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Galactic star formation occurs at a surprisingly low rate.
Yet, recent large-scale surveys of dark clouds in the Galaxy show that one
rarely finds molecular clouds without young stellar objects. Thus, star
formation must occur rapidly upon molecular cloud formation.
I will argue that this rapid onset of star formation poses stringent
constraints on the physical properties and on the boundary conditions of
the parental molecular cloud, which in turn requires a closer study
of the cloud formation process. I will discuss a series of numerical
experiments on cloud formation, demonstrating that there is no need
(and no way) for turbulent support, but that the observed turbulence
in molecular clouds is a consequence of global gravitationally driven
motions and of the cloud formation process itself. The role of the magnetic
field seems to be in parts consistent with a classical picture of support, with
the difference that it only plays a role in the diffuse molecular gas, but
not in the dense filaments. I will discuss these findings in the context
of recent observations.
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