metallicity

Metallicity is a measure of the abundance of elements heavier than helium in an astronomical object, typically a star, planet, or galaxy. In astrophysics, metals refer to all elements beyond hydrogen and helium. The quantity is commonly expressed as the mass fraction Z or, for convenient comparisons, as logarithmic abundance ratios such as [Fe/H] (iron relative to hydrogen) or [M/H] (overall metal content) relative to the Sun. Objects with Z or [Fe/H] below the solar value are sub-solar in metallicity, while those above are super-solar.

Metallicity is estimated by spectroscopy. For stars, metallicity is inferred from absorption lines, especially iron lines, and compared with solar spectra. For interstellar and circumstellar gas, metallicity is often derived from emission lines in H II regions using strong-line calibrations or the direct electron-temperature method. The Sun serves as a reference point, and solar metallicity is used to gauge whether an object is metal-poor or metal-rich.

The metallicity of stars and galaxies encodes chemical evolution. Metals are produced in stars and dispersed by winds and supernovae, enriching subsequent generations of stars. Metal-poor (Population II) stars are typically older, while metal-rich (Population I) stars are younger. Within galaxies, metallicity often varies with location, frequently higher toward the center, and correlates with stellar mass through the mass–metallicity relation. Abundance patterns (such as ratios of alpha elements to iron) also reveal enrichment timescales and the relative contributions of different supernova channels.

In practice, metallicity is a central parameter in studies of stellar evolution, galaxy formation, and cosmology, reflecting the history of star formation and chemical enrichment across the universe.