Astronomers refer to all the chemical elements heavier than hydrogen and helium as metals, even though this includes elements such as carbon and oxygen which are not considered metals in the normal sense. The metallicity of a star is therefore specified as the fraction of the star’s mass composed of these ‘metals’ – approximately 2% in the case of the Sun.

However, this definition fails to completely define the chemical composition of a star because the same metallicity can be achieved in an almost infinite number of ways. For example, a star with any given metallicity could have all of its metals as iron, while another with the same overall metallicity, could have all of its metals as oxygen. In the real world, the whole periodic table of elements is present in each star, but the relative amounts of each element varies from star to star.

In order to quantify the relative amounts of individual elements present in a star, astronomers define an ‘abundance ratio’ as the logarithm of the ratio of two metallic elements in a star relative to their ratio in the Sun. For example, the abundance ratio of magnesium to iron (written [Mg/Fe]) is defined as the logarithm of the magnesium to iron ratio in a star compared to the magnesium to iron ratio in the Sun. The following table illustrates how this works:

Chemical elements are produced through a variety of processes, meaning that abundance ratios contain useful information regarding the source of the gases making up the star. Magnesium (and other ‘light’ elements such as C and O) are produced in Type II supernovae (SNII; the explosions of massive stars), while ‘iron peak’ elements (Fe, Ni, Zn, Co, Mn, Cr) are produced in Type Ia supernovae (SNIa; the explosion of a white dwarf star in a binary system). For any given population of stars, the different types of supernova will explode at different times. This is because the massive stars that explode as SNII have short lives, while the white dwarfs that explode as SNIa are the end product of the stellar evolution of long-lived, low to intermediate mass stars. The elements produced in the different explosions are therefore incorporated into stars at different epochs during the star formation history of the galaxy, and the abundance ratios of individual stars can give us strong clues about their ages.