Formation of luminous matter in the Universe is a complicated process, which includes many processes and components. It is the vastly different scales involved in the process (from star formation on few parsec scales to galaxy clusters and superclusters on megaparsecs scales) and numerous ill-understood processes, which make the whole field a maze of unsolved, but exciting problems. We present new approximations for numerical treatment of multiphase ISM forming stars. The approximations were tested and calibrated using N-body+fluid numerical simulations. We specifically target issues related with effects of unresolved lumpinesses of the gas. We show that the degree of freedom is much smaller than naively expected because of self-regulating nature of the process of global star formation. One of the problems of numerical simulations is related with the uncertainties of approximation of the supernovae (SN) feedback. It is often assumed that the feedback is mainly due to momentum transferred by SN in to the ISM. We argue that this may not be true. We present a realistic example of gas actively forming stars with short cooling time for which the SF feedback is important, but the kinetic energy of the gas motion due to SN is negligibly small as compared with the thermal energy of gas.