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The abundances of the chemical elements (or just abundances, or chemical composition) are of interest in astrophysics, and efforts are made to match astrophysical theories regarding them to observations. Abundances within stars, planets, moons, and the interstellar medium can be worked out with some reliability through spectroscopy, and study of albedos, using knowledge of basic physical and chemical processes.
The abundances of all the objects within the solar system (including the Sun) show some consistency, showing a pattern that differs from those of other observed stars, and it is presumed that individual stars and their systems inherit abundance characteristics from the material that formed them, i.e., the material of specific molecular clouds. Given this conclusion, any divergence from the usual abundance seen throughout the solar system (e.g., the Sun's lack of lithium or Earth's lack of hydrogen and plethora of oxygen) demands reasonable explanation.
The abundances of molecular clouds, in turn, are presumed to be strongly affected by the nucleosynthesis within earlier stars and supernovae, formed from earlier molecular clouds, and so forth, back to the Big Bang, whose output is termed the primordial abundances. Current knowledge of the chemical composition of the universe is a target for th theories of the Big Bang nucleosynthesis and the subsequent chemical evolution (i.e., all the nucleosynthesis since then).
The abundance patterns (that vary among stars, etc.) include ratios between the abundance of elements (abundance ratios), as well as the relative abundances of isotopes of given elements (isotopic ratios). These are interest through the history of the universe, in the current universe, and in particular settings (e.g., on a particular planet or moon) where they help determine the history of objects and systems. The determination of such ratios for some given material is termed fractionation. Spectrography can determine some of these at a distance, lab analysis can be used for meteorites and Moon rocks, and space probes' on-board mass spectrometers for captured dust and other particles.