Abstract
The study of the very young open cluster NGC 6231 clearly shows the presence of a mass-segregation for the most massive stars. These observations, combined with those concerning other young objects, and very recent numerical simulations, strongly support the hypothesis of an initial origin for the mass-segregation of the most massive stars. These results led to the conclusion that massive stars form near the center of clusters. They are strong constraints for scenarios of star and stellar cluster formation.
