Young massive star clusters are systems of stars with more than about ten thousand solar-masses of fabric and ages less than about one hundred million years that are gravitationally bound together. In these clusters the stellar densities can reach and even exceed the densities found in stellar globular clusters,more evolved systems that host hundreds of thousands of stars in volumes only tens of light-years across. Observations suggest that the young massive clusters near in all sizes: their mass distribution ranges from low mass, open clusters with about one hundred solar-masses to tall-mass ones with a hundred million solar-masses.
Source: phys.org