stellar metallicity

When astronomers say 'metals', they mean something startling: to them, every element heavier than hydrogen and helium counts as a metal — including oxygen, carbon, and nitrogen, which no chemist would call metals. Metallicity is simply how much of a star is made of these heavier elements, as opposed to the primordial hydrogen and helium left over from the Big Bang.

Metallicity is read from a star's spectrum: heavier elements stamp absorption lines onto the starlight, and the strength of those lines reveals how abundant they are. Astronomers usually express it relative to the Sun, often via the iron-to-hydrogen ratio written [Fe/H]. A value of 0 means Sun-like; +0.3 means about twice the Sun's metals; -1 means one-tenth; -2 means one-hundredth. Even a 'metal-rich' star is overwhelmingly hydrogen and helium — the Sun is only about 1.3 percent metals by mass — so these are tiny seasonings, but ones that matter enormously.

Metallicity matters because it is a clock and a family record rolled into one. The universe began with essentially no metals; every heavier element was forged inside earlier generations of stars and scattered when they died. So a metal-poor star is old, born from gas that few previous stars had enriched, while a metal-rich star is young, made from gas already salted with the ash of countless predecessors. Metallicity thus distinguishes the old Population II stars from the younger Population I stars, traces the chemical history of galaxies, and even influences whether a star can host rocky planets.