stage of the Universe, so it follows that they have lower metallicities than younger generations of stars which formed in a more metal-rich Universe.

The metallicity of a star is most often expressed in terms of [Fe/H], which represents the logarithmic ration of iron to hydrogen relative to the Sun's value. There are several compounding reasons for why this scale has become adopted as the standard: iron abundance in a galaxy increases roughly linearly with time through successive generations of stellar nucleosynthesis and supernova enrichment,