Metallicity
Metallicity measures how rich a star or gas cloud is in elements heavier than helium, reflecting enrichment from past stellar explosions. It indicates whether a star is old (low metallicity) or young (high metallicity).
Metallicity is the measure of the abundance of elements heavier than helium within a star, gas cloud, or planetary system, typically expressed as a logarithmic ratio relative to the sun's composition. It reflects the cumulative enrichment from previous generations of stellar explosions, where heavier elements ('metals') are synthesized and dispersed into the interstellar medium. In astrophysics, low metallicity indicates older stellar populations, while high metallicity suggests younger, more evolved systems.
What this means in real life
An old star in the galactic halo might have very low metallicity because it formed billions of years ago when the universe contained mostly hydrogen and helium. A younger star in the galactic disk has higher metallicity because it formed from gas enriched by previous stellar explosions.
What it isn’t
It is not a measure of how metallic or shiny something appears. Metallicity is a technical term about elemental composition, not visual properties or density—a star with high metallicity may look identical to one with low metallicity.
Commonly misused online
Social media sometimes conflates metallicity with 'metal content' in a literal sense, or uses it metaphorically to describe how 'heavy' or 'intense' a star or galaxy seems, rather than its actual chemical abundance.