A stellar abundance can be expressed as an elemental or isotopic mass fraction, or as a number ratio to hydrogen. Inferring it from a spectral line requires a model for level populations, opacity, radiation and velocity fields. Agreement between suitable ionization stages and weak photospheric lines tests whether the adopted stellar atmosphere is adequate.
The abundance of chemical elements heavier than helium in astronomical material. A metal mass fraction measures their combined mass; spectroscopic abundance ratios are other measures of metallicity and need not be numerically identical to that fraction.
The fraction of total mass in chemical elements heavier than helium, so the hydrogen mass fraction, helium mass fraction and metal mass fraction satisfy . Despite the astronomical name, this includes elements such as carbon and oxygen that are not chemical metals.
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