Introduction

The understanding of the origin of sunlight (and starlight in general) was a nineteenth and early twentieth century development that culminated in the release of nuclear energy in human-made devices on Earth. Beyond the implications (both negative and positive) of such developments, however, lies the profound perspective gained in the latter half of the twentieth century regarding the origin of the elements of the periodic table. The existence and abundances of the 90-odd elements that make up Earth, the planets, the solar system, and the universe beyond have an explanation that lies in natural nuclear reactions that have taken place in the several generations of stars preceding the formation of the Sun and the solar system.

Stars and nuclear fusion

The observable cosmos around us is, by and large, made of stars. Stars are spheres made primarily of hydrogen and helium gas; the size of the spheres is determined by a balance between the attractive force of gravity pulling everything inward and the pressure associated with the high temperatures of stars' interiors, which is a force tending to push the material outward. Most stars eventually evolve, through nuclear processes described below, into dense spheres of carbon, oxygen, or exotic neutrons; some collapse into the mysterious and incredibly dense black holes.

The copious amounts of photons coming out of stars, including the Sun, are a signature of the enormous temperatures in their interiors. The origin of these high temperatures, and hence of sunlight or starlight, was a matter of debate throughout the nineteenth century. A hypothesis by the British physicist Lord Kelvin, that the Sun was radiating away the energy associated with its initial collapse from clouds of interstellar gas and dust, met with a timescale problem: the Sun would cool in several tens of millions of years, but various lines of evidence suggested that terrestrial rocks were older by at least a factor of 10.