The abundance of elements heavier than hydrogen and helium in a star or celestial object, expressed as a fraction of the solar metallicity. In astronomy, all elements heavier than helium are considered 'metals,' regardless of their chemical properties.
Derived from 'metal' plus the suffix '-icity,' but in astronomical context, 'metal' refers to any element beyond hydrogen and helium. This usage developed in the mid-20th century as spectroscopy revealed the chemical composition of stars and their evolutionary significance.
Metallicity is like a cosmic timestamp! Stars with higher metallicity are younger because they formed from gas enriched by previous generations of stars that forged heavy elements in their cores and scattered them through supernovae.
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