The exotic stellar population of Westerlund 1

[Image Description: A dense cluster of bright stars, each with six large and two small diffraction spikes, due to the telescope’s optics. They have a variety of sizes depending on their brightness and distance from us in the cluster, and different colours reflecting different types of star. Patches of billowing red gas can be seen in and around the cluster, lit up by the stars. Small stars in the cluster blend into a background of distant stars and galaxies on black.]