Stars generally form through a fragmented collapse of a molecular cloud as a group or cluster.
This simple picture seems to be generally true but bears many unsolved details. The complete initial mass function of such groups is still unknown. The formation of low-mass stars is presently unclear. How does the formation of the high-mass stars in a group influence the ongoing star formation. Unfortunately, the early stages of star formation, where all these processes run and stellar masses are determined, cannot be observed easily.