Science Writing Competition June 05 Runner Up
The Wonder of the Sun
by Neil Roxburgh
Few people pay a second thought to the hot shining disk which unfailingly rises each morning and sets each evening. It warms the Earth and allows life to thrive. It appears to be constant and predictable. Ever since the dawn of mankind, over five million years ago, the Sun has played a vital role in the lives of the people who have walked this Earth. It appears in countless myths and legends. Across the globe and throughout history the Sun has been worshipped as a God. Every living thing relies on the Sun, which until recent times, was beyond the scope of human comprehension. Everything about the Sun is overwhelming. Its size utterly dwarfs the Earth, its age shrinks our own faltering life spans to insignificance and its power is simply staggering. However, for the first time in our history, we are developing the tools required to describe, to explain and to understand the Sun.
Advances in technology, physics and mathematics have allowed the study of the Sun to move into a new era. The age of space travel, a new generation of telescopes and supercomputers have given scientists the ability to finally answer some of the most important questions ever asked.
Our Sun was born about 5 billion years ago from
an immense cloud of interstellar gas. The weak gravity of the cloud gradually made it condense into a rotating disk. The centre of the disk continued to collapse causing the gravity to become stronger and the gas to become hotter. At some point, the gas became so hot that the hydrogen atoms in the cloud began to fuse together to produce helium. It is this nuclear process that powers each of the 200 billion stars in the Milky Way and every other star in the universe. The outer regions of the disk collapsed to form the planets and, in this way, our solar system was born.
The Sun lies at the very centre of the solar system. All nine planets, including the Earth, revolve around this massive burning sphere. The Earth lies within a very special narrow corridor, within which the Sun provides just the right amount of energy to allow the compound H2O to exist in its liquid form. Planets closer to the Sun, Mercury for example, are scorched wastelands with the surface temperature during the day being over 300oC. Planets farther from the Sun are eternally locked in a frozen winter.
All life on earth was made possible by the presence of water. Given the slow pace of evolution, man kind, as the pinnacle of the process, has been reliant upon the Sun producing an almost constant output of energy. Thankfully, the Sun is a very average star which falls into the class of stars called Yellow Dwarfs. Given that its life expectancy is around 11 billion years, the Sun is middle-aged, and so we have another 6 billion years before the Sun dies. Thus, despite the known fate of our Sun and therefore the predicted end of life on Earth, we need not panic just yet. For the foreseeable future, humans can expect the Sun to shine as the first cavemen saw and for thousands of generations to come.
The natural curiosity of man kind lead people to observe the Sun. As early as 1000BC, the Chinese were looking at the Sun and recording what they saw. Under certain conditions, haze or fog for example, the Sun can be observed directly with the naked eye. The layer from which the Sun’s visible light comes is called the photosphere, or “sphere of light”. It is this layer that these early astronomers saw and that we see today. Not until the 20th century were people able to explain what they saw. The Sun has spots, enormous, Earth sized spots and, over the centuries, the question of what these spots actually are has been fiercely debated. Some, in believing the heavens were unblemished, argued that sunspots are planets passing between the Earth and the Sun.
With the advent of telescopes in the 17th century, Galileo proved that sunspots lie on the surface of the sun and he suggested that they were vast clouds of cool gas. In 1908, Hale made a crucial discovery. Sunspots are regions of immensely strong magnetic field, hundreds of times stronger than the Earth’s own feeble magnetic field. The heat produced in the core of the Sun radiates outwards towards the surface. Material beneath the surface is heated, making it rise and, as it rises, it cools. When the material reaches the surface it is colder than its surroundings and therefore sinks. As it sinks it is heated and, once again, when hot enough, it rises and the cycle repeats. This process, called convection, is similar to what happens when a pot of water is heated. Convection in the Sun produces huge continent sized convection cells and gives the photosphere a mottled look which is called granulation. The presence of a strong magnetic field inhibits the convection, stopping the material being heated as efficiently as field free regions. Sunspots are relatively cold. Despite still having a temperature of thousands of degrees, the material in sunspots appears dark compared to its hotter surroundings and it is for this reason that we are able to observe them.
The majority of astronomical bodies, planets,stars and galaxies have magnetic fields. The presence of these fields has baffled scientists over the years. The Earth contains a large amount of ferromagnetic material, iron for example. However, over many millions of years, the magnetic field produced by this material would have slowly diffused
away and vanished, long before the dinosaurs lived. The Sun contains no ferromagnetic material. Yet it is clear both bodies have a magnetic field, compasses still work and sunspots can be observed but where does the field come from?
Dynamo theory addresses this problem. This theory proposes a mechanism by which a celestial body can generate a magnetic field. All astrophysical bodies which contain anomalously long-lived magnetic fields also contain highly conducting fluids. The molten iron core of the Earth and the ionised gas which makes up the Sun are both extremely good conductors. The convective motions in the Sun, driven by the enormous amounts of energy produced in the core, are extremely vigorous, or “turbulent”. If the flow is sufficiently turbulent then it is believed that the energy input to the magnetic field can overcome the losses incurred by a process called “ohmic heating” as we get in any wire which carries an electrical current. In a similar fashion to the dynamo used to power a bike light which converts kinetic energy to electrical energy, the solar dynamo converts the kinetic energy of the plasma to magnetic energy. It is now widely believed that this dynamo action has maintained the magnetic field of the Earth and the Sun over millions of years.
Dynamo theory and the features produced by the Suns magnetic field, like sunspots, are the subject of intense research by academics all over the world. Research in this area falls, very generally, into two categories. On the one hand there are the observers. The incredible technological advances of the 20th century have revolutionized telescopic observations. A new generation of land-based telescopes, capable of producing images of a quality thought to be impossible only a few decades ago, are giving researchers a glimpse of small scale features previously never seen. A fleet of satellites orbiting the Sun are streaming data back to Earth twenty four hours a day and providing information that would be impossible to gain from land-based telescopes. Then, on the other hand, there are the theoreticians. Motivated by the observational data, the theoreticians attempt to explain the underlying mechanisms producing the observed features.
Some tackle a problem with a simple pen and paper while others make use of the awesome power of the world’s biggest and fastest supercomputers. Some old questions are being answered but the answers to others remain as elusive as ever. In part, it is thanks to the bewildering pace at which the speed of computers has improved that research in this area is thriving. Researchers are now able to solve problems of immense complexity and to fit more and more pieces into the jigsaw of our theoretical understanding. One widely used technique is the ability we now have to conduct realistic numerical simulations and one area in which
this technique had proved fruitful is the study of sunspots. Their very existence, their relatively long lifetimes and their intricate structure can now be explained. Magnetic field accumulates deep in the interior of the Sun. At some point an instability, called the “buoyancy instability”, occurs which allows a tube of magnetic flux to rise towards the surface. When the tube breaks through the surface two sunspots are formed. A sunspot has two very distinct parts. It has a dark central, almost circular region, called the umbra. This is surrounded by the penumbra, a striated, flamentary structure whose form resembles the way petals radiate from the centre of a flower. It is the complicated interaction of the strong magnetic field and the convective motions which cause sunspots to look the way the do.
The mathematical framework used to describe such
interaction has the grand title, Magnetohydrodynamics. Modelling an entire sunspot would be an enormous challenge, and is, at present, outside our capabilities. So a specific area of the sunspot is chosen for examination. It is relatively simple to write down the equations which describe the interaction of the ionised gas and the magnetic field but solving these equations is another matter all together. This is where the power of computers comes in. The equations are solved numerically. By doing so, the complicated non-linear behaviour of the system can be investigated and its evolution over time deduced. Previously unproven theories are becoming widely accepted thanks to the simulations producing results which match the observations remarkably well.
Despite the speed of present day supercomputers, a complete understanding of many aspects of the Sun is still extremely difficult to gain. A realistic global simulation of dynamo action, for example, is still too complex to tackle. The difficulty lies with the huge variety of scales that are involved. The turbulent convection that produces the magnetic field are relatively small scale, of the order a few kilometres. The scale of the Sun is many many times that size. Also, these motions occur very rapidly, making the time scale very small indeed. Therefore, to model the whole of the Sun and to simulate its behaviour over many hundreds, if not thousands of years is, for the moment, impossible. Advances in the field have been remarkable but there is a long way to go. Such a simulation would have to run for years on end on the fastest supercomputer. For the time being, models are simplified and assumptions are made which allow a simulation to be performed. How the results compare to reality is not known. One can extrapolate and make educated guesses at how the results can be applied to the solar dynamo but true understanding
lies with the researchers armed with the ultra-fast computers of the future.
These are just some of the many problems that are being tackled by researchers studying the Sun. With the proposal of new satellites and new telescopes it promises to be an exciting era for researchers in the field. Instruments with better resolution will be able to see even smaller features on the Sun. The bigger and faster computers a decade from now will be able to consider problems thought intractable for present day theoreticians. It is hoped that the advances in the coming years will confirm and build on our current theoretical understanding. However, new observations could
lead to old theories being discarded and the jigsaw of our understanding requiring serious modification. Thankfully, the Sun will be around for a while and as long as there remains features not well understood, the eternally inquisitive minds of humans shall endeavour to provide explanations.