Wonders of the Universe

by Professor Brian Cox

  We take for granted the sight of the Sun rising and setting on our horizon, but we now know its presence is not eternal.

  Today, 13.7 billion years after the Universe began, we are living through the most productive era that our universe will ever know. The Stelliferous Era is a time of life and death, with the constant dance between gravity and nuclear fusion creating a dynamic, ever-changing landscape in the heavens. For a human being, for whom a century is a lifetime, the changes may appear slow, but be in no doubt that you are part of the Universe at its most vibrant. As we’ve watched the stories of stars like GRB 090423 play themselves out in the night sky, we have seen at first hand that no star can last forever. Every one of those brightly burning lights has a destiny as defined and as certain as our own, and this of course includes the star at the centre of our solar system.

  The Sun was formed 4.57 billion years ago from a collapsing cloud of hydrogen and helium and a sprinkling of heavier elements. For the tiniest fraction of this time, humans have marked the passing of the days as it rose and set, and surely considered it to be an eternal presence. It was only during the twentieth century that we discovered the Sun’s fires must one day dim

  The computer-generated image shows how dramatically different the Sun will look in our heavens as it dies and dims.

  THE DEMISE OF OUR UNIVERSE

  At the moment the Sun is in the middle of its life, fusing hydrogen into helium at a rate of around 600 million tonnes every second. It will continue to do this for another five billion years; but eventually, perhaps fittingly given the grandeur and beauty it has nurtured in its empire, it won’t simply fade away. As the stores of hydrogen run dry, the Sun’s core will collapse and momentarily, as helium begins to fuse into oxygen and carbon, a last release of energy will cause its outer layers to expand. Imperceptibly at first, the extra heat of the Sun will extend towards us as its diameter increases by around 250 times. The fiery surface of our star will move beyond Mercury, towards Venus and onwards to our fragile world.

  The effects on our planet will be as catastrophic as they are certain. Gradually, the Earth will become hotter. In the distant future, if any of our descendants still remain, someone will experience the last perfect day on Earth. As the surface of the Sun encroaches, our oceans will boil away, the molecules in our atmosphere will be agitated off into space, and the memory of life on Earth will fade into someone’s history; or perhaps no one’s history if we have steadfastly remained at home.

  Long after life has disappeared, the Sun will fill the horizon; it may extend beyond Earth itself. This swollen stage in a star’s life is known as the Red Giant phase, marked by the final release of energy and the beginning of a long, long decline. In six billion years’ time, in a most beautiful display of light and colour, our sun will shed its outer layers into space to form a planetary nebula. We know this because we have seen this sequence of events unfold in the final breath of distant stars – on someone else’s sun? Written across the night sky in filamentary patches of colour are the echoes of our future.

  If in the far future, somewhere in the Universe, astronomers on a world not yet formed gaze through a telescope at our planetary nebula and reflect on its beauty, they may glimpse at its heart a faintly glowing ember; all that remains of a star we once thought of as magnificent. She will be smaller than the size of Earth, less than a millionth of her current volume and a fraction of her brightness. Our sun will have become a white dwarf – the destiny of almost all the stars in our galaxy – a fading, dense remnant, momentarily masked by a colourful cloud.

  If our planet survives, little more than a scorched and barren rock will remain, silhouetted darkly against the fading embers of a star.

  Sirius, the brightest star in our sky, sits at just over eight light years away, which makes it one of our nearest neighbours. It is so bright that on occasion it can be observed during bright twilight, partly because of its proximity but also because it is twice as big as our sun and twenty-five times as bright. It is therefore not surprising that observations of Sirius have been recorded in the oldest of astronomical records.

  For thousands of years we looked up at this beacon and assumed it was a single star, but in 1862 American astronomer Alvan Graham Clark observed a sister star hidden in the glare of Sirius’s light. It took so long to notice Sirius’s companion because, as the photograph taken by the Hubble Space Telescope (bottom right) reveals, it is so much dimmer than its sibling. Shining faintly in the lower left-hand corner, the small dot of light is an image of the white dwarf star Sirius B. This is one of the larger white dwarf stars discovered by astronomers, with a mass similar to our sun that is packed into a sphere the size of Earth. With no fuel left to burn, white dwarfs like Sirius B glow faintly with the residual heat of their extinguished furnaces. Like most white dwarfs, Sirius B is made primarily of oxygen and carbon (the remnants of helium fusion) packed tightly with a density a million times that of a younger, living star. This is the future of our star; a vision of the Sun’s death. Slowly cooling in the freezing temperatures of deep space, it is estimated that our sun will reach this phase in around 6 billion years’ time. From Earth, if indeed there is an Earth at that time, our sun will shine no brighter than a full moon on a clear night.

  Death must come to all stars. One day every light in the night sky will fade and the cosmos will be plunged into eternal night. This is the most profound consequence of the arrow of time; this structured Universe that we inhabit alongside all its wonders – the stars, the planets and the galaxies – cannot last forever. As we move through the age of stars, through the aeons ahead, countless billions of stars will live and die. Eventually, though, there will be only one type of star that will remain to illuminate the Universe in its old age

  An artist’s impression of Sirius A and its diminutive companion Sirius B in close-up. They are overlain on a real image of the night sky containing the three stars of the Summer Triangle: Vega, Deneb and Altair. As seen from Sirius, our sun would appear as a moderately bright star in this same area of sky. It is shown here just below right of Sirius A.

  NASA

  This image from NASA’s Hubble Space Telescope shows the Boomerang Nebula in early 2005 and the two lobes of matter that are being ejected from the star as it dies. The rapid expansion of the planetary nebula around this dying star has made it one of the coldest places found in the Universe so far.

  NASA

  A Hubble Space Telescope image of the dazzling Sirius A with the faint speck of Sirius B to its lower left. Sirius B is 10,000 times fainter than Sirius itself.

  NASA

  THE DEATH OF THE SUN

  Although relatively young now, the Sun, like every other star in the Universe, must one day die. In around five billion years’ time, the Sun’s stores of hydrogen will run dry and the star will begin its long, dramatic swansong. During this lengthy goodbye, the last dying bursts of extra heat will extend towards us, passing Mercury and Venus on the way and leaving a trail of destruction in its wake. Long after life has disappeared on Earth, the Sun will continue to fill the horizon as it swells in the Red Giant phase until, in about six billion years’ time, our Sun will shed its outer layers of gas and dust into space, exposing its core which will fade into a white dwarf, living on in the heavens as a shadow of its former self.

  Nathalie Lees © HarperCollins

  THE LAST STARS

  The nearest star to our solar system is Proxima Centauri. Although only a mere 4.2 light years away, Proxima Centauri is not visible to the naked eye from Earth and doesn’t even stand out against more distant stars in many of the photographs that have been taken of it. The reason for this is that Proxima Centauri is small, very small when compared to our sun – having just 12 per cent of the Sun’s mass – so to our eyes this star would appear to shine 18,000 times less brightly than our sun.

  Proxima Centauri is a red dwarf star – the most common type of star in our universe. Red dwarfs are diminutive and cold, with surface temperatu
res in the region of 4,000K, but they do have one advantage over their more luminous and magnificent stellar brethren: because they’re so small, they burn their nuclear fuel extremely slowly, and consequently they have life spans of trillions of years. This means that stars like Proxima Centauri will be the last living stars in the Universe.

  If we do in fact survive into the far future of the Universe, it is possible to imagine our distant descendants building their civilisations around red dwarfs in order to capture the energy of those last fading embers of stars. Just as our ancestors crowded around campfires for warmth on cold winter nights, so some time long in the future humans may take their warmth from a red dwarf as the last available energy in the Universe.

  The rate of the fusion reactions in the cores of these red dwarfs that is needed to provide the thermal pressure to resist the inward pull of their weak gravity is very low, which enables them to live longer. Even so, these are still active stars, and their surfaces are whipped up into turmoil by the turbulent convective currents that constantly churn their interiors. Amongst all this activity, explosive solar flares occur almost continually, blasting bursts of light and X-rays out into space.

  Ultimately, though, the frugality of these stars is no defence against the arrow of time. Four trillion years from now, at 300 times the current age of the Universe, Proxima Centauri’s fuel reserves will finally run out and the star will slowly collapse into a white dwarf. After trillions of years of stellar life and death, only white dwarfs and black holes will remain in the Universe, and then, in around 100 trillion years’ time, this age of the stars will draw to a close and the cosmos will enter its next phase: The Degenerate Era. And yet, even after 100 trillion years of light, the vast majority of the Universe’s history still lies ahead. Bleak, lifeless and desolate, our universe will go on, as it enters the dark

  These computer-generated images reveal how Proxima Centauri will meet its end. Over the next four trillion years, this red dwarf will gradually collapse into a much dimmer white dwarf.

  * * *

  After trillions of years of stellar life and death, only white dwarfs and black holes will remain in the Universe, and then, in around 100 trillion years’ time, this age of the stars will draw to a close and the cosmos will enter its next phase: The Degenerate Era.

  * * *

  A white dwarf is visible amongst brighter, living stars in this enhanced image, taken by NASA’s Galaxy Evolution Explorer, of Z Camelopardalis, a binary star system.

  NASA

  THE BEGINNING OF THE END

  On the northern coast of Namibia, where the cold waters of the South Atlantic meet the Namib Desert, lies one of the most inhospitable places on Earth. The Skeleton Coast has been feared for as long as sailors have travelled near its shores; seventeenth-century Portuguese mariners used to call this place ‘the gates to hell’, and the native Namib Bushmen named it ‘the land God made anger in’. Today, you can just about make it to the coast in a sturdy 4x4, or effortlessly cruise in from the port city of Walvis Bay in a helicopter. But even so, when you stand on the sands beside the South Atlantic, the gods still have anger left. Each morning a dense ocean fog rolls along the coastline, fed by the upwelling of the cold Benguela current. Coupled with the constantly shifting shape of the sandbanks in the intense Atlantic winds, this toxic navigational conspiracy has meant that over the years thousands of ships have been wrecked along the Skeleton Coast. The decaying carcasses of the rusting ships and the bleached bones of marine life swept ashore by the currents all add to the coast’s gothic feel. The name Skeleton Coast also reflects the large number of human lives lost here over the centuries; even if you made it ashore after a shipwreck, the onshore currents are so strong that there is no way of rowing back out to sea, and the only route to safety is through hundreds of miles of inhospitable desert. This genuinely was a place of no return: if you were shipwrecked here, this was the end of your universe.

  * * *

  Just as the ship’s iron will eventually rust and be carried away by the desert winds, so we think the last matter in the Universe will eventually be carried off into the void.

  * * *

  One of the ships to end her days here was the Eduard Bohlen, a 91-metre (300-foot), 2,272-tonne steamship that ran aground here on the 5 September 1909 on a journey from Germany to West Africa. A century’s shifting sands have carried her hundreds of metres inland and the Atlantic winds have attacked her carcass, leaving her rusting and skeletal. When we arrive she is guarded by a phalanx of jackals who are less wary of us than I expected. She forms an abstract backdrop to our story; the symbolism is immediate, brutal even, and for me surprisingly powerful. These wrecks, complex structures dismantled by the passage of time, are like our last stars.

  The Skeleton Coast: one of the most inhospitable places on Earth, where humans have perished for centuries, and where only jackals and the strongest life forms remain.

  In the far future of the cosmos, the last remaining beacons of light will no more be permitted to evade the second law of thermodynamics than the Eduard Bohlen. Even the white dwarfs must fade as the laws of physics methodically dismantle the Universe. Slowly, as the glowing embers of the last stars lose their warmth to space, they will cease to emit visible light. After trillions of years, the final beacons burning in the cosmic sky will turn cold and dark – their remnants are known as black dwarfs.

  Black dwarfs are dark, dense, decaying balls of degenerate matter. Nothing more than the ashes of stars, they take so long to form that after almost 14 billion years, the Universe is currently too young to contain any at all. Yet despite never seeing one, our understanding of fundamental physics allows us to make concrete predictions about how they will end their days. Just as the iron that makes up the ships of the Skeleton Coast will eventually be carried away by the desert winds, so it is thought that the matter inside black dwarfs, the last matter in the Universe, will eventually evaporate away and be carried off into the void as radiation, leaving nothing behind. The processes by which matter might, given enough time, decay, are not understood. Physicists need a more advanced theory of the forces of nature, known as a Grand Unified Theory, to speak with certainty about the behaviour of protons, neutrons and electrons over trillion-year timescales. There are reasons to expect that such a theory may exist, and that a mechanism for even the most stable sub-atomic particles to decay into radiation might be present in nature. For this reason, experiments to measure the lifetime of protons are ongoing in laboratories around the world, but as yet nobody has observed proton decay, and we are therefore now in the realm of speculation. But here is one possible, and given our understanding of physics today, probable, story of how our universe will end.

  NASA

  This composite X-ray image from the Chandra X-ray Observatory shows gas blowing away from a central supermassive black hole in the active galaxy NGC 1068.

  NASA

  * * *

  Once the last remnants of the last stars have decayed away to nothing…the story of our universe will finally come to an end.

  * * *

  In trillions of years, our universe will be littered with black dwarfs. From the ashes of stars, dark, dense and decaying balls of degenerate matter will form.

  With the black dwarfs gone, there will not be a single atom of matter left in the Universe. All that will remain of our once-rich cosmos will be particles of light and black holes. After an unimaginable expanse of time, it is thought that even the black holes will evaporate away, and the Universe will consist of a sea of light; photons all tending to the same temperature as the expansion of the Universe cools them towards absolute zero. When I say unimaginable period of time, I really mean it: ten thousand trillion trillion trillion trillion trillion trillion trillion trillion years. In scientific notation, that’s 10100 years. That is a very big number indeed; if I were to start counting with a single atom representing one year, there wouldn’t be enough atoms in all the stars and planets in all the galaxies in the entir
e observable universe to get anywhere near that number.

  Once the last remnants of the last stars have decayed away to nothing and everything reaches the same temperature, the story of our universe will finally come to an end. For the first time in its life the Universe will be permanent and unchanging. Entropy finally stops increasing because the cosmos cannot get any more disorganised. Nothing happens, and it keeps not happening forever.

  This is known as the heat death of the Universe, an era when the cosmos will remain vast, cold, desolate and unchanging for the rest of time. There’s no way of measuring the passing of time, because nothing in the cosmos changes. Nothing changes because there are no temperature differences, and therefore no way of moving energy around to make anything happen. The arrow of time has simply ceased to exist. This is an inescapable fact, written into the fundamental laws of physics. The cosmos will die; every single one of the hundreds of billions of stars in the hundreds of billions of galaxies in the Universe will expire, and with them any possibility of life in the Universe will be extinguished

  A VERY PRECIOUS TIME

  The fact that the Sun will die, incinerating Earth and obliterating all life on our planet, and that eventually the rest of the stars in the Universe will follow suit to leave a vast, formless cosmos with no possibility of supporting any life or retaining any record of the living things that brought meaning to its past, might sound a bit depressing to you. You might legitimately ask questions about the way our universe is put together. Surely you could build a universe in a different way? Surely you build a universe such that it didn’t have to descend from order into chaos? Well, the answer is ‘no’, you couldn’t, if you wanted life to exist in it.