the metallicity of the Sun ; conversely, those with a value of −1 have , while those with a value of 0 have the same metallicity as the Sun, and so on.

Young population I stars have significantly higher iron-to-hydrogen ratios than older population II stars. Primordial population III stars are estimated to have metallicity less than −6, a millionth of the abundance of iron in the Sun.The same notation is used to express variations in abundances between other individual