  Direct N-body calculations are presented of the early evolution of exposed
clusters to quantify the influence of gas expulsion on the time-varying surface
brightness. By assuming that the embedded OB stars drive out most of the gas
after a given time delay, the change of the surface brightness of expanding
star clusters is studied. The influence of stellar dynamics and stellar
evolution is discussed. The growth of the core radii of such models shows a
remarkable core re-virialisation. The decrease of the surface mass density
during gas expulsion is large and is only truncated by this re-virialisation
process. However, the surface brightness within a certain radius does not
increase noticeably. Thus, an embedded star cluster cannot reappear in
observational surveys after re-virialisation. This finding has a bearing on the
observed infant mortality fraction.
