However, not just any molecular cloud will collapse.  
The current theory is that stars form from the collapse of molecular clouds. However, not just any molecular cloud will collapse.  The mass at which a given molecular cloud will collapse within a radius R or the density at which a cloud will collapse are both known by names that commemorate the same person, who also corrected a small error in Rayleigh's radiation law. For ten points, name either the mass or the density at which a molecular cloud will begin to collapse.
The current theory is that stars form from the collapse of molecular clouds.
Jeans Mass or Jeans Density