For billions of years, a globular cluster orbiting a distant galaxy has been slowly torn apart by gravity. The result is a faint, narrow ribbon of stars stretching through space—an ancient stellar stream that has now become the first of its kind detected beyond the Milky Way. The discovery, made using archival observations from NASA’s Hubble Space Telescope and confirmed with ground-based data, gives astronomers a new way to investigate the invisible dark matter believed to dominate the galaxy’s mass.
The stellar stream was found around UGC 9050-Dw1, an ultra-diffuse galaxy located approximately 115 million light-years from Earth. Unlike bright spiral galaxies packed with luminous stars, ultra-diffuse galaxies contain relatively few visible stars spread across a large area. That sparse environment creates a darker cosmic background, allowing astronomers to identify extremely faint structures that would otherwise be lost in the glare of a more crowded galaxy. Within Hubble images of UGC 9050-Dw1, researchers noticed a thin, curved arc that appeared to follow the kind of orbit expected from stars escaping a globular cluster.
Globular clusters are dense, ancient systems containing thousands or sometimes millions of stars bound together by gravity. They can survive for billions of years while orbiting through the gravitational fields of their host galaxies. However, their stars are not permanently protected. As a cluster moves through a galaxy, tidal forces—differences in gravitational strength across the cluster—gradually pull stars from its outer regions. Once removed, those stars continue traveling along almost the same orbit as the original cluster, producing an elongated stream rather than dispersing randomly through space.
These streams act as natural records of galactic gravity. Every star in a stream carries information about the path followed by its parent cluster and the forces that altered that path over time. By analyzing the stream’s length, curvature, width and density, astronomers can work backward to estimate the gravitational field of the host galaxy. Because gravity is determined by mass, this method can reveal not only the contribution from visible stars but also the presence and distribution of dark matter, which emits no light and cannot be observed directly.
The research team generated thousands of computer simulations to determine which combinations of cluster properties and galactic mass distributions could reproduce the observed structure. The models accounted for the stream’s orbit, its apparent shape and the way stars would have been stripped from the cluster over billions of years. The best-fitting scenarios indicated that UGC 9050-Dw1 contains a substantial dark matter component, consistent with expectations for an ultra-diffuse galaxy. The analysis demonstrates that a globular cluster stream can serve as a sensitive gravitational probe even in a galaxy far outside the Milky Way.
“ The stars in a stellar stream all travel along nearly the same orbit, and that orbit is shaped by the galaxy’s gravity,” said Tjitske Starkenburg, a research assistant professor at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics and a coauthor of the study. “By modeling that gravity, we can estimate the galaxy’s total mass. We already know roughly how much of that mass comes from visible matter like stars, so the rest must be dark matter.”
Dark matter is thought to make up roughly 85 percent of all matter in the universe, yet its fundamental nature remains unknown. It does not absorb, emit or reflect light in a detectable way, so scientists study it through its gravitational effects on stars, galaxies and large-scale cosmic structures. Stellar streams offer a particularly precise method because they are narrow and dynamically coherent: even small changes in the gravitational field can alter their paths, create gaps in their density or produce clumps of stars along their length.
Those irregularities may eventually allow astronomers to test how dark matter is distributed on small scales. If compact concentrations of dark matter pass near a stellar stream, their gravity could pull stars away from the stream’s main path, leaving behind visible gaps or overdense regions. Similar features have been debated in streams within the Milky Way, where it can be difficult to determine whether they were created by dark matter, interactions with molecular clouds or other astrophysical processes. Finding and studying streams in external galaxies could provide an independent test of these competing explanations.
The discovery is especially important because astronomers had previously identified dozens of globular cluster streams in the Milky Way but had not clearly observed one in another galaxy. Extragalactic streams are usually too faint and too distant for existing telescopes to distinguish from background light. The successful detection in Hubble’s archival data suggests that other examples may already be hidden in observations collected for different scientific purposes. Researchers expect the number of known streams to rise sharply as new observatories begin surveying much larger regions of the sky.
The European Space Agency’s Euclid mission and NASA’s Nancy Grace Roman Space Telescope are designed to map enormous areas of the universe with high sensitivity and resolution. Roman, in particular, will survey an area roughly 100 times larger than Hubble’s field of view, greatly expanding the search for delicate stellar structures around distant galaxies. A larger sample of streams could allow astrophysicists to compare dark matter distributions across different galaxy types, test models of galaxy formation and determine whether ultra-diffuse galaxies consistently contain large dark matter halos. What began as a faint arc in an old Hubble image may therefore become the first step toward a new era of extragalactic dark matter mapping.
Subject of Research: Globular cluster stellar streams, dark matter distribution and galactic gravitational fields
Article Title: Evidence for the first globular cluster stellar stream beyond the Milky Way
News Publication Date: 12-Aug-2026
Web References: https://ciera.northwestern.edu/directory/tjitske-starkenburg/; https://ciera.northwestern.edu/; https://doi.org/10.1038/s41586-026-10878-w
References: Nature, DOI: 10.1038/s41586-026-10878-w
Image Credits: Hubble Space Telescope and Holm et al. (2026)
Keywords
Dark matter, stellar streams, globular clusters, galaxies, ultra-diffuse galaxies, Hubble Space Telescope, stellar dynamics, observational astrophysics, stellar evolution, galactic gravity
