Stars are generally not formed in isolation, but rather in clusters of hundreds or even thousands. Such clusters form all their stars at the same time and from the same gas cloud, meaning that all of the stars have the same age and metallicity. This is known as a ‘single stellar population’ (SSP).

Modelling the properties (colours, luminosities and spectra) of SSPs is important for research into the stellar populations of galaxies since, in theory, we can reconstruct the complex histories of these galaxies by combining individual SSPs. However, the usefulness of SSP models to date has been limited by age-metallicity degeneracy.