Young massive clusters in the Gaia era

After the detection of gravitational waves and the realisation of the wide diversity among core-collapse supernova explosions, Interest in massive stars has been rekindled. Young open clusters are our natural laboratories to explore the lives and deaths of massive stars, and most of our current understanding comes from their study. Until recently, we have been strongly limited by practical difficulties and small number statistics. Most open clusters contain only a handful of evolved massive stars, while we have to probe a vast parameter space in terms of metallicity, age, initial rotational velocity and binary properties. Very massive clusters, with larger populations, are rare and, in most cases, difficult to observe. In the past five years, however, the Gaia mission has brought a global revolution for the study of open clusters. Exquisite proper motions can be used to determine bulk properties and identify large samples of certain cluster members. Accurate distances allow us to beat extinction and gain a much better handle on cluster ages. Although high-quality astrometric data are still limited to relatively nearby (up to 2-3 kpc) clusters, this revolution is allowing us to look back to our old photometry and spectroscopy and extract enormous amounts of valuable information that laid hidden. I will show some examples of ways in which this wealth of information can be exploited to characterise young massive open clusters and effectively use them to constrain the evolution of massive stars.