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 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