What is the most massive young star cluster known in the Milky Way?

The most massive young star cluster known in the Milky Way is Westerlund 1.

Also called Wd1 or the Ara Cluster, Westerlund 1 lies roughly 12,000 light-years away in the southern constellation Ara. Astronomer Bengt Westerlund discovered it in 1961, but thick interstellar dust made the cluster difficult to study for decades.

The cluster is exceptionally rich in massive stars, including Wolf–Rayet stars, red supergiants, yellow hypergiants, and other short-lived stellar giants. Its estimated mass is tens of thousands of times the Sun’s mass, with some studies placing its original mass near 100,000 solar masses. These stars likely formed in a single burst about 3.5–5 million years ago.

A common mix-up is the Arches Cluster, which is younger and contains some of the most massive individual stars in the Galaxy. Westerlund 1 is distinguished by its overall population and total mass. Astronomers expect it may eventually evolve into something resembling a globular cluster.