  Star formation is inefficient. Only a few percent of the available gas in
molecular clouds forms stars, leading to the observed low star formation rate
(SFR). The same holds when averaged over many molecular clouds, such that the
SFR of whole galaxies is again surprisingly low. Indeed, considering the low
temperatures, molecular clouds should be highly gravitationally unstable and
collapse on their global mean freefall timescale. And yet, they are observed to
live about 10-100 times longer, i.e., the SFR per freefall time is only a few
percent. Thus, other physical mechanisms must provide support against quick
global collapse. Magnetic fields, turbulence and stellar feedback have been
proposed as stabilising agents controlling star formation, but it is still
unclear which of these processes is the most important and what their relative
contributions are. Here I present high-resolution, adaptive-mesh-refinement
simulations of star cluster formation that include turbulence, magnetic fields,
and protostellar jet/outflow feedback. These simulations produce nearly
realistic star formation rates consistent with observations, but only if
turbulence, magnetic fields and feedback are included simultaneously.
