Abundances of Elements

Definition

The relative amount (or fraction) of a given nuclide in a sample of matter is called the abundance of that nuclide. It can be expressed either in absolute terms (i.e., with respect to the total amount of matter in the sample) or in relative terms (with respect to the amount of some key element, e.g., the most abundant one, in the sample). Similarities and differences in the elemental and isotopic composition of stars and galaxies are key ingredients for understanding their origin and evolution.

Overview

The composition of remote objects (the Sun, stars, interstellar gas, and galaxies) is determined through spectroscopy, which usually allows the determination of elemental abundances; in rare cases, particularly for interstellar clouds, some isotopic abundances may be determined in those objects. For Earth, lunar, and meteoritic samples, nuclear mass spectroscopy allows precise determination of most isotopic abundances; this is also the case for cosmic rays, albeit only for...