What is "metallicity"

n. (context astronomy English) The abundance of elements heavier than helium in stars as a result of nucleosynthesis

In astronomy and physical cosmology, the metallicity or Z'' is the fraction of mass of a star or other kind of astronomical object that is not in hydrogen (X'') or helium (Y). Most of the physical matter in the universe is in the form of hydrogen and helium, so astronomers use the word "metals" as a convenient short term for "all elements except hydrogen and helium". This usage is distinct from the usual physical definition of a solid metal. The rationale for the astronomical usage is that in the high-temperature and pressure environment of a star, atoms do not undergo chemical reactions and effectively have no chemical properties, including that of being a metal as usually understood. For example, stars and nebulae with relatively high abundances of carbon, nitrogen, oxygen, and neon are called "metal-rich" in astrophysical terms, even though those elements are non-metals in chemistry.

The distinction between hydrogen and helium on the one hand and metals on the other is relevant because the primordial universe is believed to have contained virtually no metals, which were later synthesised within stars.

Metallicity within stars and other astronomical objects is an approximate estimation of their chemical abundances that change over time by the mechanisms of stellar evolution, and therefore provide an indication of their age. In cosmological terms, the universe is chemically evolving. According to the Big Bang Theory, the early universe first consisted of hydrogen and helium, with trace amounts of lithium and beryllium, but no heavier elements. Through the process of stellar evolution stars first generate energy by synthesising metals from hydrogen and helium by nuclear reactions, then disperse most of their mass by stellar winds or explode as supernovae, dispersing the new metals into the universe. It is believed that older generations of stars generally have lower metallicities than those of younger generations, having been formed in the metal-poor early universe.

Observed changes in the chemical abundances of different types of stars, based on the spectral peculiarities that were later attributed to metallicity, led astronomer Walter Baade in 1944 to propose the existence of two different populations of stars. These became commonly known as Population I (metal-rich) and Population II (metal-poor) stars. A third stellar population was introduced in 1978, known as Population III stars. These extremely metal-poor stars were theorised to have been the 'first-born' stars created in the universe.