In this talk, I will show that the variety of Kennicutt-Schmidt (KS)
relations reported in the literature, from molecular clouds in the Milky
Way (MW) to resolved and unresolved galaxies, is naturally explained by
the very collapse of the molecular clouds. I will also show that the low
values and the constancy of the “efficiency per free-fall time” arise
naturally when the clouds collapse. Our results imply that locally, star
formation is fast and efficient and that molecular cloud turbulence, the
pressure in the midplane of the galaxies, their rotation, or other
physical agents are not the primary drivers of the star formation law
but only a slight modification over the primary law, namely, that the
star formation rate is proportional to the mass in collapsing gas.
However, the filamentary structure of clouds is relevant in lowering the
star formation efficiency, and the stellar feedback is crucial to halt
star formation. I will also show the kinematic differences between young
stars born in collapsing vs born in turbulent molecular clouds and show
that Gaia data is consistent with stars born from collapsing clouds
rather than clouds supported by turbulence.
