In this chapter we explore the basic principles governing the structure of stars.

We have already realized that the Sun – and like it, every star – is nothing more than a ball of superheated gas. You might think that such a ball would be pretty uninteresting, but you would be wrong. Indeed, stars are quite remarkable things.

Perhaps the most remarkable thing about them is the enormous amount of energy they emit. In a tiny fraction of a second, for instance, the Sun emits more energy than the human race has used in all the centuries since the industrial revolution. For many years scientists were at a loss to account for all this energy. Only recently have we realized that stars are powered by nuclear reactions occurring in their cores. So in a very real sense, what we term “solar energy” is in fact nuclear energy. The only difference is that, in this case, the nuclear reactor is floating off in space, 93 million miles away.

With this insight in hand, scientists have constructed computer models of stars. These models balance the enormous inward pull of gravity by the equally enormous outward push of gas pressure. Similarly, the generation of energy from nuclear reactions is balanced by the emission of energy via starlight. These models have revealed that main sequence stars are in what we might call the first phase of their evolution, in which they derive their energy from nuclear reactions involving hydrogen. Our own Sun is such a star. In the next chapter we will consider what happens once the supply of hydrogen runs out.