General introduction

The existence and distribution of the chemical elements and their isotopes is a consequence of nuclear processes that have taken place in the past in the Big Bang and subsequently in stars and in the interstellar medium (ISM) where they are still ongoing. These processes are studied theoretically, experimentally and observationally. Theories of cosmology, stellar evolution and interstellar processes are involved, as are laboratory investigations of nuclear and particle physics, cosmochemical studies of elemental and isotopic abundances in the Earth and meteorites and astronomical observations of the physical nature and chemical composition of stars, galaxies and the interstellar medium.

Figure 1.1 shows a general scheme or ‘creation myth’ which summarizes our general ideas of how the different nuclear species (loosely referred to hereinafter as ‘elements’) came to be created and distributed in the observable Universe. Initially – in the first few minutes after the Big Bang – universal cosmological nucleosynthesis at a temperature of the order of 109 K created all the hydrogen and deuterium, some 3He, the major part of 4He and some 7Li, leading to primordial mass fractions X ≃ 0.75 for hydrogen, Y ≃ 0.25 for helium and Z = 0.00 for all heavier elements (often loosely referred to by astronomers as ‘metals’!).