Imagine a cosmic tug of war that has been playing out for billions of years, where our galaxy's immense gravity slowly pulls apart some of its oldest stellar neighborhoods. For the first time, astronomers have captured clear visual evidence of this dramatic process, thanks to the European Space Agency's cutting edge Euclid space telescope.

Ancient Cities of Stars Under Siege

Globular clusters are like the grand old cities of our galaxy. These spectacular spherical collections of stars can contain hundreds of thousands of suns packed into regions just a few dozen light years across. What makes them truly special is their age: many formed when the universe was still young, making them nearly 13 billion years old.

Scientists have long suspected that these stellar cities don't remain intact forever. Just as ocean tides are caused by the Moon's gravitational pull on Earth, the Milky Way's gravity should create "tides" that stretch and eventually tear apart globular clusters. However, actually seeing this process in action has been remarkably difficult until now.

In September 2023, Euclid set its sights on two of these ancient stellar systems: NGC 6254 and NGC 6397. What it found has given astronomers their clearest view yet of how galaxies slowly cannibalize their own star clusters.

Why This Discovery Matters

Think of globular clusters as cosmic fossils. They preserve information about the early universe like ancient artifacts buried in archaeological sites. Understanding how they evolve and eventually dissolve helps scientists piece together the history of galaxy formation and the fundamental laws governing how gravity shapes the universe.

For decades, researchers have searched for evidence that globular clusters develop "tidal tails," long streams of stars pulled away by gravitational forces. These tails are incredibly faint and spread across vast regions of sky, making them extremely challenging to detect. It's like trying to spot a whisper thin thread stretched across a football field while standing at one end.

A Telescope Built for the Task

Euclid brings a unique set of capabilities to this challenge. Unlike previous space telescopes that excel at either capturing wide views or zooming in for detailed close ups, Euclid does both simultaneously. Its cameras can image an area of sky nearly as large as the full Moon while maintaining sharp enough resolution to distinguish individual stars.

The telescope observes in both visible light and near infrared wavelengths, capturing images through four different filters. This multi color vision allows astronomers to distinguish between actual cluster members and the millions of other stars that happen to lie in the same direction but are much closer or farther away.

Catching NGC 6254 in the Act

When researchers analyzed the Euclid images of NGC 6254, located about 5,000 light years from Earth, they discovered something remarkable. By carefully mapping where member stars were located and filtering out background galaxies and unrelated foreground stars, they created detailed maps showing the cluster's shape.

In the inner regions, NGC 6254 appears nearly round, just as you would expect for a stable ball of stars held together by their mutual gravity. But farther out, the story changes dramatically. The cluster's outer edges become distinctly elongated, stretched out in a northwest direction like taffy being pulled.

This distortion becomes progressively more extreme at greater distances from the cluster's center. At about 18 minutes of arc from the core (roughly half the apparent width of the full Moon), the elongation reaches its maximum, with the cluster stretched into an elliptical shape with an axis ratio of about 0.85 to 1.

When Simulations Meet Reality

To verify what they were seeing, the research team created computer simulations that modeled NGC 6254's journey through the Milky Way over the past 2 billion years. These simulations tracked hundreds of thousands of virtual stars, calculating how the galaxy's gravitational field would affect them over time.

The match between simulation and observation was striking. The computer models predicted that NGC 6254 should develop elongation in the same northwest direction observed in the real data, and that this stretching should become apparent at approximately the same distance from the cluster center.

This agreement provides powerful confirmation that what Euclid is seeing represents actual tidal distortion caused by the Milky Way's gravity. The cluster is being pulled apart, and these observations have caught it in the act.

Reading the Stellar Census

Beyond revealing the cluster's shape, Euclid's sharp vision allowed astronomers to create extraordinarily detailed stellar censuses. By measuring both the brightness and color of individual stars, they constructed what astronomers call color magnitude diagrams, essentially maps showing what types of stars live in the cluster.

These diagrams revealed the cluster's stellar population in unprecedented detail, from relatively bright stars down to dim objects with just 15% the mass of our Sun. The data quality was so good that researchers could detect subtle color variations among faint stars, possibly related to an intriguing phenomenon where globular clusters contain multiple generations of stars with slightly different chemical compositions.

The observations pushed to surface brightness levels of about 30 magnitudes per square arcsecond. To put that in perspective, this is more than a magnitude fainter than any previous measurements of these clusters, allowing detection of stellar populations that were essentially invisible before.

The Case of NGC 6397

The second target, NGC 6397, presented both opportunities and challenges. Located only about 2,500 light years away, it's one of the closest globular clusters to Earth. This proximity means its stars appear more spread out across the sky, making it harder to observe the entire system in one shot.

Euclid's observations covered approximately half of NGC 6397's extent, reaching out to about 600 arcseconds from its center. The data revealed a mild elliptical shape in the outer regions, with the long axis oriented north to south. This matches what previous studies had found, but the limited coverage meant researchers couldn't definitively say whether this represents tidal distortion or some other effect.

Computer simulations suggest that NGC 6397 should indeed show tidal features, but that they become prominent only at larger distances than what these initial observations covered. It's like having a map that shows part of a river but ends just before the river splits into multiple channels.

Stars at the Bottom of the Barrel

One of Euclid's remarkable achievements in these observations was detecting some of the faintest, lowest mass stars in both clusters. For NGC 6254, the data reached stars with masses around 0.16 times that of the Sun. For the closer NGC 6397, astronomers could trace the stellar population down to objects with just 0.12 solar masses.

These low mass stars are crucial for understanding globular cluster evolution. Internal dynamics within clusters tend to push lighter stars toward the outer regions over time, while heavier stars sink toward the center. This means the faint stars at the cluster edges are precisely the ones most likely to be torn away by tidal forces.

By mapping these vulnerable populations in detail, Euclid provides the information needed to understand not just whether clusters are losing stars, but which stars are being lost and at what rate.

Technical Wizardry Behind the Scenes

Creating these stunning results required sophisticated analysis techniques. The research team used specialized software to model how the telescope's optics blur starlight, then essentially reversed this process to measure the true brightness of each star.

They had to account for differential reddening, the way dust between us and the cluster absorbs more blue light than red light, with the amount of absorption varying across the field of view. They also had to distinguish between stars that are actually part of the cluster and the much larger number of unrelated stars that happen to lie in the same direction.

The final member star samples used for the morphology analysis included careful cuts based on multiple parameters: how point like each source appeared, whether its colors matched what you would expect for a cluster star, and quality metrics from the fitting process. Only sources that passed all these tests made it into the final analysis.

What Lies Beyond the Frame

The simulations of NGC 6254 hint at an exciting possibility. If the tidal distortions detected within Euclid's field of view continue to larger scales, as the models predict, then this cluster should have developed full fledged tidal tails stretching several degrees across the sky.

These tails would contain stars that have been completely stripped from the cluster, now streaming along its orbit around the Milky Way like debris trailing behind a comet. Detecting these extended structures will require observations covering larger areas of sky, but the morphological distortions seen in the Euclid data make a compelling case that they should be there.

A Preview of Things to Come

These observations of NGC 6254 and NGC 6397 represent just a small taste of what Euclid will accomplish during its planned six year mission. The telescope's wide survey will eventually cover about 14,000 square degrees of sky, roughly one third of the entire celestial sphere.

Within this enormous footprint lie more than 20 known Milky Way globular clusters. For each one, Euclid will provide the same combination of wide area coverage, high resolution, and multi wavelength depth that made these early results possible.

This systematic survey will allow astronomers to search for tidal features around many clusters, building up a statistical picture of how common these phenomena are and how they relate to cluster properties like mass, orbit, and position in the galaxy.

The Dark Matter Question

The prevalence or absence of tidal tails carries implications that extend beyond the clusters themselves. Some theoretical models suggest that if globular clusters are surrounded by their own halos of dark matter, this invisible material could help them retain stars that would otherwise escape.

In such scenarios, instead of developing long tidal tails, clusters might accumulate diffuse envelopes of stars that remain loosely bound. Systematic observations with Euclid will test these ideas, potentially offering new insights into whether globular clusters contain dark matter and how it affects their evolution.

The morphology of tidal features also encodes information about the cluster's history. The length, orientation, and structure of tidal tails depend on the cluster's orbit, the Milky Way's gravitational field, and how much mass the cluster has lost over time. By comparing observations with simulations, astronomers can work backwards to reconstruct each cluster's evolutionary path.

A New Window on Galactic Archaeology

Globular clusters serve as tracers of galactic history in multiple ways. Their orbits carry information about the distribution of matter in the Milky Way, including both normal matter and dark matter. The chemical composition of their stars records conditions in the early universe when they formed.

Now, with Euclid's ability to map tidal features in detail, astronomers have a new tool for galactic archaeology. The patterns of tidal disruption reveal not just the present day gravitational field of the galaxy, but also how it has changed over billions of years as the Milky Way grew and evolved.

Some globular clusters may have formed in smaller satellite galaxies that were later absorbed by the Milky Way. The properties of their tidal features could help identify which clusters have this origin versus those that formed within the Milky Way itself.

Combining Forces with Other Missions

While Euclid excels at imaging faint stars over wide areas, it works even better in combination with other observatories. The European Space Agency's Gaia mission has measured precise positions and velocities for billions of stars, including many in globular clusters.

By combining Euclid's deep imaging with Gaia's motion measurements, astronomers can identify cluster members with even greater confidence and trace how stars move as they transition from being bound cluster members to tidal tail escapees. This synergy between missions promises insights that neither could achieve alone.

Ground based surveys also contribute to the story. Large telescopes with wide field cameras can cover even bigger areas of sky than Euclid, though typically with less depth and resolution. Together, these observatories are building a comprehensive picture of globular cluster systems throughout the galaxy.

The Human Element

Behind these cosmic discoveries lies years of work by teams of scientists, engineers, and technicians. Designing and building Euclid required solving countless technical challenges, from crafting mirrors that maintain their shape in the harsh environment of space to developing detectors sensitive enough to capture the faint light from distant stars.

Analyzing the data requires sophisticated software and careful validation. Every step of the process, from removing instrumental artifacts to selecting cluster members, involves choices that must be justified and tested. The researchers cross checked their photometry against existing observations from the Hubble Space Telescope, finding excellent agreement that validated their techniques.

Looking to the Future

As Euclid continues its mission, it will build up an unprecedented database of high quality imaging across a large fraction of the sky. This archive will serve astronomers for decades to come, enabling studies that haven't even been imagined yet.

For globular cluster science specifically, the coming years promise a golden age of discovery. The combination of Euclid's wide field imaging, Gaia's astrometry, and spectroscopy from ground based telescopes will reveal these ancient stellar systems in unprecedented detail.

Future observations might extend to covering the full extent of nearby clusters like NGC 6397, capturing their tidal features across multiple degrees of sky. Deeper exposures could push even farther down the stellar mass function, revealing the faintest dwarf stars and perhaps even brown dwarfs.

Why Should We Care?

You might wonder why we should invest resources in studying these distant star clusters. Beyond the inherent human drive to understand our cosmic origins, this research has practical value for advancing our knowledge of fundamental physics.

Globular clusters serve as laboratories for testing theories of gravity, stellar evolution, and cosmology under extreme conditions that cannot be reproduced on Earth. They help calibrate the cosmic distance scale and the age of the universe. They trace the assembly history of galaxies and the distribution of dark matter.

Each new discovery adds another piece to the puzzle of how the universe works. The tidal distortions captured by Euclid reveal gravity in action on scales of thousands of light years, providing tests of our theoretical models and potentially revealing new phenomena.

A Cosmic Perspective

These observations remind us of the dynamic nature of the universe. What appears eternal and unchanging on human timescales is actually in constant flux when viewed over millions or billions of years. Even these ancient stellar cities, survivors from the universe's youth, are slowly being dismantled by the relentless pull of gravity.

Yet this destruction is also part of the cycle of cosmic evolution. The stars torn from globular clusters don't disappear; they become part of the Milky Way's stellar halo, the diffuse cloud of stars surrounding our galaxy. In this sense, the clusters are gradually merging with the larger structure of which they were always a part.

The images captured by Euclid offer a glimpse into this grand process, freezing a moment in a transformation that unfolds over timescales far longer than human civilization has existed. They showcase both the power of modern technology to reveal the universe's secrets and the ongoing journey of discovery that defines our species.

Publication Details

Published: 2025 (Online)
Journal: Astronomy & Astrophysics
Publisher: EDP Sciences
DOI: https://doi.org/10.1051/0004-6361/202449696

Credit and Disclaimer: This article is based on original research published in Astronomy & Astrophysics. The content has been adapted for a broader audience while maintaining scientific accuracy. For complete details, comprehensive data, full methodology, and in-depth analysis, readers are strongly encouraged to access the original peer-reviewed research article through the DOI link provided above. All factual information, data interpretations, and scientific conclusions presented here are derived from the original publication, and full credit goes to the research team and their contributing institutions.