Metallicity

Metallicity is the abundance of elements heavier than helium in an astronomical object. It traces the object’s chemical enrichment beyond the primordial hydrogen and helium produced by the Big Bang.

Spectroscopy: The study of matter and radiation through their interaction across wavelengths. Astronomers infer metallicity from element-specific absorption and emission features in spectra.
Big Bang nucleosynthesis: The production of light nuclei in the early universe during its first few minutes. It established the hydrogen- and helium-rich starting composition against which later enrichment is measured.
Stellar nucleosynthesis: The creation of chemical elements through nuclear reactions inside stars. Stellar fusion and subsequent burning stages produce many elements that raise metallicity.
Metallicity distribution function: The distribution of stellar metallicities within a population or region. Its shape records the enrichment history and assembly of stellar systems.
Metallicity versus iron abundance: The distinction between total heavy-element abundance and the abundance of iron relative to hydrogen. Iron is often used as a proxy, but stars with unusual element ratios can break that equivalence.
Stellar spectral classification: A system for classifying stars by the features and temperature of their spectra. Spectral type must be separated from metal-line strength when estimating a star’s composition.
Chemical element: A substance defined by atoms with the same number of protons. Metallicity groups all elements heavier than helium together, regardless of their individual chemistry.
Supernova nucleosynthesis: The formation and dispersal of elements during stellar explosions. Supernova ejecta enrich surrounding gas with elements synthesized in massive stars.
Age–metallicity relation: The statistical relationship between stellar age and metallicity in a population. It tests whether older stars formed from less-enriched gas in a given system.
Alpha enhancement: An elevated abundance of alpha-capture elements relative to iron compared with the solar ratio. Two stars with equal iron abundance can differ in total metallicity because their alpha-element abundances differ.

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