For the first time, astronomers using NASA’s Fermi Gamma-ray Space Telescope have traced a budding outflow of gas from a cluster of young stars in our galaxy — insights that help us understand how the universe has evolved as NASA explores the secrets of the cosmos for the benefit of all.

The cluster, called Westerlund 1, is located about 12,000 light-years away in the southern constellation Ara. It’s the closest, most massive, and most luminous super star cluster in the Milky Way. The only reason Westerlund 1 isn’t visible to the unaided eye is because it’s surrounded by thick clouds of dust. Its outflow extends below the plane of the galaxy and is filled with high-speed, hard-to-study particles called cosmic rays.

“Understanding cosmic ray outflows is crucial to better comprehending the long-term evolution of the Milky Way,” said Marianne Lemoine-Goumard, an astrophysicist at the University of Bordeaux in France. “We think these particles carry a large amount of the energy released within clusters. They could help drive galactic winds, regulate star formation, and distribute chemical elements within the galaxy.”

A paper detailing the results published Dec. 9 in Nature Communications. Lemoine-Goumard led the research with Lucia Härer and Lars Mohrmann, both at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany.

Super star clusters like Westerlund 1 contain more than 10,000 times our Sun’s mass. They are also more luminous and contain higher numbers of rare, massive stars than other clusters.

Scientists think that supernova explosions and stellar winds within star clusters push ambient gas outward, propelling cosmic rays to near light speed. About 90% of these particles are hydrogen nuclei, or protons, and the remainder are electrons and the nuclei of heavier elements.

Because cosmic ray particles are electrically charged, they change course when they encounter magnetic fields. This means scientists can’t trace them back to their sources. Gamma rays, however, travel in a straight line. Gamma rays are the highest-energy form of light, and cosmic rays produce gamma rays when they interact with matter in their environment.

Most gamma-ray observations of stellar clusters have limited resolution, so astronomers effectively see them as indistinct areas of emission. Because Westerlund 1 is so close and bright, however, it’s easier to study.

In 2022, scientists using a group of telescopes in Namibia operated by the Max Planck Institute called the High Energy Spectroscopic System detected a distinct ring of gamma rays around Westerlund 1 with energies trillions of times higher than visible light.

Lemoine-Goumard, Härer, and Mohrmann wondered if the cluster’s unique properties might allow them to see other details by looking back through nearly two decades of Fermi data at slightly lower energies — millions to billions of times the energy of visible light.

Fermi’s sensitivity and resolution allowed the researchers to filter out other gamma-ray sources like rapidly spinning stellar remnants called pulsars, background radiation, and Westerlund 1 itself.

What was left was a bubble of gamma rays extending over 650 light-years from the cluster below the plane of the Milky Way. That means the outflow is about 200 times larger than Westerlund 1 itself.

The researchers call this a nascent, or early stage, outflow because it was likely recently produced by massive young stars within the cluster and hasn’t yet had time to break out of the galactic disk. Eventually it will stream into the galactic halo, the hot gas surrounding the Milky Way.

Westerlund 1 is located slightly below the galactic plane, so the researchers think the gas expanded asymmetrically, following the path of least resistance into a zone of lower density below the disk.

“One of the next steps is to model how the cosmic rays travel across this distance and how they create a changing gamma-ray energy spectrum,” Härer said. “We’d also like to look for similar features in other star clusters. We got very lucky with Westerlund 1, though, since it’s so massive, bright, and close. But now we know what to look for, and we might find something even more surprising.”

“Since it started operations 17 years ago, Fermi has continued to advance our understanding of the universe around us,” said Elizabeth Hays, Fermi’s project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “From activity in distant galaxies to lightning storms in our own atmosphere, the gamma-ray sky continues to astound us.”

By Jeanette Kazmierczak
NASA’s Goddard Space Flight Center, Greenbelt, Md.

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Claire Andreoli
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NASA’s Goddard Space Flight Center, Greenbelt, Md.