  The formation of stars shapes the structure and evolution of entire galaxies.
The rate and efficiency of this process are affected substantially by the
density structure of the individual molecular clouds in which stars form. The
most fundamental measure of this structure is the probability density function
of volume densities (rho-PDF), which determines the star formation rates
predicted with analytical models. This function has remained unconstrained by
observations. We have developed an approach to quantify rho-PDFs and establish
their relation to star formation. The rho-PDFs instigate a density threshold of
star formation and allow us to quantify the star formation efficiency above it.
The rho-PDFs provide new constraints for star formation theories and correctly
predict several key properties of the star-forming interstellar medium.
