Massive Galactic clusters (> 1000 M(sun)) exhibit a clear correlation between cluster density, size and age and can be sorted in two categories, i.e. starburst and leaky clusters. The reason for the existance of two types of massive clusters is an open question. However, the answer is probably connected to a different formation histories of the two types.
In this study we concentrate onleaky clusters only and investigate possible formation scenarios and gas expulsion phase.
This is done by using existing observational data and numerical results of embedded cluster properties.
Assuming that a clear correlation between cluster density, size and age exists, it is shown that the density-radius development over time for embedded clusters can be approximated by ρ ≈ 100*r-1.3M(sun) pc-3. The consequences for the star formation process in leaky clusters are discussed and found to favour an inside-out star formation scenario with an initially low but later accelerated star formation rate. It is shown how the leaky clusters form in a unique sequential manner and that rapid gas expulsion is responsible for the 80-90% mass loss over the next 20 Myr.