Astronomers have observed "droplets," "columns," and elongated shells in the Milky Way's largest star cluster.

Bright massive stars have a short and turbulent lifespan. In clusters, the distances between objects are small, allowing stellar winds and the material ejected by these stars to interact actively, creating regions conducive to the formation of new generations of stars. The more massive the cluster, the greater the amount of matter available for these processes.

Young star clusters with a mass exceeding ten thousand solar masses are commonly referred to as supermassive. Such clusters are frequently found in galaxies undergoing a burst of star formation. In our Milky Way, fewer than 10 remain, all located more than eight thousand light-years away from Earth. The vast distances and high density of very bright stars hinder our ability to detect less massive stars and faint structures in their interstellar space. The James Webb Space Telescope has significantly enhanced observational capabilities.

The Westerlund 1 cluster, situated 13,800 light-years from us, features a rich assortment of massive stars across various spectral classes at different stages of evolution. It hosts 24 Wolf-Rayet stars, several blue and yellow supergiants, four red supergiants, a bright blue variable star, and over a hundred hot, massive blue stars of spectral classes O and B. There are no analogs to the Westerlund 1 cluster in the Milky Way.

Astronomers have yet to reach a consensus on the age and mass of the Westerlund 1 cluster. Early estimates suggest it may be around five million years old, with a mass potentially exceeding hundreds of thousands of solar masses, making it the most massive star cluster in the Milky Way. Recent assessments indicate an age of about 10 million years, which would imply a lower mass. Regardless, it impresses with its diversity of massive stars.

A total of 23.6 hours of observation time with the James Webb Space Telescope was allocated to this remarkable object. In a paper published openly on the arXiv website, astronomers presented new results from their data analysis.

Scientists were particularly surprised by the abundance of bright and dense nebulae surrounding the center of the Westerlund 1 cluster. The bright massive stars often “blow away” material from the interstellar medium, creating “holes.” This phenomenon is also observed in younger and less massive clusters.

The authors of the study identified several regions of notable “cloudiness” in Westerlund 1. The “Eastern droplets” are elongated toward the massive stars in the eastern group and toward the cluster's center. In the “Western clouds,” there are significantly fewer such “droplets,” yet there is a “pillar” directed toward the cluster's center.

Judging by the distortion of the “central clouds,” they are influenced by massive stars from all sides. Additionally, around the oldest massive stars, astronomers observed ejected shells and outflows of material.

The abundance of nebulae in the cluster raises questions about their nature. According to the authors' hypotheses, these may be remnants of the molecular cloud from which the cluster formed, ejected material from massive stars, or remnants of supernovae.

It is unlikely that material from the molecular cloud could have persisted in clumps so close to the center of the Westerlund 1 cluster. However, as noted by astronomers, the “pillar” extending about 3.2 light-years in the western region stands out against the other nebulae. It may indeed have a direct connection to the molecular cloud.

The study of the Westerlund 1 cluster is conducted as part of the EWOCS program, aimed at investigating the evolution of stars and planets in regions of active star formation, focusing on the nearest young massive star clusters, Westerlund 1 and Westerlund 2. The goal of the new observational campaign was to obtain sufficiently accurate data to isolate low-mass stars and even brown dwarfs within the clusters. More detailed analyses of all notable features and objects in the Westerlund 1 cluster will be published in forthcoming articles.