Globular clusters are often described as ancient, tightly bound fossils of the early Milky Way. It has been believed that these dense balls of stars have survived and remained unchanged for billions of years. 

However, new observations suggest that at least some of them are far from stable. Astronomers have now caught one such cluster, NGC 6569, actively losing its stars to the Milky Way’s central bulge. 

Using detailed stellar fingerprints, the study reveals that this 13-billion-year-old cluster is slowly being torn apart by the galaxy’s gravity, feeding stars into the surrounding bulge at a measurable rate. 

“We have conducted the first wide-field, medium-resolution spectroscopic survey of NGC 6569. Our findings provide evidence that NGC 6569 is actively losing stars through tidal stripping,” the study authors note.

This discovery provides rare, direct evidence of how globular clusters dissolve over time and reshape the stellar population of the Milky Way.

Detecting the cluster’s hidden star loss

NGC 6569 lies about 35,500 light-years from Earth, deep inside the Milky Way’s crowded central bulge (a region of our galaxy where millions of stars are squeezed into a small space). It is relatively massive, about 230,000 times the mass of the Sun, and contains stars richer in heavy elements than many other globular clusters. 

Studying such clusters is difficult because the bulge is packed with stars, making it hard to tell which stars belong to the cluster and which are just passing by. 

To overcome this challenge, the researchers used the Anglo-Australian Telescope (AAT) as part of the Milky Way Bulge Extra-Tidal Star Survey (MWBest). The goal of this survey is to understand how globular clusters slowly break apart under the strong gravitational forces near the center of the galaxy.

The researchers collected medium-resolution spectra of 303 stars in and around NGC 6569. These spectra reveal both the chemical composition of stars and how fast they are moving. 

By combining this information with precise motion data, the team could identify stars that once belonged to the cluster but are now drifting away.

How much loss is happening?

This approach allowed the astronomers to identify 40 stars located well outside the cluster’s main body, between 7 and 30 arcminutes from its center, that share the same chemical and motion signatures as NGC 6569. 

These stars are interpreted as genuine tidal debris—stars that have been pulled out of the cluster by the Milky Way’s gravitational tides. Five of these stars appear to form a faint halo of debris surrounding the cluster.

The team also compared stars still bound to the cluster with the surrounding field stars. They found that about 35 percent of nearby stars share NGC 6569’s motion, suggesting the cluster is embedded in its own stripped-off material. 

By combining chemical data with dynamical modeling, the researchers estimated that NGC 6569 is losing stars at a rate equal to the mass of about one to one-and-a-half Suns every million years, which adds up to about 5.6 percent of the cluster’s total mass over a billion years.

Why this globular cluster discovery matters

These findings help solve a long-standing puzzle in astronomy. Theoretical models have long predicted that globular clusters near the galactic center should lose most of their mass over time, but clear observational evidence has been rare, especially for clusters in the bulge. 

While about a quarter of halo globular clusters show tidal tails, such features are seldom detected in bulge clusters due to extreme crowding and dust. This study shows that tidal stripping is not just possible in the bulge; it is actively happening.

The results also suggest that globular clusters like NGC 6569 play an important role in building up the Milky Way’s bulge by steadily donating stars to it. 

However, the study also has limits. The idea that the cluster is moving through a tube of its own tidal debris remains a hypothesis and needs to be tested with detailed N-body simulations that can model the cluster’s long-term evolution.

Next, the researchers plan to expand this chemo-dynamical approach to other bulge clusters. By doing so, they hope to build a clearer picture of how many globular clusters are slowly dissolving—and how much of the Milky Way’s central stellar population comes from these ancient star systems. 

“Extending this approach to the full MWBest sample will ultimately clarify the cumulative role of GC-dissolution in building the Galactic bulge,” the study authors concluded.

The study is published in arXiv.