Astronomers using the Hubble Space Telescope (HST) have studied one of the largest young star clusters in our galaxy, Westerlund 1. Their findings, recently published on the pre-print server arXiv, provide new insights into the cluster’s shape, movement, and formation history.
Star clusters are groups of stars that form from the same giant cloud of gas and dust. Open clusters, like Westerlund 1, are loosely bound groups of stars that share a common origin. Some of the largest and youngest of these are called superstar clusters (SSCs), which contain thousands of massive stars and have total masses exceeding 10,000 times that of our Sun.
Westerlund 1 is one of the most massive SSCs in our Milky Way. Located about 13,800 light-years from Earth, it has a mass between 50,000 and 100,000 times that of the Sun and spans about 3.26 light-years in radius. It is relatively young, estimated to be between 5 and 10 million years old. Because of its rich variety of stars and its proximity, it is an ideal place to study how young, massive star clusters evolve.
To learn more about Westerlund 1, a team led by Lingfeng Wei from the University of California, San Diego, analyzed images from Hubble taken over multiple years. They measured the movements of 10,346 stars in the cluster and created a detailed map of its structure.
Their analysis showed that Westerlund 1 is not perfectly round but stretched out in a northeast-southwest direction, aligning with the plane of the Milky Way. It has a high eccentricity of 0.71, meaning it is more oval-shaped than circular. Interestingly, they found that this eccentricity decreases for more massive stars, which tend to be more evenly distributed.
The researchers suggest that this shape might be due to the way the cluster originally formed. It could have inherited its elongated structure from the cloud of gas and dust that gave birth to it. Another possibility is that smaller star-forming regions merged together, creating the overall shape seen today.
The study also measured how fast stars are moving within the cluster. They found that Westerlund 1 has a velocity dispersion of 3.42 km/s—lower than what would be expected if the cluster were in a stable gravitational balance, also known as virial equilibrium.
This suggests that the cluster is “subvirial,” meaning it may have formed with an unusually high efficiency. The team estimates that more than 56% of the original gas that formed the stars was converted into stars, or that gas was expelled from the cluster in a way that did not disrupt its structure.
The researchers also calculated two important timescales for the cluster. The crossing time, which is how long it takes a typical star to travel across the cluster, is about 300,000 years. The relaxation time, which measures how long it takes for the cluster to settle into a stable state, is much longer—about 260 million years.
Based on their findings, the team estimates that Westerlund 1 is about 10.7 million years old. These numbers suggest that the cluster has already undergone significant evolution, with the heaviest stars moving toward the center while lighter ones spread outward.
Analysis and Implications
This study provides important new details about Westerlund 1, improving our understanding of massive young star clusters. The cluster’s elongated shape hints at its chaotic formation history, while its relatively low internal velocity suggests it formed with high efficiency or experienced minimal disruption after its birth.
One of the most interesting findings is that heavier stars are more evenly distributed compared to lighter stars. This supports the idea of mass segregation, a process where more massive stars gradually move toward the center over time while lighter stars drift outward.
The fact that Westerlund 1 is subvirial suggests that it either formed in an exceptionally efficient way or that gas expulsion did not significantly disrupt it. If true, this could challenge previous ideas about how massive star clusters evolve.
Further observations, especially using infrared and radio telescopes, could help clarify how Westerlund 1 formed and whether similar processes shape other young star clusters. Studying clusters like this can also provide insight into the early history of galaxies, where such massive clusters were common.
By using Hubble’s ability to track stellar movements with incredible precision, this research brings us one step closer to understanding the complex lives of massive star clusters—and the forces that shape them.
The research findings can be found in arXiv.
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