Chinese astronomers have found a pair of young star clusters formed within a circumgalactic high-velocity cloud on the outskirts of the Milky Way—the first direct evidence of star formation in such an environment.

High-velocity clouds are neutral hydrogen structures moving at extreme velocities that cannot be explained by Galactic rotation models.

The finding, published on Wednesday in the journal Nature Astronomy, shows that the binary open cluster, located 45,000 light-years from Earth, was born from an internal cloud-cloud collision 11 million years ago.

The discovery was made by a research team at the China West Normal University in Nanchong, Sichuan province, using astrometry from European Space Agency's Gaia space observatory, spectroscopy from China's Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), and neutral hydrogen cloud data from the Effelsberg-Bonn HI Survey.

He Zhihong, an associate researcher at the university and the first and corresponding author of the study, said the two clusters, designated Emei-1 and Emei-2, have a combined mass roughly 2,000 times that of the Sun.

"The discovery of the Emei clusters ends a long-standing mystery about whether high-velocity clouds can produce stars, revealing that the circumgalactic medium can sustain star formation," he said.

It offers an important opportunity to study star formation in near-pristine, low-metallicity environments and suggests that some hypervelocity stars and streams may originate in high-velocity clouds, according to the study.

The study also explains why stars have never been found in high-velocity clouds before: they escape too quickly.

"Computer simulations show that stars leave their birth clouds in less than 20 million years," he said. "The Emei clusters were detectable precisely because they are extremely young."