An international team of astronomers, utilizing the power of the James Webb Space Telescope (JWST), has made significant progress in understanding the supermassive Galactic open cluster known as Westerlund 1. The results of their observational campaign, detailed in a paper published on the arXiv preprint server on November 20, provide valuable insights into the structure, properties, and surrounding environment of this exceptional cluster.
Open clusters (OCs) are groups of stars that form from the same giant molecular cloud and remain loosely bound together by gravity. The Milky Way is home to over 1,000 known open clusters, with more being discovered regularly. These clusters play a crucial role in understanding stellar formation, evolution, and the dynamics of our galaxy. While most OCs are relatively small, some larger, more massive clusters—called superstar clusters (SSCs)—contain a high concentration of young, massive stars and exhibit significantly larger masses. Typical SSCs have masses exceeding 10,000 solar masses, making them vital for understanding star formation on a galactic scale.
Westerlund 1, located approximately 13,800 light-years from Earth, is one of the most massive and intriguing SSCs known in the Milky Way. With an estimated mass ranging from 50,000 to 100,000 solar masses and a radius of about 3.26 light-years, Westerlund 1 is believed to be the largest and most massive star cluster in our galaxy. It contains a mix of massive stars, including Wolf-Rayet stars, supergiants, and potentially the most massive stars in the galaxy. The cluster is relatively young, with an estimated age of only 5–10 million years, which is quite young in cosmic terms. These characteristics make Westerlund 1 an excellent target for studying star formation and the evolution of massive clusters.
To investigate this stellar behemoth, a team of astronomers led by Mario Giuseppe Guarcello of the Palermo Astronomical Observatory in Italy turned to JWST’s Mid-Infrared Instrument (MIRI) and Near Infrared Camera (NIRCam). These advanced instruments are capable of capturing high-resolution infrared images, which are essential for observing the cooler and more obscured regions of the cluster, such as brown dwarfs and protoplanetary disks, that are often hidden from view in visible light.
The team’s observations revealed a diffuse nebulosity surrounding the core of Westerlund 1, likely a product of the intense radiation and stellar winds produced by the cluster’s massive stars. This nebulosity appeared to be composed of droplet-like features that point toward the massive stars in the cluster. Further analysis of the surrounding nebula identified an elongated trunk stretching about 3.3 light-years toward the center of the cluster. This structure is thought to be influenced by the powerful outflows and radiation from the cluster’s massive stars, which shape the surrounding interstellar material.
In addition to the large-scale nebulosity, the team also detected a small group of cloud fragments that wrap around the group of massive stars. These fragments may be sites of ongoing star formation, where new stars and planetary systems could be emerging from the dense gas and dust. This discovery highlights the dynamic nature of Westerlund 1, where stellar feedback from the massive stars influences the surrounding environment, potentially triggering the formation of new stars and planets.
One of the most interesting findings was the detection of extended shells around several M-type supergiants in the cluster. These shells appear as large, spherical structures surrounding the stars. In some cases, the shells were elongated in the opposite direction relative to the cluster center, suggesting that the stars’ winds and radiation may be shaping the surrounding material in complex ways. In other cases, narrow outflows were detected, possibly indicating the presence of massive stellar winds or other types of material ejected from the supergiants.
These findings are significant because they provide new insights into the interactions between massive stars and their surrounding environments. The feedback from massive stars, including strong radiation and stellar winds, plays a crucial role in shaping the interstellar medium and influencing the formation of new stars and planetary systems. In the case of Westerlund 1, the JWST observations suggest that the cluster is an active site of stellar evolution, where the feedback from young, massive stars is actively shaping the surrounding nebula and potentially triggering further star formation.
The team’s observations also offer new opportunities to study the lower mass members of the cluster, including brown dwarfs and stars at the edge of the cluster’s mass range. These objects are important for understanding the full spectrum of star formation in such massive clusters, as they may provide clues to the initial conditions and processes that govern the formation of both massive and low-mass stars. The JWST’s sensitivity to infrared light makes it uniquely suited for studying these cooler, fainter objects that are typically difficult to observe with optical telescopes.