Astronomers have detected a faint, elongated stream of stars wrapped around a galaxy far beyond the Milky Way, marking the first confirmed observation of a globular cluster stellar stream in another galaxy. The discovery, reported in Nature, provides a new way to investigate the invisible dark matter that dominates the mass of galaxies. Until now, these delicate structures had been observed only within the Milky Way, where their relative proximity makes them faint but accessible targets. Finding one in an external galaxy demonstrates that the same gravitational tool can be used on a much broader cosmic scale—and could transform the way scientists study the hidden architecture of galaxies across the Universe.
The newly identified stream lies in UGC9050-Dw1, an ultra-diffuse galaxy, a remarkably faint and extended system whose stars are spread over a large area. Unlike bright spiral galaxies such as the Milky Way, ultra-diffuse galaxies contain relatively few stars for their size, making their light difficult to distinguish from the background sky. The stellar stream itself is even harder to see because it consists of stars gradually pulled away from a globular cluster. In images, it appears as a subtle trail stretching through the galaxy, a remnant of a long gravitational interaction that has been unfolding over time.
Globular clusters are dense, ancient systems containing thousands, and in some cases hundreds of thousands, of stars bound together by gravity. As a cluster orbits its host galaxy, the galaxy’s gravitational field exerts unequal forces across the cluster. The side facing the galactic centre experiences a slightly stronger pull than the far side, producing tidal stress. Over millions or billions of years, this process can remove individual stars from the cluster. The escaped stars do not immediately disperse randomly; instead, they continue following similar orbits, forming a narrow stellar stream that traces the cluster’s path through the galactic gravitational field.
That path carries information about the invisible mass shaping the galaxy. A stellar stream behaves like a gravitational test structure: its position, width, curvature and density reflect the forces acting on it. By modelling the stream’s orbit and the way it has been stretched, researchers can infer how much mass lies inside the galaxy and how that mass is distributed. Because most of a galaxy’s mass is not made of stars or gas, the stream becomes an indirect probe of its dark matter halo. Dark matter does not emit, absorb or reflect light, but its gravity influences the motion of visible matter, including stars in globular clusters and the streams they leave behind.
Julie Kiel Holm, a PhD student at the Niels Bohr Institute, and Sarah Pearson, an associate professor at DTU Space, led an international team that identified the structure in UGC9050-Dw1. Their analysis establishes that a method developed through studies of stellar streams in the Milky Way can be applied to a galaxy outside our own. The researchers used the stream’s observable properties to estimate the external galaxy’s mass distribution and dark matter content. Their results indicate that UGC9050-Dw1 contains a substantial amount of dark matter, consistent with earlier findings about ultra-diffuse galaxies, but obtained through an entirely different measurement technique.
The significance of the result extends beyond the discovery of one exceptionally faint line of stars. In the Milky Way, astronomers have used streams to search for irregularities in the dark matter distribution, including possible small-scale clumps that could disturb a stream’s otherwise smooth structure. Applying the same approach to other galaxies could reveal whether dark matter behaves similarly in environments with different sizes, shapes, star-formation histories and gravitational conditions. It could also help resolve a long-standing question surrounding ultra-diffuse galaxies: why do some appear to contain far more dark matter than their modest populations of stars would suggest?
Detecting the stream required overcoming two major challenges at once. The first was its intrinsic faintness. A globular cluster stream contains stars dispersed over a vast region, so its light is spread thinly across the sky. The second was the low surface brightness of the host galaxy itself. In such observations, the signal can easily be overwhelmed by foreground stars, background galaxies, instrumental effects and variations in the sky brightness. Sophisticated imaging and analysis methods are therefore essential. Researchers must distinguish a coherent, physically connected pattern from random alignments of unrelated stars or faint background features, then test whether the structure is consistent with a stream shaped by the galaxy’s gravitational field.
The discovery also offers a glimpse of what the next generation of astronomical surveys may uncover. The Euclid Space Telescope is designed to map billions of galaxies and investigate the large-scale distribution of matter, while NASA’s Nancy Grace Roman Space Telescope will deliver exceptionally sharp and wide-field observations of distant regions of the sky. Their data could reveal many more faint streams around external galaxies, creating a new population of targets for dark matter studies. Instead of relying almost exclusively on the Milky Way as a laboratory, astronomers may soon compare the gravitational structures of galaxies across a wide range of environments.
For Holm and her colleagues, the observation is therefore both a first and a proof of concept. A stellar stream born in a globular cluster has now been shown to survive as a detectable structure outside the Milky Way and to provide quantitative information about the dark matter halo of its host galaxy. The result turns an extraordinarily faint feature into a potential cosmic measuring instrument. As larger telescopes and more powerful data-analysis techniques expose the hidden outskirts of other galaxies, these fragile trails of stars could become one of the most revealing ways to investigate the unseen matter that governs the evolution of the Universe.
Subject of Research: Globular cluster stellar streams, ultra-diffuse galaxies and dark matter mapping beyond the Milky Way.
Article Title: Evidence for the first globular cluster stellar stream beyond the Milky Way
News Publication Date: 12-Aug-2026
Web References: Nature article; DOI: 10.1038/s41586-026-10878-w
References: Holm, J. K., Pearson, S., Nibauer, J., Sand, D. J., Price-Whelan, A. M., Starkenburg, T., Hendel, D. and Fielder, C., “Evidence for the first globular cluster stellar stream beyond the Milky Way,” Nature.
Image Credits: Hubble Space Telescope and Holm et al. (2026).
Keywords
Globular cluster stellar stream, dark matter, ultra-diffuse galaxy, UGC9050-Dw1, Hubble Space Telescope, Milky Way, stellar astronomy, galactic evolution, Euclid Space Telescope, Nancy Grace Roman Space Telescope
