Massive stars are found, and, because of their age, are probably formed, in cluster cores. When the stars begin to shine, strong stellar winds of ≳ 103 km s-1 exert a tremendous force on any remaining gas, which has at the same time been heated and expands at the sound speed (~10 km s-1). The gas will be rapidly expelled from cluster core in much less than a core crossing time. This reduces the potential of the core, dramatically affecting the dynamics of the core stars. This process is simulated using NBODY2 to follow the distributions of massive stars over a few crossing times for various initial gas fractions. The core of massive stars is found to remain intact for a core gas fraction of 4 (or equivalently, a star formation effeciency of 20%). The space and velocity distributions are also investigated.