Imagine looking up at the night sky and noticing not just the bright stars, but the faint glow between them—a ghostly light that tells stories of cosmic collisions and galactic destruction spanning billions of years. That's exactly what scientists have captured using the Euclid space telescope, revealing an invisible tapestry of stars floating freely between galaxies in one of our nearest galaxy clusters.

The Perseus cluster, located about 240 million light-years from Earth, is a cosmic metropolis containing thousands of galaxies bound together by gravity. But scattered throughout this cluster is something far more mysterious: a diffuse ocean of stars that don't belong to any single galaxy. This so-called "intracluster light" has fascinated astronomers for decades, yet studying it has been nearly impossible. Until now.

Light from Nowhere

Picture a bustling city at night. The streetlights are the galaxies—bright, distinct, easy to spot. But between those lights exists a subtle glow, perhaps from scattered light bouncing off buildings or distant reflections. That faint background illumination is what intracluster light represents on a cosmic scale. It's incredibly dim—fainter than the darkest sky you've ever seen—making it extraordinarily difficult to detect and measure.

These orphaned stars were once part of normal galaxies. Over billions of years, as galaxies crashed into each other or passed too close, gravity ripped stars away from their homes and flung them into the space between galaxies. They became cosmic refugees, wandering the cluster without a galaxy to call home. Tracking these homeless stars helps scientists understand how galaxy clusters evolve and even reveals the hidden scaffolding of dark matter that holds everything together.

But there's a problem: photographing something this faint from Earth is nearly impossible. Our atmosphere, light pollution, and even scattered sunlight within our solar system create a glowing curtain that drowns out these whispers of light. Space telescopes offered some improvement, but their small fields of view meant piecing together the big picture was like trying to see a mural through a keyhole.

A New Eye on the Universe

Enter Euclid, a European Space Agency telescope launched specifically to study the dark universe. With its wide field of view—capable of capturing an area 40 times larger than the full moon in a single image—and exquisitely controlled scattered light, Euclid is perfectly designed for hunting faint structures. Its sharp resolution in visible light and deep infrared vision combine to create an unprecedented tool for mapping intracluster light.

For their first deep observations, scientists pointed Euclid at the Perseus cluster's heart. What they found was breathtaking: a vast web of faint light extending over 600,000 light-years from the cluster's central galaxy, NGC 1275. That's roughly one-third of the cluster's total radius, far beyond what previous telescopes could trace.

The images revealed something unexpected. The center of this diffuse light isn't positioned directly over NGC 1275, the brightest galaxy in the cluster. Instead, it's shifted westward by about 60,000 light-years, pointing toward several other massive galaxies. This offset suggests something dynamic is happening—perhaps the cluster is still digesting a recent merger, with the dark matter halo, orphaned stars, and central galaxy all responding differently to the cosmic traffic accident.

Stellar Detectives

But the researchers didn't stop at mapping the light. Scattered throughout this faint glow are globular clusters—ancient, densely packed balls of stars that orbit galaxies like loyal satellites. Euclid's sharp vision allowed scientists to identify roughly 70,000 of these stellar beacons within the central region of Perseus, with about 51,000 floating freely in intracluster space rather than bound to any single galaxy.

These globular clusters serve as forensic evidence. Just as crime scene investigators use fingerprints to identify culprits, astronomers use the properties of globular clusters to identify which galaxies contributed stars to the intracluster population. The team discovered that the orphaned globular clusters come in two distinct families: one matching the properties of massive galaxies, another matching smaller dwarf galaxies.

Here's where the story gets interesting. The chemical composition of the intracluster light—measured through its color—revealed that these stars are relatively metal-poor, meaning they formed in smaller galaxies or in the outer regions of larger ones. Yet there's a puzzle: dwarf galaxies simply don't have enough stars to account for all the intracluster light observed. The solution? Most of these homeless stars were likely stripped from the outer halos of intermediate-mass galaxies during close encounters, or came from dwarf galaxies that were first captured by larger galaxies before ultimately being shredded and released into the cluster.

Tracing Invisible Matter

Perhaps the most profound discovery is what the intracluster light reveals about dark matter—the invisible substance that makes up 85% of the universe's matter but has never been directly detected. The distribution of intracluster stars closely mirrors what theoretical models predict for dark matter halos. The elliptical shape, the specific slope of how the light fades with distance, even the orientation—all match expectations for how dark matter should be arranged.

This matters enormously because dark matter cannot be seen; it only reveals itself through its gravitational effects. Luminous tracers like intracluster light and orphaned globular clusters offer a rare window into mapping dark matter's distribution across an entire galaxy cluster. The measurements suggest these stars are excellent proxies for the invisible dark matter web that binds galaxy clusters together.

The offset between NGC 1275 and the center of intracluster light also provides clues about the cluster's dynamics. Computer simulations suggest two possible scenarios: either NGC 1275 itself is moving through the cluster, disturbed by a past merger but not yet settled back into equilibrium, or the dark matter halo's core is still oscillating after a collision, with the orphaned stars tracing this cosmic wobble. The velocity measurements of dwarf galaxies in Perseus support the latter interpretation—the entire cluster core may be gently rocking back and forth as it recovers from a recent merger.

A Window into Cosmic History

Studying intracluster light is like cosmic archaeology. Each star carries a record of its birthplace and journey. The abundance of metal-poor stars suggests many came from the outskirts of galaxies where fewer heavy elements exist. The specific numbers and types of globular clusters indicate which galaxy masses contributed most to the orphaned population. Even the colors reveal stellar ages and compositions, painting a picture of how this cosmic ecosystem assembled over billions of years.

The fact that about 38% of all the light within the central 500,000 light-years of Perseus comes from the diffuse intracluster component is remarkable. In terms of mass, assuming typical star-to-light ratios, roughly two-thirds of the stellar mass in this region doesn't belong to any individual galaxy. It's as if a major city discovered that most of its population lives not in buildings but scattered throughout parks and streets.

This has implications beyond Perseus. If intracluster light is this abundant in one cluster, it likely pervades all galaxy clusters throughout the universe. That means previous estimates of how many stars exist in galaxies versus floating freely in space may need revision. It also affects how we understand galaxy evolution—clearly, the stripping and destruction of galaxies is a major process shaping cosmic structure.

The Road Ahead

Euclid's Perseus observations represent just the beginning. The telescope will eventually survey thousands of galaxy clusters across one-third of the sky, each potentially harboring similar diffuse stellar populations. This enormous dataset will allow statisticians to examine how intracluster light varies with cluster mass, age, and dynamical state. Does every cluster show the same offset between its brightest galaxy and the intracluster light peak? Do younger, more dynamically active clusters have more chaotic distributions?

The ability to trace dark matter through luminous proxies also opens new avenues for testing dark matter theories. If alternative models of dark matter predict different halo shapes or density profiles, mapping intracluster light across many clusters could distinguish between possibilities. It's an indirect but powerful way to probe the universe's most mysterious component.

For the general public, these findings remind us how much remains unknown in our cosmic backyard. Even in a well-studied cluster just 240 million light-years away, Euclid revealed structures and patterns never seen before. The universe continually surprises us, hiding secrets in plain sight—or in this case, in the spaces between.

The images themselves are stunning. The diffuse glow of intracluster light, the delicate elliptical contours stretching across hundreds of thousands of light-years, the thousands of tiny globular clusters scattered like fireflies throughout—all captured with unprecedented clarity. These aren't just data points; they're portraits of cosmic violence and beauty, showing how destruction on galactic scales creates something hauntingly ephemeral.

Why It Matters

Beyond scientific curiosity, understanding galaxy clusters has practical implications for cosmology. Galaxy clusters are among the universe's largest gravitationally bound structures, making them sensitive probes of cosmic expansion and the properties of dark energy. Accurately measuring their masses requires knowing how much matter they contain—including the often-overlooked intracluster component. Better mass estimates improve our models of how the universe has evolved and will continue to evolve.

There's also a humbling perspective embedded in this research. Every star in the intracluster light was once part of a galaxy, perhaps orbiting peacefully around its galactic center, possibly hosting planets, maybe even civilizations. Then gravitational tides tore them away, casting them into the void between galaxies. They now wander the cluster for billions of years, witnesses to the grand but violent processes shaping the cosmos. If stars could tell stories, these orphans would have the most dramatic tales.

The technical achievement shouldn't be understated either. Measuring surface brightness levels fainter than one photon per second per square degree requires exquisite control over every aspect of telescope design—from minimizing internal scattered light to carefully modeling and removing contaminating signals like dust in our own solar system. The fact that Euclid succeeded despite observing Perseus near the dusty plane of the Milky Way demonstrates the telescope's capabilities.

The Euclid space telescope has given us a new way to see the universe—not by finding new objects, but by revealing the faint threads connecting the bright ones. In doing so, it illuminates both the visible and invisible architecture of the cosmos, tracing the echoes of ancient galactic collisions and mapping the dark matter web that orchestrates it all.

As Euclid continues its mission, we'll discover whether Perseus is typical or unusual, refining our understanding of how galaxy clusters form and evolve. Each observation adds pieces to the cosmic puzzle, bringing us closer to understanding our place in this vast, interconnected universe. And somewhere in that faint glow between galaxies, countless stars continue their lonely journey, silent testament to the dynamic, ever-changing nature of the cosmos.

Publication Details

Published: 2025
Journal: Astronomy & Astrophysics
Publisher: EDP Sciences
DOI: https://doi.org/10.1051/0004-6361/202450772

Credit and Disclaimer

This article is based on original research published in Astronomy & Astrophysics as part of the Euclid Consortium's Early Release Observations special issue. The research involved contributions from institutions across multiple countries including Germany (Max Planck Institute for Extraterrestrial Physics and Ludwig-Maximilians-Universität München), the United Kingdom (University of Nottingham), Spain (Instituto de Astrofísica de Canarias), the United States (University of Florida), France (CEA Paris-Saclay), and numerous other international partners. The content has been adapted for a general audience while preserving scientific accuracy. For complete methodological details, comprehensive statistical analyses, full datasets, detailed photometric measurements, and in-depth theoretical frameworks, readers are strongly encouraged to consult the original peer-reviewed research article through the DOI link provided above. All scientific findings, data interpretations, and conclusions presented here are derived directly from the original publication, and full credit belongs to the Euclid Consortium research team and their institutions.