The previous chapters have touched on the scale of the universe and the nature of the smallest pieces of matter. The structure of the universe is determined not just by the matter contained within it, but by the forces that both bind matter together and compel it to move apart. These forces, which act at the macroscopic and microscopic levels, are thought to be carried by certain types of subatomic particles. In the case of electro-magnetism the force-bearing particle is called the photon.
We have learned most of what we know of the universe around us by studying the light coming from objects; our most information-filled sense is that of vision, and we have augmented it through the use of devices that can measure in detail the energy distribution of the light. This energy distribution from celestial bodies reveals much about their chemical composition and physical condition. Light from one such self-luminous body, the Sun, is the primary power source for Earth's climate and for life on the planet. The light by which the Sun and other stars shine is not generated by chemical reactions, but by reactions involving the nuclei of atoms at enormous pressures and temperatures deep within these gaseous objects' interiors; these are called nuclear reactions.
The nuclear reactions powering stars have, over time, generated essentially all of the natural elements except hydrogen, the most abundant element, and some of the helium (the remainder having been made from hydrogen in the primordial Big Bang). Thus the elements that make up life today (carbon, nitrogen, oxygen, phosphorus, etc.), with the exception of hydrogen, were manufactured by the very same process that today provides the energy source sustaining life on the planet. This chapter sets us on an evolutionary course that joins up eventually with the history of Earth and life, as we consider the processes by which elements are made.