Astronomers using archival data from the Hubble Space Telescope have discovered the first globular cluster stellar stream outside the Milky Way, located approximately 115 million light-years away in the ultra-diffuse galaxy UGC 9050-Dw1, according to a study published on August 12, 2026, in Nature.
A faint, winding ribbon of stars in a distant galaxy has opened an entirely new window onto the universe’s most elusive substance. For the first time, researchers have spotted a globular cluster stellar stream beyond our galactic neighborhood, giving astronomers a fresh method to map dark matter where traditional approaches have hit walls. The discovery centres on UGC 9050-Dw1, an ultra-diffuse galaxy that sits roughly 115 million light-years from Earth.
Spotting a Faint Ribbon Across 115 Million Light-Years
Stellar streams form when a host galaxy’s immense gravity tugs at a dense ball of stars, slowly pulling individual members away to create a long, narrow trail that shares a common orbit. While astronomers have cataloged dozens of these structures within the Milky Way, finding them in external galaxies has long remained out of reach. Extragalactic streams are usually far too faint to detect against the glare of conventional galaxies.
That obstacle disappeared when researchers examined archival observations from the Hubble Space Telescope. As study co-author David Hendel reviewed images of UGC 9050-Dw1, he spotted a delicate, curved arc. The galaxy’s sparse stellar population provided an unusually dark backdrop, allowing the subtle stellar trail to stand out despite the vast distance.
“This is the first time such a stream has been observed outside our own galaxy, the Milky Way, which makes the discovery particularly exciting.”
Julie Kiel Holm, doctoral fellow at the Niels Bohr Institute
The finding was detailed in a study published in Nature. The research team was co-led by Julie Kiel Holm of the University of Copenhagen’s Niels Bohr Institute and Sarah Pearson of DTU Space at the Technical University of Denmark, alongside an international team including Northwestern University astrophysicist Tjitske Starkenburg and University of Arizona astronomers David Sand and Catherine Fielder.
Using Cosmic Tracers to Map Dark Matter
Beyond the visual milestone, the discovery provides a sensitive new probe for weighing galaxies and measuring the invisible scaffold of the cosmos. Because a host galaxy’s gravitational field dictates the shape and motion of a stellar stream, researchers can model that trail to reconstruct the gravity acting upon it.
That gravitational modeling allows scientists to calculate a galaxy’s total mass, separating the visible portion made of stars from the rest. The research team reconstructed the gravitational field to infer how dark matter shapes stellar orbits. Dark matter accounts for roughly 80% to 85% of the universe’s total matter, yet it remains fundamentally mysterious because it does not emit, absorb, or reflect light.
“The stars in a stellar stream all travel along nearly the same orbit, and that orbit is shaped by the galaxy’s gravity. By modelling 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.”
Tjitske Starkenburg, research assistant professor at Northwestern University
Through thousands of computer simulations, the team tested various combinations of globular cluster properties and dark matter distributions to replicate the stream’s observed shape. The best-fitting models confirmed that UGC 9050-Dw1 contains substantial quantities of dark matter, aligning with established expectations for ultra-diffuse systems. The results matched older, orthodox measurement techniques, validating the new extragalactic tool.
Why Ultra-Diffuse Galaxies Are Prime Targets
The rare detection highlights a fortunate convergence of physical traits in ultra-diffuse galaxies. These systems are spread out and emit very little light, creating the necessary dark backdrop to spot faint stellar debris. At the same time, researchers emphasize that pulling stars out of a parent cluster requires immense gravitational muscle.
Sarah Pearson noted that ultra-diffuse galaxies combine extreme faintness with massive gravitational fields, making them ideal hunting grounds for stellar streams.
Preparing for a New Generation of Telescopes
The identification of UGC 9050-Dw1’s stellar stream is expected to open the floodgates for similar discoveries across a wider variety of galactic types. Because Hubble’s field of view is relatively small, finding this initial stream required a measure of serendipity. Researchers emphasize that upcoming astronomical hardware will dramatically expand the search area.
Along with the Euclid Space Telescope, these instruments promise a surge in observable extragalactic streams.
Keep reading
