State-of-the-art cosmological hydrodynamical simulations have star particles with typical mass between $sim 10^8$ and $sim 10^3$ M$_{odot}$ according to resolution, and treat them as simple stellar populations. On the other hand, observations in nearby galaxies resolve individual stars and provide us with single star properties. An accurate and fair comparison between predictions from simulations and observations is a crucial task. We introduce a novel approach to consistently populate star particles with stars. We developed a technique to generate a theoretical catalogue of mock stars whose characteristics are derived from the properties of parent star particles from a cosmological simulation. Also, a library of stellar evolutionary tracks and synthetic spectra is used to mimic the photometric properties of mock stars. The aim of this tool is to produce a database of synthetic stars from the properties of parent star particles in simulations: such a database represents the observable stellar content of simulated galaxies and allows a comparison as accurate as possible with observations of resolved stellar populations. With this innovative approach we are able to provide a colour-magnitude diagram from a cosmological hydrodynamical simulation. This method is flexible and can be tailored to fit output of different codes used for cosmological simulations. Also, it is of paramount importance with ongoing survey data releases (e.g. GAIA and surveys of resolved stellar populations), and will be useful to predict properties of stars with peculiar chemical features and to compare predictions from hydrodynamical models with data of different tracers of stellar populations.
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