Metallicity



In astronomy and physical cosmology, the metallicity is the fraction of mass of a star or other kind of astronomical object, beyond hydrogen and helium. Most of the physical matter in the universe is in the form of hydrogen and helium, so astronomers conveniently use the blanket term "metals" to refer to all other elements. For example, stars or nebulae that are relatively rich in carbon, nitrogen, oxygen, and neon would be "metal-rich" in astrophysical terms, even though those elements are non-metals in chemistry. This term should not be confused with the usual physical definition of solid metals.

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 also 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, where stars at the end of their lives discard most of their mass by stellar winds or explode as supernovae, the metal content of the Galaxy and the universe increases. It is postulated that older generations of stars generally have lower metallicities than those of younger generations.

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