We argue that massive stars are the dominant sources of energy for the turbulent motions within giant molecular clouds , and that the primary agent of feedback is the expansion of H II regions within the cloud volume .
This conclusion is suggested by the low efficiency of star formation and corroborated by dynamical models of H II regions .
We evaluate the turbulent energy input rate in clouds more massive than 3.7 \times 10 ^ { 5 } solar masses , for which gravity does not significantly affect the expansion of H II regions .
Such clouds achieve a balance between the decay of turbulent energy and its regeneration in H II regions ; summed over clouds , the implied ionizing luminosity and star formation rate are roughly consistent with the Galactic total .
H II regions also photoevaporate their clouds : we derive cloud destruction times somewhat shorter than those estimated by Williams and McKee .
The upper mass limit for molecular clouds in the Milky Way may derive from the fact that larger clouds would destroy themselves in less than one crossing time .
The conditions within starburst galaxies do not permit giant molecular clouds to be supported or destroyed by H II regions .
This should lead to rapid cloud collapse and the efficient formation of massive star clusters , which may explain some aspects of the starburst phenomenon .