Using the Hubble Space Telescope (HST), astronomers have observed a supermassive galactic open cluster designated Westerlund 1. Results of the study, published Jan. 28 on the pre-print server arXiv, yield essential details regarding the structure of this cluster.
Open clusters (OCs), formed from the same giant molecular cloud, are groups of stars loosely gravitationally bound to each other. It is assumed that most star formation takes place in massive clusters of stars, known as superstar clusters (SSCs). They are very massive young OCs usually containing a very large number of young, massive stars. The total mass of a typical SSC exceeds 10,000 solar masses.
Westerlund 1 is a supermassive SSC located some 13,800 light years away. It has an estimated total mass of 50,000–100,000 solar masses and a radius of about 3.26 light years. With an estimated age of 5–10 million years, Westerlund 1 is one of the most massive young star clusters in our galaxy.
Due to its youth, proximity, and rich population of stars across a wide range of masses, Westerlund 1 is an excellent site to study a starburst environment in detail. That is why a team of astronomers led by Lingfeng Wei of the University of California, San Diego (UCSD) decided to explore this cluster with HST.
“Westerlund 1 was observed with the Hubble Space Telescope at multiple epochs to measure proper motions (PMs) and in multiple filters to obtain multi-color photometry,” the researchers wrote in the paper.
Hubble allowed Wei’s team to measure proper motions of 10,346 observed stars and to determine their kinematic cluster memberships. Afterward, the astronomers constructed the stellar density map of Westerlund 1 corrected by a spatial reddening map and cluster membership of each star.
Based on the stellar density map, the researchers found that Westerlund 1 is elongated in a northeast-southwest direction, aligning with the galactic plane. The cluster has an eccentricity of 0.71 and a semi-major axis at a position angle of about 56 degrees east of north, which aligns with the galactic plane. Moreover, it turned out that the eccentricity decreases with increasing mass.
The astronomers assume that the high eccentricity of Westerlund 1 may be inherited from the molecular cloud from which the cluster formed, or it could be a result of a formation process during which multiple substructures merged.
Furthermore, the study found that Westerlund 1 has a velocity dispersion of approximately 3.42 km/s, which is smaller than the values expected for the cluster if it were in virial equilibrium. This indicates that Westerlund 1 is subvirial, which points to an exceptionally high star formation efficiency (likely above 56% at its formation) or inefficient stellar feedback driven by local gas expulsion before stars reach the cluster.
The study also calculated the crossing and relaxation times for Westerlund 1, which were found to be 300,000 years and 260 million years, respectively. According to the authors of the paper, Westerlund 1 is 10.7 million years old, which, together with crossing and relaxation times, means mass segregation for the stars in the cluster with mass down to 10 solar masses. https://phys.org/news/2025-02-westerlund-star-cluster-hubble-unveils.html
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