Rubin Observatory Uncovers 180,000‑Light‑Year Stellar 'Tail' on Messier 61

Rubin Observatory's first image reveals a faint stellar stream

Mere months after its much‑anticipated debut, the Vera C. Rubin Observatory is already delivering on its promise to reshape our picture of the universe. Although the observatory’s enormous camera on a Chilean mountaintop has not yet begun its formal survey, astronomers analysing the first test exposure discovered a surprising feature: a faint trail of stars — a stellar stream — trailing from the well‑known galaxy Messier 61.

“This is the first stellar stream detected from Rubin,” says Sarah Pearson, an astrophysicist at the University of Copenhagen. “And it’s just a precursor for all of the many, many features we’ll find like this.”

Published in the Research Notes of the American Astronomical Society, the finding came after the team carefully processed Rubin’s inaugural image to remove scattered and excess light. That processing revealed a stream roughly 55 kiloparsecs long — about 180,000 light‑years — placing it among the longer stellar streams known.

Messier 61, a starburst galaxy in the Virgo Cluster first catalogued in 1779, has long attracted attention for its high rate of star formation and numerous recorded supernovae. Powerful observatories such as the Hubble Space Telescope and the James Webb Space Telescope have studied the galaxy extensively, yet this faint stream was only visible after Rubin’s deep, wide‑field imaging and careful background filtering.

The authors suggest the stream most likely originated when a small dwarf galaxy was torn apart by Messier 61’s gravity. Such accretion events are expected to be common and can inject fresh gas and stars into larger galaxies, potentially triggering elevated star formation and explaining some of Messier 61’s unusual properties.

Rubin’s first test frame already contains roughly ten million galaxies, hinting at what the observatory will achieve. Over the coming decade, Rubin is expected to image light from about 20 billion galaxies, enabling astronomers to detect many more faint features like stellar streams and probe how galaxies assemble over cosmic time.

“The expectation is that every single galaxy should be surrounded by these streams. It's a fundamental part of how the galaxies are made,” says Aaron Romanowsky, an astronomer at San José State University and a co‑author of the report. “We just need to look fainter, and that’s the hope with Rubin.”

Why it matters: Discovering faint stellar streams helps astronomers reconstruct past merger events, trace a galaxy’s growth history, and test models of dark matter and galaxy formation. Rubin’s combination of depth, field of view, and image quality makes it uniquely suited to find these low‑surface‑brightness features at large scale.