Stellar chemical abundance
= Stellar chemical abundance
{title2=$X_i,\quad n_i/n_{\rm H}$}
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.