We discuss an extended set of Tree+SPH (smoothed particle hydrodynamics) simulations of the formation of clusters of galaxies, with the goal of investigating the interplay between numerical resolution effects and star formation/feedback processes. Our simulations were all carried out in a concordance Λ cold dark matter (ΛCDM) cosmology and include radiative cooling, star formation and energy feedback from galactic winds. The simulated clusters span the mass range Mvir~= (0.1-2.3) × 1015h-1Msolar, with mass resolution varying by several decades. At the highest achieved resolution, the mass of gas particles is mgas~= 1.5 × 107h-1Msolar, which allows us to resolve the virial region of a Virgo-like cluster with more than two million gas particles and with at least as many dark matter (DM) particles. Our resolution study confirms that, in the absence of an efficient feedback mechanism, runaway cooling leads to about 35 per cent of baryons in clusters to be locked up in long lived stars at o
