The Fundamental Building Blocks of Galaxies
  Most stars form in compact, dense embedded clusters with memberships ranging from a dozen stars to many millions of stars. Embedded clusters containing more than a few hundred stars also contain O stars that disrupt the nebula abruptly. There are observational indications that the expulsion of gas from such clusters may occur within, or even quicker than, a stellar crossing-time scale. Because the star-formation efficiency is empirically deduced to be typically lower than 40 per cent, such clusters are left severely out of dynamical equilibrium with a super-virial velocity dispersion when they are exposed. As a consequence, they expand drastically.   The velocity dispersion of expanding star-burst clusters may achieve values of a few tens km/s, which would have a significant impact on the stellar velocity distribution function and therefore the morphology of a galaxy. An application of these ideas to the vertical structure of the Milky Way (MW) disk suggests that such popping star clusters may constitute the missing heat source for understanding the age-velocity-dispersion relation. If the early MW disk experienced a vigorous epoch of star formation then perhaps even the thick disk may have formed from popping clusters. Thus we have an alternative hypothesis to understanding thick disks which does not rely on, or may act in addition to, sinking galaxy satellites or sinking cosmological sub-structures. The GAIA mission will test this hypothesis by allowing the kinematical fields around young clusters to be mapped to exquisite precision. 
Pavel Kroupa (UniBonn)