
In astronomy, all elements heavier than hydrogen and helium.In astronomy, 'metals' refers to all chemical elements heavier than hydrogen and helium. This terminology reflects the fact that hydrogen and helium were created in the Big Bang, while heavier elements are forged in the interiors of stars and dispersed through supernova explosions and stellar winds. The 'metallicity' of a star or galaxy—its abundance of these heavier elements—is a key indicator of its age and the history of star formation in its environment.
The term 'metals' in this astronomical sense was popularized in the mid-20th century, notably by Walter Baade, who used it to distinguish between Population I stars (metal-rich) and Population II stars (metal-poor).
Metals are produced through nucleosynthesis in stars: elements up to iron are created by fusion in stellar cores, while elements heavier than iron are produced mainly through neutron capture processes (the s-process and r-process) during supernovae or neutron star mergers. These metals are then ejected into the interstellar medium, enriching the gas from which new stars form. Thus, each generation of stars has a higher metallicity than the previous one, making metallicity a cosmic clock.
Metallicity measurements are used to date stellar populations, trace galactic chemical evolution, identify the most ancient stars in the universe, and even to infer the presence of planets, since planet formation is more likely around metal-rich stars.
The most metal-poor star ever found, SMSS J031300.36-670839.3, has an iron abundance about 10 million times lower than the Sun's, indicating it formed from gas enriched by a single early supernova.



















