OB associations are loose groups of young, massive stars. They constitute the last stage of the massive starformation process, and the context in which new stars are formed. Although OB... Show moreOB associations are loose groups of young, massive stars. They constitute the last stage of the massive starformation process, and the context in which new stars are formed. Although OB associations have long been studied, it is not clear how they form and disperse in the field and what are the characteristics of their stellarpopulations. Further, not all O and B stars can be associated to groups or clusters, and a fraction of them moves at very high velocity: these are the so-called runaway stars.The data of the Gaia satellite are crucial to unravel the structure and star formation history of the young associations, as they allow to study their spatial structure, kinematics, and ages with unprecedented precision.In this thesis, I studied the properties of the stellar groups in the Orion OB association and the three dimensional arrangement of young stars in the solar neighbourhood, providing a view of the spatial configuration of young star forming regions within 500 pc from the Sun.I further analysed the properties of massive runaway stars, aiming at a better understanding of massive star evolution and of the dynamical processes occurring within dense stellar clusters Show less
