Wonders of the Universe

by Professor Brian Cox

  The building itself was a gutted husk, a brick skeleton; all the windows, if it ever had them, were gone. The cells were dormitories of twenty or thirty concrete bunk beds in close rows. Each had a single tiny bathroom, some with ragged pieces of cloth still draped across the entrance, paying lip service to privacy. The walls of the cells were a grotesque patchwork of ripped colour, papered with glamour girls mixed with the odd football team. I found it disturbing for two reasons. First, you can’t stop wondering about incarceration there; the centre of a hot, humid city like Rio is not the place to spend years inside a steel and concrete cage. The second was less cerebral: the prison was wired with live explosives. From inside the shell the bright outside pressed and glowed like a stellar surface, impossible to view against the internal black. The light won’t come in. It stays outside in the city. I could feel the analogy as I descended down holed, cement-dusted precarious stairwells into the dense heart of the dying star. It is here, inside a violent, condemned structure, far from the light of the surface, that the elements of living things are meticulously assembled. In here, the star transforms from matter consumer to matter producer.

  Stars exist in an uneasy equilibrium. Their gravity acts to compress them, which heats them up until the electromagnetic repulsion between the hydrogen atoms is overcome and they fuse together to make helium. This releases energy, which keeps the star up. When the hydrogen runs out, the outward pressure disappears; gravity regains the upper hand and the structure of the star changes dramatically. The core collapses rapidly, leaving a shell of hydrogen and helium behind. Within the shrinking core the temperature rises until, at 100 million degrees Celsius, a new fusion process is triggered. At these temperatures helium nuclei can overcome their mutual electromagnetic repulsion and wander close enough together to fuse – the star begins to burn helium. This transfer from hydrogen to helium fusion has two profound effects: firstly, sufficient energy is released to halt the stellar collapse, so the star stabilises and rapidly swells. This is the beginning of its life as a red giant. Secondly, it fuses into existence the element vital for life. At first sight the fusion of two helium nuclei, each consisting of two protons and two neutrons, should only be able to produce the isotope beryllium-8, composed of four protons and four neutrons. This is an unstable isotope of beryllium that quickly breaks down, but in the intense temperatures of a dying star, as the core exceeds 100 million Kelvin, these nuclei live just long enough to fuse with a third helium nucleus, creating the precious element carbon-12. This is where all the carbon in the Universe comes from; every carbon atom in every living thing on the planet was produced in the heart of a dying star.

  Just as in a dying star, the structure of a building and the elements that keep it standing become unstable over time. This prison was given a helping hand to its destruction, but a dying star will detonate itself as it reaches the end of its life, producing spectacular planetary nebulae. It took seconds to demolish the prison block, which is the same length of time it takes for a red giant star to collapse.

  The helium-burning phase doesn’t end with the alchemic synthesis of carbon, because during the same intensely hot phase in the star’s life the conditions allow a nucleus of helium to latch onto a newly minted carbon nucleus to create another element vital for life. Oxygen makes up 21 per cent of the air we breathe, is a prerequisite for water, the solvent of life, and is the third-most common element in the Universe after hydrogen and helium. As you breathe in around two and a half grams of oxygen each minute, it’s worth remembering that all this life-giving gas was created in an environment as far away from our understanding of what is habitable as you can get.

  Compared with the lifetime of a star, this stellar production line of carbon and oxygen is over in the blink of an eye. Within about a million years the helium supply in the core is used up, and for many stars that’s where fusion stops. Any average-sized star, like our sun, has by now reached the end of its productive life. When our sun reaches this stage, in about ten billion years’ time, there won’t be enough gravitational energy to compress the core any further and restart fusion. Instead, the star becomes more and more unstable, huge pressure points will build up, until eventually the whole stellar atmosphere explodes, hurling the precious cargo of oxygen, carbon, hydrogen, and all, on its journey into space. For at this brief moment in time, no more than a few tens of thousands of years, a dying star will create one of the most beautiful structures in our universe: a planetary nebula.

  Once this brief cosmic light show is over, an average-sized star will shrink to an object no bigger than Earth. A white dwarf is the fate of such stars and billions like it, but for massive stars like Betelgeuse the action is far from over. If a star has a mass half as big again as our Sun, it will continue down the chemical production line. As helium fusion slowly comes to an end, gravity takes over and the collapse of the core restarts. The temperature rises, launching the third stage in the birth of our universe’s elements, and with temperatures reaching hundreds of millions of Kelvin, carbon fuses with helium to make neon, neon fuses with more helium to make magnesium, and two carbon atoms fuse to make sodium. With more and more elemental ingredients entering the cooking pot, and temperatures rising, the heavier elements are produced one after another. The core continues to collapse, the temperature continues to rise, and the next stage of fusion begins, leaving layers of newly minted elements behind.

  With the first twenty-five elements now created within the star, the runaway production line hits a block at the twenty-sixth element, iron, created from a complex cascade of fusion reactions fuelled by silicon. At this stage the temperature of the star is at least 2.5 billion Kelvin, but it has nowhere else to go. The peak of nuclear stability has been reached, and no more energy can be released by adding more protons or neutrons to iron. The final stage of iron production lasts only a couple of days, transforming the heart of the star into almost pure iron in a desperate bid to release every last gasp of nuclear binding energy and stave off gravity. This is where the fusion process stops; once the star’s core has been fused into iron, it has only seconds left to live. Gravity must now win, and the star collapses under its own weight forming a planetary nebula.

  As I walked away from the prison for the cameras, a button was pressed and the building fell. The demolition took seconds – the same time it takes a red giant star like Betelgeuse to collapse

  PLANETARY NEBULAE

  This dying star, IC 4406, like many planetary nebulae, is highly symmetrical. It is known as the ‘Retina Nebula’ because the tendrils of dust emitted from it that have been compared to the eye’s retina.

  NASA

  About 5,000 light years (4,700 trillion kilometres/2,900 trillion miles) from Earth lies the Calabash Nebula. This image, captured by the Hubble Space Telescope, shows material being ejected from the star.

  The Eskimo Nebula is so-called because of its resemblance to a head surrounded by fur-lined hood when viewed from Earth. It was discovered in 1787 by astronomer William Herschel.

  This composite image depicts the Helix Nebula. This planetary nebula resembles a doughnut, as seen from Earth, but new evidence suggests that the Helix in fact consists of two gaseous discs.

  MyCn18 is a young planetary nebula which was discovered in the early twentieth century. However, it was this Hubble Telescope image in January 1996 that revealed the nebula’s hourglass shape with intricate engravings.

  NASA

  The aptly named Cat’s Eye Nebula (officially known as NGC 6543) was one of the first planetary nebulae to be discovered (in 1786 by William Herschel). It is one of the most complex nebulae known to exist in the Universe.

  Imaged on 20 July 1997, Mz3 has been dubbed the Ant Nebula because its outline resembles the head and thorax of an ant when seen through telescopes on Earth. On close inspection, the ant’s body appears to consist of two fiery lobes.

  This planetary nebula is known as Kohoutek 4-55 (or K 4-55), named after its discoverer, Czech astronomer Lubos Koho
utek. It is unusual for its multi-shell structure.

  THE RAREST OF ALL

  Once the centre of the great American gold rush, the 16-1 mine is one of the few gold mines still operating in the state of California today. Digging for gold there with the miners was an enlightening experience. As I peered at seemingly ordinary rocks, I could see glints and glimmers of a familiar yellow colouring, revealing the stones’ precious hidden cargo of gold.

  The first twenty-six of the elements are forged in the cores of stars and are distributed through the Universe in their inevitable collapse. But what of the other seventy-two – some of which are vital for life, and many of which we hold most precious? If they are not formed within stellar furnaces, what could their origin possibly be?

  In the remote forests of northwestern California, the mountains still hide a secret that made the quiet pine woods the ultimate destination for fortune seekers only a century ago. Although they’re empty today, in the late nineteenth century this was the centre of the California gold rush. Hundreds of thousands of people arrived here, trying anything and everything to get rich, from simple panning to the most advanced mining techniques available. Gold worth billions of dollars was extracted, fuelling the rise of one of the world’s great cities, San Francisco. The insatiable appetite for gold has waned today, but in the forests around Lake Tahoe, the 16–1 mine remains one of the few gold mines still operating in the state of California.

  For almost 100 years, miners have been digging for gold in the 16–1, and it is still one of the richest gold deposits in the world, due to a quirk in the local geology. The unique thing about California is that it sits on the divide between the North American tectonic plate and the Pacific tectonic plate. The whole region is one enormous fault line, with thousands of smaller faults running through the rocks of the mountains. When you travel into the mine, which is nothing more than a series of horizontal tunnels at gentle gradients hollowed out of the mountainside, you can see these fault lines everywhere; they reveal their presence as visible boundaries between rock and quartz – a maze of mini-faults. One hundred and forty million years ago, in the Jurassic period when the dinosaurs were running around above the mine, hot water bubbled up and flowed through this rock, carrying a precious cargo. Its water was laden with gold brought up from deep within the Earth, deposited through the seams of quartz. For the last 100 years all the miners have had to do is to follow quartz seams laced with shimmering gold.

  The gold that runs all the way through the quartz in the 16–1 mine is unusually pure, at anything up to 85 per cent, and the thick tendrils snaking through the rock glint and glimmer that familiar yellow in the sunlight. The rest is about 14.5 per cent silver, with traces of heavier metals. The area is so rich in gold that it can even be found as simple pure nuggets that can be picked up off river beds, and at the 2010 price of around £900 per troy ounce, it’s obvious why mines like this are still in operation.

  If you stop to think about it though, there’s something a bit odd about the value we attach to gold. Throughout history people have gone to extraordinary lengths to get their hands on it, which is odd because it isn’t particularly useful for anything. Copper and iron will help you survive, but gold is next to useless. Most of the gold that we’ve struggled to extract has ended up as jewellery. The only thing that gold has going for it, other than being shiny, is that it is incredibly rare, and this is what drives up its price. All the gold dug out of the ground throughout all of human history – with all the associated tragedy and elation, hardship and riches – would just about fill three Olympic-sized swimming pools.

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  All the gold dug out of the ground throughout all of human history would just about fill three Olympic-sized swimming pools. It is this almost vanishing scarcity that makes gold so valuable.

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  It is this almost vanishing scarcity (three swimming pools relative to the size of a planet) that makes gold so valuable; it is just one of many rare elements that are to be found in the most minute of traces within the Earth.

  There are over sixty elements heavier than iron in the Universe, some are valuable, such as gold, silver and platinum; some are vital for life, such as copper and zinc; and some are just useful, such as uranium, tin and lead. Very massive stars can produce very tiny amounts of the heavier elements up to bismuth-209 (element number 89) in their cores by a process called neutron capture, but it is known that this makes nowhere near enough to account for the abundances we observe today. There simply haven’t been enough massive stars in the Universe.

  The conditions necessary to produce large amounts of the elements beyond iron are only found in the most rare of all celestial events. Blink and you’ll miss them, because in a galaxy of 100 billion stars the conditions violent enough to form substantial amounts of these elements will exist on average for less than two minutes in every century

  SUPERNOVA: LIFE CYCLE OF A STAR

  All stars are born from clouds of gas, but the length of their life and their eventual fate are governed by their mass (i.e. how much gas they contain). Stars dozens of times heavier than the Sun live for only a few million years before swelling into supergiants and exploding as supernovae (top row). However, stars like the Sun live longer and die more gently, shining steadily for billions of years before swelling into red giants and losing their outer layers as a planetary nebula (middle row). The core of the star, exposed as a white dwarf, then continues to glow for billions of years more before gradually fading out. The least massive stars, the red dwarfs (bottom), simply fade out over tens of billions of years.

  Nathalie Lees © HarperCollins

  THE BEGINNING AND THE END

  This computer-generated sequence of images shows what will happen when Betelgeuse goes supernova. Deep in the heart of the star, the core will succumb to gravity and fall in on itself, then rebound with colossal force. The blast wave emitted generates the highest temperatures in the Universe. Over millions of years the scattered elements of the exploded star will become a nebula, at the heart of which is a super-dense core that is Betelgeuse the neutron star.

  After a few million years of life, the destiny of the largest stars in our universe is a dramatic one. Having run out of hydrogen and burnt through the elements all the way to iron, giant stars teeter on the edge of collapse. Yet even in this dilapidated state these stars have one last violent act, and it is a generous one. It occurs with such intensity that it allows for the creation of the heavy elements.

  If we could gaze deep into the heart of one of these dying giants, we would see the core finally succumb to gravity. As fusion grinds to a halt, this giant ball of iron falls in on itself with enormous speed, contracting at up to a quarter of the speed of light. This dramatic collapse causes a rapid increase in temperature and density as the core shrinks to a fraction of its original size. The inner core may eventually shrink to 30 kilometres (19 miles) in diameter. At this point, with temperatures nearing 100 billion Kelvin and densities comparable to those inside an atomic nucleus, quantum mechanics steps in to abruptly halt the collapse. By now most of the electrons and protons in the core have been literally forced to merge together into neutrons. Neutrons, in common with protons and electrons, obey something called the Pauli exclusion principle, which effectively prevents them from getting too close to one another (in more technical terms, no two neutrons can be in the same quantum state). This has the effect of making a ball of neutrons the most rigid material in the Universe – 100 million million million times as hard as a diamond. When the neutrons can be compressed no more, the contraction must stop and all the superheated collapsing matter rebounds with colossal force. A shockwave shoots out through the star and as this blast wave runs into the outer layers of the star it generates the highest temperatures in the Universe – 100 billion degrees. The precise mechanism for this rapid heating is not fully understood, but it is known that for a matter of seconds the conditions are intense enough to form all the heaviest elements we see in our u
niverse, from gold to plutonium. This is a Type II supernova – the most powerful explosion we know of.

  Supernovae are so rare that since the birth of modern science we have never had the chance to see one close up. The last supernova explosion seen from Earth in our galaxy was in 1604, a few years before the invention of the astronomical telescope. On average, it is expected there should be around one supernova explosion in the Milky Way per century, but for the last 400 years we’ve had no luck. It’s long overdue and astronomers are always searching the skies for stars which they think might be the most likely candidate to go supernova.

  One of the prime candidates is Orion’s shining red jewel, Betelgeuse. With so many telescopes trained on this nearby star, we have been able to follow its every move for decades. Charting its brightness, we have discovered that it is extremely unstable; it has dimmed by about 15 per cent in the past decade. As supernova candidates go, Betelgeuse is top of the list. It is generally thought that Betelgeuse could go supernova at any time. It is a relatively young star, perhaps only ten million years old, and has sped through its life cycle so rapidly because it is so massive. However, when you’re ten million years old, the end of your life can be quite drawn out and a phrase like ‘any time soon’ in stellar terms is not quite what you might expect. It means that Betelgeuse should go supernova at some point in the next million years, but equally it could explode tomorrow. What we do know is that when it does go it will provide us with quite a show. Betelgeuse is only 500 light years away, almost uncomfortably close, which means that the explosion will be incredibly bright. It will be by far the brightest star in the sky and it may even shine as brightly as a full moon at night and fill the sky as a second sun during the day.