Metal abundances
Metallicity
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. 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(NFe/NH)star - log10(NFe/NH)Sun, where NFe and NH 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, with respect to that of hydrogen which, by convention, is 12.00. Stellar metallicity is often expressed in mass fraction. Nebular metallicity is often defined as the relative abundance of oxygen: (NO/NH)neb/(NO/NH)Sun, where N_O and N_H represent the numbers of oxygen and hydrogen atoms per unit volume.
Chemical abundances