At the center of the Milky Way lies a realm of cosmic extremes—dense star clusters, raging black holes, and colossal clouds of gas and dust where stars are born. Among these lies one of the galaxy’s most remarkable stellar nurseries: Sagittarius B2, a giant star-forming cloud located about 27,000 light-years from Earth and only 330 light-years, in projection, from the galactic center. For decades, astronomers have known this region as a hotbed of star birth, but much of its inner workings remained hidden behind thick veils of dust.
Now, thanks to the extraordinary sensitivity of the James Webb Space Telescope (JWST), astronomers have peeled back these layers to reveal a breathtaking new view of this cosmic nursery. Using JWST’s Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI), a team led by Nazar Budaiev of the University of Florida has uncovered hidden stars, glowing ionized gas, and the first hints of how star formation is unfolding inside Sagittarius B2. Their findings, published September 15 on the arXiv preprint server, promise to reshape our understanding of how stars form in the most extreme regions of the galaxy.
A Stellar Nursery Unlike Any Other
Sagittarius B2 is no ordinary cloud. Stretching across dozens of light-years, it is one of the most active star-forming sites in the entire Milky Way, producing new stars at a rate of about 0.04 solar masses per year. Previous observations revealed a staggering population: more than 700 young stellar objects (YSOs), over 50 regions of ionized hydrogen gas known as H II regions, numerous molecular outflows, and hundreds of masers—cosmic beacons that shine when molecules amplify microwave radiation.
Yet despite these discoveries, much about Sagittarius B2 remained obscured. Radio telescopes had given astronomers hints of what was happening deep within the cloud, but they could not reveal the full picture. The dense dust of Sagittarius B2 blocked visible light and even some infrared radiation, leaving astronomers to wonder what secrets were still hidden.
This is where JWST comes in. With its unparalleled ability to see through dust in infrared light, the telescope has now opened a new chapter in the story of Sagittarius B2.
JWST’s First Glimpse of Sagittarius B2
When JWST turned its golden mirrors toward Sagittarius B2, it revealed a cloud of extraordinary complexity and structure. For the first time, astronomers could distinguish between regions at different stages of stellar birth.
On the western side of the cloud, where past observations had already noted signs of star formation, JWST confirmed the presence of the earliest stages of stellar growth. But in a surprising twist, the telescope also uncovered new star formation in the eastern, less dense regions of the cloud—areas once thought to be quieter. This discovery suggests that Sagittarius B2 is not a uniform factory of stars but a patchwork of environments, each undergoing its own phase of cosmic evolution.
One particularly striking feature was a region of the cloud backlit by extended recombination line emission, which revealed a sharp, glowing edge where ionized gas interacts with surrounding material. This kind of detail had never been seen before, showing how JWST can illuminate the hidden architecture of interstellar clouds.
Hidden Giants Among the Stars
Perhaps the most groundbreaking revelation was JWST’s ability to unveil populations of massive stars previously invisible to other telescopes. Astronomers identified both a low-extinction population—massive stars visible in relatively clear lines of sight—and a hidden, high-extinction population, buried behind dense dust.
Even more exciting, JWST revealed new candidate H II regions around massive stars that had been missed in radio surveys. These regions mark the places where massive stars, only a few million years old, unleash intense radiation that strips electrons from surrounding hydrogen atoms, creating vast bubbles of glowing plasma.
This finding demonstrates that JWST is not just complementing radio observations—it is surpassing them, uncovering a new layer of galactic structure that had remained hidden until now.
Escaping Light in a Dense Universe
Another remarkable discovery came from JWST’s MIRI instrument, which observed the star-forming region known as Sgr B2 N. Here, astronomers found that radiation is escaping along outflow cavities—channels carved by young stars as they spew jets and winds into their surroundings.
This escape of infrared photons carries profound implications. Until now, many astronomers assumed that light produced in such dense star clusters would be largely trapped, bouncing endlessly within the cloud. But JWST shows that light can find geometric escape routes, even in some of the densest star-forming environments in the galaxy. This means that radiation from newborn stars can influence surrounding gas more effectively than previously thought, shaping the future of star formation within the cloud.
Star Formation in Its Infancy
For all its new revelations, JWST also raised a surprising puzzle. Despite its extraordinary sensitivity, the telescope did not detect a widespread population of young stellar objects across Sagittarius B2. This suggests that, contrary to expectations, star formation in the cloud may have only just begun.
Instead of being a fully matured nursery packed with newborn stars, Sagittarius B2 might be in the very earliest stages of a stellar boom—an awakening giant that will continue to give birth to stars for millions of years to come.
A Transformative View of Stellar Birth
The University of Florida team and their collaborators emphasize that these first results are only the beginning. JWST has already shown its ability to penetrate dense galactic environments and reveal hidden stars, glowing hydrogen regions, and the structural complexity of star-forming clouds.
“JWST unveils previously hidden massive stars and ionized structures, offering a transformative view of how stars form under some of the most extreme galactic conditions,” the researchers conclude.
In other words, Sagittarius B2 is more than just a star-forming cloud. It is a laboratory for understanding the birth of stars in environments that resemble the conditions of the early universe.
Looking Ahead: The Future of Galactic Exploration
The observations of Sagittarius B2 highlight the unique power of JWST to explore the dusty corners of the cosmos where stars are born. Future studies will likely combine JWST’s infrared vision with data from radio and millimeter observatories such as ALMA, creating an unprecedented multi-wavelength portrait of stellar nurseries.
Each new glimpse brings us closer to answering some of astronomy’s most profound questions: How do massive stars form in such extreme environments? How does their radiation shape the clouds around them? And how do starbursts in the heart of the Milky Way compare to the furious star formation that took place in the early universe?
As JWST continues its mission, Sagittarius B2 serves as a reminder that the galaxy is alive with creation, and that the light of new stars—once hidden in darkness—can now reach us across the gulf of space and time.
More information: Nazar Budaiev et al, JWST’s first view of the most vigorously star-forming cloud in the Galactic center—Sagittarius B2, arXiv (2025). DOI: 10.48550/arxiv.2509.11771
