Abstract
Photoionization by massive stars creates expanding H II regions, which eventually evaporate most of the gas in a star-forming molecular cloud. This is a very efficient cloud destruction mechanism that can also control the maximum number of stars that are formed within a molecular cloud. The resulting star-formation efficiency at low galactic pressures has an overall value of about 5%, but this efficiency value can increase above 50% in the inner regions of star-forming galaxies.
