Astronomers detect first stellar stream beyond Milky Way in ultra-diffuse galaxy

September 23, 2026 at 05:03 PM2 min

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An international team led by Julie Kiel Holm of the Niels Bohr Institute and Sarah Pearson of DTU Space has detected a globular cluster stellar stream in an ultra-diffuse galaxy beyond the Milky Way. The discovery, published in Nature, marks the first time such a stream has been observed outside our galaxy. The stream's faint signal and location in a low-luminosity galaxy make the detection particularly notable.

Key Facts

The stellar stream was detected in an ultra-diffuse galaxy, a type of galaxy that emits very little light.
The discovery was published in Nature and led by PhD student Julie Kiel Holm of the Niels Bohr Institute and Associate Professor Sarah Pearson of DTU Space.
Dark matter represents approximately 80-85 percent of all matter in the universe.
The researchers show for the first time that globular cluster stellar streams can be used to measure dark matter in galaxies outside the Milky Way.

Detection and Significance

An international team led by PhD student Julie Kiel Holm of the Niels Bohr Institute and Associate Professor Sarah Pearson of DTU Space reported the discovery in Nature. The stream is located inside an ultra-diffuse galaxy, which gives off very little light, making the detection particularly challenging. Julie Kiel Holm stated that this is the first time such a stream has been observed outside the Milky Way. Sarah Pearson noted that the discovery opens new possibilities for searching for stellar streams in other galaxies and measuring dark matter in ultra-diffuse galaxies.

Stellar Streams and Dark Matter

A globular cluster stellar stream is a long, narrow trail of stars that originated in a globular cluster, a dense collection of stars held together by gravity. These streams develop when the gravity of the surrounding host galaxy gradually pulls stars away from the globular cluster through tidal stripping. Because gravity determines the shape and motion of stellar streams, astronomers can study them to learn how mass is distributed throughout a galaxy. Dark matter represents approximately 80-85 percent of all matter in the universe, and stellar streams act as tracers of this invisible matter.

Implications for Galaxy Studies

The researchers show for the first time that globular cluster stellar streams can be used to measure dark matter in galaxies outside the Milky Way. Improvements in massive astronomical datasets and advanced analysis methods are turning stellar streams into one of the most promising tools for studying galaxies. The discovery may enable long-term measurements of dark matter content in more ultra-diffuse galaxies.