Wonders of the Solar System Text Only. Andrew Cohen

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СКАЧАТЬ 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^-15 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=mc². 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 Paraná and the Argentine province of Misiones is the Iguaçu river. Stretching for over one thousand kilometres, the Iguaçu 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 Iguaçu 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 СКАЧАТЬ