In the preceding chapter we explained how in a star like the Sun helium is steadily formed from the fusion of hydrogen. In this chapter we sketch some of the basic ideas of ‘nuclear astrophysics’, a subject which seeks to understand all the nuclear processes leading to energy generation in stars in the various stages of stellar evolution, and to account for the observed relative abundances of the elements in the Solar System in terms of these processes.

The accepted theory of the Universe is that it is expanding, and began with an intensely hot and dense ‘big bang’ between 10 × 109 and 20 × 109 years ago. A few hundred thousand years after the big bang, the expanding material had cooled sufficiently for it to condense into a gas made up of hydrogen and helium atoms in a ratio of about 100:7 by number, together with photons and neutrinos. Apart from a small amount of lithium, it is thought that the proportion of heavier elements produced in this first explosion was insignificant (essentially because there are no stable nuclei with A = 5 or A = 8). If this is so, we must conclude that all the heavier nuclei in the Solar System have been produced in previous generations of stars and then thrown out into space again, perhaps in the explosion of supernovae.