  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.
