An Etymological Dictionary of Astronomy and Astrophysics

In a star, nebula, or galaxy, the proportion of the material that is made up of → metals, that is elements heavier than → helium. It is generally denoted by Z. The term “metallicity” is a misnomer used in astrophysics.

1. In practice, the metallicity of stars is usually expressed by the number ratio of → iron atoms to → hydrogen atoms per unit volume, with respect to the solar values:
   [Fe/H] = log₁₀(N_Fe/N_H)_star - log₁₀(N_Fe/N_H)_Sun, where
   N_Fe and N_H are the numbers of iron and hydrogen atoms per unit volume. In fact it is taken to be equal to the iron → abundance with respect to the solar value.
   The solar logarithmic iron abundance is 7.50 ± 0.04 (Asplund et al. 2009, ARAA 47, 481), with respect to that of hydrogen which, by convention, is 12.00. Stellar metallicity is often expressed in mass fraction. See also → solar metallicity.
   

2. Nebular metallicity is often defined as the relative abundance of → oxygen:
   (N_O/N_H)_neb/(N_O/N_H)_Sun, where N_O and N_H represent the numbers of oxygen and hydrogen atoms per unit volume.

See also: From metallic, from → metal + → -ity.