We have seen how the Solar System came to be, and how it has changed in the billions of years since it was born. Now it is time to take a different journey – a journey into the future of the Solar System. This is the subject of Part 4.

We think of the Sun as all-powerful and everlasting. Indeed, on a human timescale it is. Deep within its fiery interior, though, the numbers speak for themselves. On the main sequence the Sun converts a phenomenal 600 million tonnes – the mass of a small mountain – of hydrogen into helium every second, just to keep itself balanced against gravity. At the moment, there is no need for us to worry about this alarming appetite. For the Sun has enough hydrogen to keep its nuclear fires stoked for a good few billion years into the future – long after mankind has vanished. But the day will come when the Sun's fuel heap will run dry, and its useable hydrogen has been totally consumed. When that happens, the Sun will start to die – and with it, the rest of the Solar System.

With the emergence and subsequent evolution of the planetary atmospheres, the Solar System was almost complete. Only two things remained to be added: the rings of the giant planets, and some of the smaller, irregular satellites. The irregular satellites were probably acquired early in the history of the Solar System, when the giant planets captured icy planetesimals from the thinning Solar Nebula. Some are no doubt of more recent origin. The origins of the rings, however, are more difficult to pin down.

The most famous ring system is Saturn's. Consisting of countless boulder-sized, and smaller, icy chunks in individual orbits about the planet, the rings are exceedingly thin – with relative dimensions like those of a sheet of paper the size of a football pitch. But Saturn is not alone, because each of the other giant planets has similar accoutrements, albeit with different characteristics. Indeed, research has shown that no two systems are alike: they differ from each other in terms of diameter, brightness, and in the sizes and compositions of the particles that constitute them. This is a clue to their formation. But the biggest hint is that most of the rings surround their planetary hosts inside their respective ‘Roche limits’. This is the distance from a given planet at which gravitational forces tear apart any body held together mostly by gravity. These clues could mean that the rings are the unassembled ruins of moons that strayed within this danger zone and got ripped to shreds, or the remains of comets that got too close and suffered a similar fate.

Over tens of thousands of years, the gases inside the globule continued to fall away from the inside edge of the cocoon, pulled inexorably towards that dense core at the centre. By now, the core of the globule was taking on a definite shape – a gargantuan ball, about the size of the present-day Solar System out to Pluto. Its surface was still too cold to glow optically. But, at last, its central regions had warmed up significantly – to about 10 000 Celsius – and the molecules there had split into atoms of hydrogen.

This marked an important point in the development of the Sun. At this temperature, the cloud core was now hot enough for the radiation it emitted to carry a significant punch. Radiation is composed of tiny packets of energy called photons, each of which can be likened to a subatomic particle. If there are enough of these photons emitted every second they can hit like a hail of bullets, a barrage of electromagnetic force known as radiation pressure. Before this point the core of the globule had been emitting too few photons to exert a noticeable force. Now, though, as the growing waves of radiation streamed away from the warming core they slammed into the outermost regions of the globule where the gases were less dense, and slightly hindered their inbound journey. Thus the contraction of the core slowed, but it did not stop, so overwhelming was the inward pull of gravity. The very centre of the core was also dense enough now that it was beginning to become opaque to the heat radiation generated inside it.

By 3 million years or thereabouts – about 1 million years after the initial collapse of the globule – the protosun had shrunk to a few solar radii. Its temperature at the centre was now around 5 million degrees Celsius, while the surface seethed and bubbled at around 4500 Celsius. At last the object had crossed the line that separates protostars from true stellar objects. It joined the ranks as what astronomers call a T-Tauri star.

Named after a prototypical young stellar object in the constellation Taurus, the T-Tauri phase is one of extreme fury. And as with all T-Tauri stars, this earliest form of solar activity would have been driven – at least in part – by a powerful magnetic field. Because the gases inside the young star were by now fully ionised – a soup of positively and negatively charged elements – their movement as the star rotated effectively amounted to a series of gigantic electric currents. Thus the spinning star developed a global magnetic field in the same way that a wire carrying an electric current does – just as the Sun generates its field even today. During the Sun's T-Tauri phase, though, the star would have been spinning very quickly – once in 8 days compared with once in 30 days – spun up by the swirling gases that had ploughed into it earlier. This means that the T-Tauri Sun's magnetic field was much mightier than at present, and this is what made this phase in the Sun's formation so violent. The Sun was still surrounded by its protoplanetary disc. So, as the Sun whirled around, it dragged its magnetic field through this disc.

Time goes by. And the dead Solar System continues to orbit the galactic centre just as it does today. Its journey takes it past stars young and old; carries it through the ghostly ruins of stars already dead – shrouds of gas ejected by planetary nebulae, or during the cataclysmic explosions known as supernovae. Ultimately the ashes of the star once known as the Sun are spread throughout the entire Milky Way galaxy, mixed indistinguishably with the remains of other stars, replenishing the interstellar medium from which they sprang billions of years earlier.

Then one day…

A massive star reaches the end of its life. It blows itself apart. Shockwaves from the supernova spread outwards from the epicentre through the interstellar medium like concentric rings on the surface of a lake. The waves compress the gas clouds through which they propagate. And eventually, somewhere, part of the cloud begins to contract under its own gravity. Millions of years later, a new star shines in the galaxy, born from the ashes of those long dead – including the Sun. Perhaps planets will also form around this new star – even life. And so it could be that the very atoms that currently comprise our bodies will one day find themselves part of a very different, alien creature. For the Universe is the ultimate recycling machine. We have come full circle.

At last, after a period of perhaps 30 to 50 million years – astronomers still cannot agree on their numbers – the Sun's contraction finally came to an end. Why? Because the Sun's internal temperature had reached an all-time high of 15 000 000 Celsius – and something had begun to happen to its supply of hydrogen.

Hydrogen is the simplest of all elements. Each atom contains just a single subatomic particle called a proton in its nucleus, positively charged. Orbiting this, meanwhile, is a single much smaller particle with exactly the opposite electric charge: an electron. Inside the Sun, these atoms are ionised: the electrons are detached and roam freely in the sea of hydrogen nuclei or protons. Very often, two of these hydrogen nuclei come together. Just as two magnetic poles of like polarity repel each other, so too do two protons. But not if they are brought together with sufficient speed. The speed of particles in a gas can be measured by the gas's temperature. And at 15 000 000 Celsius, the positively charged hydrogen nuclei at the Sun's core were now moving so quickly that when they smashed together they overcame their electrostatic repulsion, and fused as stronger nuclear forces took over. At last, the hydrogen was being consumed, gradually converted into helium in the Sun's core via a chain of nuclear reactions. Energy is a by-product of these reactions. And so the Sun now began to generate a significant amount of power in its core.

The Sun, its nine planets and their satellites, the asteroids and the comets – together, these are the elements that comprise the Solar System. In this book we shall meet them in detail. We shall come to know their properties, their place in the Solar System, what they look like and how they compare with one another. We will learn what they are made of, when and how they were made. We will discover what the Solar System's various contents have endured since their fiery birth. And, lastly, we shall see what will happen to them – to the Solar System as a whole – in the far, distant future, billions of years from now, as the tired star we call the Sun passes into old age, and beyond. These and other issues are all part of a great story – the story of the Solar System.

Overview of the Solar System

What is the shape of the Solar System? Where are the various objects within it to be found, and how do they move in relation to each other? These are important questions. For, unless we can answer them as accurately as possible, we shall be doomed to failure in our treatment of an even more fundamental issue, dealt with in detail in this book: the origin of the Solar System. So perhaps it would be prudent to spend a little time putting together what we currently know about the Solar System of which we are all a part.

The first thing to establish is that the centre of our planetary system is solar territory.

The subject of this book is astrophysical accretion, especially in those circumstances where accretion is believed to make an important contribution to the total light of an astrophysical system. Our discussion therefore centres mainly on close binary systems containing compact objects and on active nuclei. The reader is assumed to possess a basic knowledge of physics at first degree level, but only a rudimentary experience of astronomy is required. We have tried to concentrate on those features, particularly the basic physics, that are probably more firmly established; but the treatment is necessarily somewhat heterogeneous. For example, there is by now a tolerably coherent line of argument showing that the formation of an accretion disc is very likely in many close binaries, and giving a plausible picture of what such a disc is like, at least in some simple cases. In other areas, such as accretion on to the surface of a compact object, or in active nuclei, we are not so fortunate, and we must work back and forth between theory and observation. Our aim is that the book should provide a systematic introduction to the subject for graduate students. We hope it may also serve as a reference for interested astronomers in other fields, and that selected material will be suitable for undergraduate options in astronomy.

In Chapters 2 and 3 we present introductory material on fluid dynamics and plasma physics. Many excellent texts exist in these areas, but they tend to be too detailed for our needs; we have tried to extract just those basic ideas necessary for the subsequent discussion, and to set them in an astrophysical context.

Introduction

Whenever we need to consider the behaviour of a gas on lengthscales comparable to the mean free path between collisions, we must use the ideas of plasma physics. In this chapter we shall briefly introduce some of the concepts that will be important to our study of accretion.

A plasma differs from an atomic or molecular gas in that it consists of a mixture of two gases of electrically charged particles: an electron gas and an ion gas, with very different particle masses me and mi.

The electrons and ions interact with each other through their electrostatic Coulomb attractions and repulsions. These Coulomb forces decrease only slowly (∞ r-2) with distance and do not have a characteristic lengthscale. Thus, a plasma particle interacts with many others at any one instant, and this makes the description of collisions more complicated than in atomic or molecular gases, where the interparticle forces are very short-range. A further complication arises from the great difference in particle masses me and mi. Since collisions between particles of very different masses can transfer only a small fraction of the kinetic energy of order me/mi ≪ 1, it is possible for electrons and ions to have significantly different temperatures over appreciable timescales. These two properties – the long-range nature of the Coulomb force and the disparity in electron and ion masses – give the physics of plasmas its particular character. A further series of complex phenomena occurs when the plasma is permeated by a large-scale magnetic field; this is particularly relevant for the study of gas accreting on to highly magnetized neutron stars and white dwarfs.