Oyashio: The First Globular Cluster Stellar Stream Discovered Beyond the Milky Way

For decades, astronomers have mapped the Milky Way’s dark matter halo using the faint, shredded remains of star clusters — stellar streams. But no such stream had ever been confirmed around another galaxy. That changed in August 2026 with the discovery of Oyashio, the first globular cluster stellar stream ever found beyond the Milky Way .

Located roughly 115 million light-years from Earth, Oyashio is a faint, curved ribbon of stars unraveling from a globular cluster in the ultra-diffuse galaxy UGC 9050-Dw1 . Its discovery, published in Nature, gives scientists a powerful new tool for probing the invisible dark matter that dominates galaxies across the universe .

Where It Was Found: The Ultra-Diffuse Galaxy UGC 9050-Dw1

The stream resides in UGC 9050-Dw1, an ultra-diffuse galaxy (UDG) — a type of faint, low-surface-brightness dwarf galaxy . UDGs are thought to be extremely rich in dark matter, making them ideal targets for studying the substance .

• Host galaxy: UGC 9050-Dw1
• Distance: ~115 million light-years (35.2 megaparsecs) from Earth
• Distance measurement uncertainty: 35.2 ± 2.5 Mpc

How It Was Identified: Archival Hubble Data + Fresh Eyes

The stream was so faint that standard surveys missed it entirely. The breakthrough came when a team led by PhD student Julie Kiel Holm (Niels Bohr Institute) and associate professor Sarah Pearson (DTU Space) re-analyzed archival images from the Hubble Space Telescope using advanced processing techniques .

What they found was an extremely thin, curved structure stretching about 2 kiloparsecs (roughly 6,500 light-years) from a compact globular cluster candidate . Its narrow, elongated shape and alignment with the surviving cluster confirmed it as a tidal stellar stream — stars being stripped by the host galaxy’s gravity .

The stream was named Oyashio, after a cold ocean current in the North Pacific, following a convention of naming extragalactic streams after ocean currents .

Why Oyashio Matters: A New Window on Dark Matter

A stellar stream is like a cosmic breadcrumb trail. Because all the stars in the stream travel along nearly the same orbit, the stream’s shape, width, and curvature directly trace the total gravitational field of its host galaxy . Astronomers can model that orbit to measure the galaxy’s mass distribution — including its invisible dark matter component.

• First extragalactic application: Hundreds of globular cluster streams are known in the Milky Way and have been used to map our galaxy’s dark matter halo. Oyashio proves this technique can work for galaxies far beyond our own .
• Testing dark-matter-rich UDGs: Ultra-diffuse galaxies like UGC 9050-Dw1 are believed to be dominated by dark matter. Oyashio provides a direct, quantitative way to test how much dark matter the galaxy contains and how it is distributed, offering new constraints on galaxy formation models .
• Generative stream modeling: The team used dynamical models fitted directly to the stream’s shape to constrain the mass of the progenitor globular cluster and produce the first stream-based halo constraint for an ultra-diffuse galaxy .

As Northwestern University astrophysicist Tjitske Starkenburg, a co-author of the study, explained: "The stars in a stellar stream all travel along nearly the same orbit, and that orbit is shaped by the galaxy’s gravity." By modeling that gravity, researchers estimate the galaxy’s total mass and, crucially, how much of that mass is dark matter .

What Comes Next

Oyashio is just the beginning. Astronomers expect that many more extragalactic stellar streams await discovery in existing and future telescope archives — especially with the James Webb Space Telescope and next-generation observatories. Each new stream will offer another chance to weigh the invisible matter that shapes galaxies .

"We have discovered a globular cluster stellar stream in another galaxy. This is the first time such a stream has been observed outside our own galaxy, the Milky Way, which makes the discovery particularly exciting," Holm said .