Wonders of the Solar System Text Only. Andrew Cohen

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