A Colossal Gamma-Ray Bubble Erupts from the Milky Way's Brightest Star Cluster

A colossal bubble of gamma rays is billowing into space beneath our galaxy's disk. The source is Westerlund 1, the Milky Way's most massive and luminous super star cluster. Located 12,000 light-years away in the constellation Ara, this cluster is now the first of its kind observed creating such an outflow in gamma rays.

The discovery marks a breakthrough in understanding how cosmic rays shape galaxies. These powerful outflows help control star formation, fuel galactic winds, and spread heavy elements across vast distances.

Westerlund 1 is blowing a gamma-ray bubble that stretches 650 light-years from the cluster. This outflow is roughly 200 times larger than the cluster itself. Unlike typical expansions, it is growing downward, following the path of least resistance into lower-density regions beneath the galactic plane.

Scientists have long studied similar phenomena in other star clusters. Around 20-30 known globular clusters in the Milky Way have been examined for gamma-ray outflows, mainly using data from the Fermi-LAT telescope up to 2025. So far, only the globular cluster 47 Tucanae has shown confirmed gamma-ray emissions from millisecond pulsar winds. Most others have revealed only upper limits on such activity.

This detection is the first time a super star cluster has been caught producing a gamma-ray outflow. The findings highlight how such energetic processes influence the structure and evolution of galaxies.

The gamma-ray bubble from Westerlund 1 provides direct evidence of how massive star clusters interact with their surroundings. By driving these outflows, they help regulate star formation and distribute essential elements across the galaxy. Further observations could reveal whether similar processes occur in other clusters, deepening our understanding of cosmic ray dynamics in the Milky Way.