2 were about to embark on a very special mission: to visit all four of the Solar System’s gas giants – Jupiter, Saturn, Uranus and Neptune. Normally such a journey would take thirty years to complete, but by a stroke of good fortune these spacecraft were designed at a time when the planets were uniquely aligned, allowing the probes to complete their grand tour in less than twelve years. Today, over thirty years after their launch, both spacecraft are alive and well, and remarkably Voyager 1 is still reporting back to Earth – the ultimate and most wonderful example of mission creep in the history of space exploration. Voyager 1 is currently the furthest man-made object from Earth. Travelling at seventeen kilometres (eleven miles) per second, this extraordinary spacecraft is just over seventeen billion kilometres (eleven billion miles) from home and delivering knowledge that it was never designed or expected to uncover. Listening to Voyager 1 is the sensitive ear of the Goldstone Mars station in the Mojave desert, California; one of the few telescopes in the world that is capable of communicating over such vast distances. Voyager is so far away that it takes the signal around fifteen hours to arrive, travelling at the speed of light. It may appear as little more than a blip on a screen, but the information Voyager is sending is providing the first data from the frontier of our solar system, from the edge of the heliosphere, and constantly measuring the solar wind as it fades away. Voyager 1 has now reached the point where this wind that emanated so powerfully from the surface of the Sun has literally run out of steam. The heliopause is the boundary at which the solar wind is no longer strong enough to push against the stellar winds of the surrounding stars. Beyond this point Voyager will leave its home and head off into interstellar space. With the batteries expected to struggle on until 2025, this spacecraft will continue to feed us data as it becomes the first man-made object to leave our solar system. FROM EARTH TO THE OORT CLOUD Our journey through the Sun’s Empire doesn’t end at this distant frontier, seventeen billion kilometres (eleven billion miles) away, where the solar wind meets the interstellar wind. The Sun has a final, invisible force that reaches out much further. Our star is by far the largest wonder in the Solar System. In fact, it alone makes up 99 per cent of the Solar System’s mass. It is this immensity that gives the Sun its furthest reaching influence – gravity. This is the full extent of the Sun’s empire; the lightest gravitational touch that retains a cloud of ice that encloses the Sun in a colossal sphere. Beyond this Oort cloud there is nothing. Only sunlight escapes; light that will take four years to reach even the Sun’s closest neighbour, Proxima Centauri – a red dwarf star among the 200 billion others that make up the Milky Way. And it’s by looking here, deep into our local galactic neighbourhood, that we’re learning to read the story of our own star’s ultimate fate. INVESTIGATING THE FUTURE OF OUR SUN The Sun’s empire is so vast and so ancient, and its power so immense, that it seems an audacious thought to imagine that we could even begin to comprehend its end – the death of our sun. However, that is exactly what astronomers are trying to do, and many of them head to the most arid and barren desert on Earth, the Atacama, in Chile, looking for answers. There, high up on an the sides of an extinct volcano at an altitude of 2,635 metres (8,643 feet), sits Paranal Observatory, home to the world’s most powerful array of telescopes. On arrival we were given ‘important information for a safe stay on Paranal’. As the observatory is about two and a half kilometres (one and a half miles) in the air, we were advised that if we experienced any of the following, we should consult a paramedic immediately: headache and dizziness, breathing problems, ringing or blocking of the ears, or seeing stars. It honestly said that if you saw stars at the Paranal Observatory you should consult a paramedic immediately! Perched high above the clouds is the reason why so many astronomers venture to this desert. Here, four colossal instruments make up the European Southern Observatory’s ‘Very Large Telescope’, or VLT. If you look up at the sky with these mighty machines you quickly notice that the stars are not just white points of light against the blackness of the sky, but are actually coloured. Through these lenses, orangey-red, yellow and bluey-white stars fill the clear Chilean sky. However, this beauty is not just one of the wonders of our night sky, it has also revealed something much deeper. To gaze upon the galaxy full of stars is to observe them at all the stages of their lives – from youthful bright stars to middle-aged yellow stars very similar to the Sun. Contained within the night sky we can see a colour code that allows us to plot the life cycle of every star, including our own. If you look up at the sky with these mighty machines you quickly notice that the stars are not just white points of light against the blackness of the sky, but are actually coloured. Through these lenses, orangey-red, yellow and bluey-white stars fill the clear Chilean sky. THE HERTZSPRUNG-RUSSELL DIAGRAM For the last 100 years astronomers have meticulously charted the nearest ten thousand stars to Earth and arranged each according to its colour and brightness. From this was born the Hertzsprung-Russell diagram; a powerful and elegant tool that allows astronomers to predict the history and evolution of stars, and in particular the future life of our sun. Most of the stars, including our own, are found in the ‘main sequence’ – the band of stars that runs from the top left to the bottom right. The Sun will spend most of its life there, steadily burning its vast reserves of hydrogen fuel, which will last for another five billion years. After which, it will pass through a Red Giant phase. THE DEATH OF THE SUN Eventually, like all stars, the Sun’s fuel will run out, its core will collapse and our star will begin its final journey. At this stage you may expect it to slowly burn out and splutter its way into oblivion, but there is a final, remarkable twist to our Sun’s ten-billion-year story. When the fuel does finally run out, the nuclear fusion reactions in the Sun’s core will grind to a halt and gravity will be master of our star’s fate once more. The Sun will no longer be able to support its own weight and it will begin to collapse. Just as in its formation, this collapse will start to heat the Sun once more, until the layers of plasma outside the core become hot enough for fusion to begin again – but this time on a much bigger scale. Our star’s brightness will increase by a factor of a thousand or more, causing it to swell to many times its current size. The Sun will then drift off the main sequence and into the top right-hand side of the Hertzsprung-Russell diagram, into the area known as the Giant Branch. As the outer layers expand, the temperature of the surface will fall and its colour will shift towards red. Mercury will be little more than a memory as it is engulfed by the expanding red sun, which will grow to two hundred times its size today. As it swells the Sun will stretch all the way out to the Earth’s orbit, where our own planet’s prospects are dim. So it seems that the wonder that has remained so constant throughout all of its ten billion years of life will end its days as a giant red star. For a few brief instants the Sun will be two thousand times as bright as it is now, but that won’t last long. Eventually our star will shed its outer layers and all that will be left will be its cooling core – a faint cinder, or White Dwarf, that will glow pretty much to the end of time, fading slowly into the interstellar night. As it does so, all its wonders – the aurorae that danced through the atmospheres of planets of the Solar System, and its light that sustains all the life here on Earth – will be gone. The gas and dust of the dying Sun will drift off into space, and in time they will form a vast dark cloud primed and full of possibilities. Then, one day, another star will be born, perhaps with a similar story to tell, the greatest story of the cosmos. CHAPTER 3 THE CLOCKWORK SOLAR SYSTEM The story of the Solar System is the story of the emergence of order out of chaos, guided by the simplest law of physics: gravity. The planets and their moons exist in relatively stable orbits because of a delicate interplay between gravity and angular momentum, and this beautiful natural balance is written before our eyes in the spinning patterns and rhythms of the heavens. In the small ancient city of Kairouan on the north-eastern plains of Tunisia lies the fourth most holy site in the Islamic world. Founded by Arabs in 670 CE, this city of just 150,000 people is home to the oldest place of Muslim worship in the Western world. The great mosque of Kairouan is both impressive in its beauty and also in its scale. Covering over 9,000 square metres (97,000 square feet), the Mosque resembles a great fortress as well as a place of worship. At its heart is a vast courtyard and near the centre is a beautiful piece of astronomical engineering – an ancient sundial. Humans have used sundials like this one to follow the brightest star in the heavens for over 5,500 years. For the last fourteen centuries, the sundial at the centre of this great mosque has measured the relentless passing of the days, marking out the passage of time as the Sun travels across the sky, plotting the call to prayers before dawn, at sunrise, at noon, at sunset and in the evening. The sundial is a beautifully simple piece of technology. Originally nothing more complicated than a stick in the ground or the length of a human shadow, sundials have enabled us to measure time by following the movement of the Sun across our sky. For thousands of years this movement appeared to confirm the Earth’s position at the centre of the Universe. From the most simple of observations it seemed to make perfect sense that the Sun orbits the Earth every 23 hours and 56 minutes. Yet the simple, regular rhythm that each one of us witnesses every day is nothing more than an illusion. It is not the Sun that’s moving, what we are observing is the rotation of the Earth as it travels through space. It’s wonderful to think that across the planet the rhythms of our lives are governed by our journey through space. From waking up to going to bed, eating strawberries in July or a tangerine in December. Travelling at 108,000 kilometres an hour, on a 900-million-kilometre journey around the Sun, our planet completes this epic journey once every 365.25 days. The year in the life of our planet is just one of the endless rhythms by which we live our life – and all of these are governed by the seemingly clockwork motion of our planet. It carries us through cycles of night and day as it turns on its axis, rotating at 1,700 kilometres an hour every twenty-four hours. The length of the day at a particular place on the Earth’s surface is dictated by the precise angle of our planet in relation to the Sun. We have seasons here, too, due to the fact that the Earth’s axis is tilted by twenty-three degrees. As we journey around the Sun this angle creates the changing dynamic that defines the cycles of many of the creatures that live both on the land and in the oceans. In the Northern Hemisphere the summer months coincide with the North Pole leaning towards the Sun; when, at this time of year, the angle favours the northern half of our planet with extra energy from our star. By winter the dynamic has changed; the North Pole is pointing away from the Sun and the Southern Hemisphere is bathed in additional sunlight. It’s wonderful to think that across the planet the rhythms of our lives are governed by our journey through space. From waking up to going to bed, wearing a jumper one month and a T-shirt the next, eating strawberries in July or a tangerine in December, each of these everyday events is intimately connected to a journey through space that catapults us at 108,000 kilometres (67,000 miles) per hour around a star, but leaves most of us completely unaware of this rollercoaster ride through the cosmos. It’s not only Earth that is subject to these rhythms – the whole Solar System is full of these cycles, with each planet orbiting the Sun at its own distinct tempo. Mercury is the fastest; closest to the Sun, it reaches speeds of 200,000 kilometres (124,000 miles) per hour, completing its orbit in just eighty-eight days. Venus rotates so slowly that it takes longer to spin on its axis (225 days) than it does to go around the Sun, so that on Venus (and also on Mercury) a day is longer than a year. Further out, the planets orbit more and more slowly. Mars completes one orbit of the Sun every 687 days, just a couple of months short of two Earth years. Jupiter, the largest planet, takes twelve Earth years to complete each orbit, Saturn almost thirty years, Uranus eighty-four years and at the very furthest reaches of the Solar System, four and a half billion kilometres from the Sun, Neptune travels so slowly that by 2009 it hadn’t completed a single orbit since it was discovered in 1846. 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