Elemental Abundances

One of the central issues to be understood in astronomy is how the elements came to be. About 114 elements are known (there is some uncertainty about the heaviest man-made elements). Of these, 81 are radioactively stable and occur naturally on the Earth; another 10 are unstable but occur in small concentrations naturally; one (promethium) is produced only in the fission of very heavy elements. The remaining elements are short-lived and produced in either man-made or natural reactions, but do not occur with any sustained concentration in nature. The abundance of elements in the Solar System is illustrated in the following diagram as a function of the atomic mass number.

This is a logarithmic scale, so there is an enormous difference (about 10¹²) between the most abundant and least abundant elements. The high abundance around mass 55-60 is called the iron peak, because this peak is dominated by isotopes of elements near iron in atomic number. The iron peak reflects the stability of these nuclei, which have the highest nuclear binding energies of any known elements. The process by which the elements are built up from the lightest to the heaviest is called nucleosynthesis. As we now discuss, there are two sources for nucleosynthesis of the elements, the big bang, and stars.