The unique feature of Westerlund 1 lies in its large, dense, and diverse population of massive stars, which is unmatched in other clusters of the Milky Way. It hosts a full range of star classifications: from Wolf-Rayet astronomical objects to yellow hypergiants.
Westerlund 1 contains so many massive stars that astronomers believe over 1500 supernovae will explode within a period of less than 40 million years.