In this chapter we describe the nuclear reactions that power the Sun and thus make possible life on Earth. In contrast to the power from fission discussed in the preceding chapter, the radiance of the Sun comes from the fusion of the lightest element, hydrogen, into helium. We then examine the possibility of controlled nuclear fusion for power production on Earth.

10.1 The Sun

In stars, the gravitational, the weak, the electromagnetic, and the strong interactions all play an active and essential role. Our Sun and its planets are thought to have condensed out of a diffuse mass of material, mostly hydrogen and helium atoms, some 5 x 1O9 years ago. Table 10.1 gives the estimated proportions of the ten most abundant nuclei in that mass of material.

The major attributes of the Sun, determined from a wide variety of observations, are as follows:

(The luminosity of a star is the total rate of emission of electromagnetic energy.)

Because of the long range and universally attractive nature of gravity, a homogeneous mass of gas at sufficiently low temperature is unstable to contraction into objects like stars. During contraction of a mass of gas, gravitational potential energy is converted into kinetic energy and radiation energy, and the temperature of the gas rises. The rate of collapse is determined by the extent to which the build-up of pressure in the hot, dense interior can balance the incessant pressure of gravitational contraction.