106. The remarkable cluster Westerlund 1

At a distance of about 4 kpc, the young compact star cluster Westerlund 1 contains many rare, evolved, high-mass stars, including red supergiants, yellow hypergiants, one of the largest known stars, and 24 Wolf-Rayet stars. Gaia is pinpointing its distance and its members, and resolving the puzzling question of its age.