  Recent surveys of star forming regions have shown that most stars, and
probably all massive stars, are born in dense stellar clusters. The mechanism
by which a molecular cloud fragments to form several hundred to thousands of
individual stars has remained elusive. Here, we use a numerical simulation to
follow the fragmentation of a turbulent molecular cloud and the subsequent
formation and early evolution of a stellar cluster containing more than 400
stars. We show that the stellar cluster forms through the hierarchical
fragmentation of a turbulent molecular cloud. This leads to the formation of
many small subclusters which interact and merge to form the final stellar
cluster. The hierarchical nature of the cluster formation has serious
implications in terms of the properties of the new-born stars. The higher
number-density of stars in subclusters, compared to a more uniform distribution
arising from a monolithic formation, results in closer and more frequent
dynamical interactions. Such close interactions can truncate circumstellar
discs, harden existing binaries, and potentially liberate a population of
planets. We estimate that at least one-third of all stars, and most massive
stars, suffer such disruptive interactions.
