Star formation is a fundamental physical process in our Universe. Stars light up the cosmos, and give rise to planets, some of which may support life. While humans have no doubt wondered about stars since prehistoric times, new technological tools like the Milky Way have taken our natural curiosity to a whole new level. Now we can peer inside obscured regions and detect young stars in their dusty cocoons.
Our intellectual wondering became more formalized at least as far back as ancient Greece, when Democritus proposed that the Milky Way was made of stars. Two millennia later, the German philosopher Immanuel Kant thought specifically about how stars form. In his 1755 work “Universal Natural History and Theory of the Heavens,” he proposed that stars formed when rotating nebular matter collapsed gravitationally, a remarkably accurate assessment for the time.
Now astrophysicists have extremely detailed models of star formation, and of how stars age and how they meet their demises. But there are still plenty of questions, and one of the reasons that NASA, the ESA, and the CSA built the JWST was to seek answers to those questions. One of the space telescope’s science themes is the Birth of Stars and Protoplanetary Systems.
As part of its effort to answer questions about star formation, the JWST observed Sagittarius B2, the most massive and active star-forming gas cloud in the entire Milky Way. It’s near the galaxy’s center, only about four hundred light years away from the Milky Way’s supermassive black hole, Sagittarius A-star. Sagittarius B2 (Sgr B2) is about 150 light years across and contains about three million solar masses.
Sgr B2 is replete with glowing dust lit up by a brilliant collection of massive stars. But it’s the cloud’s gas content that tells the real story. Sgr B2’s hydrogen density is up to 40 times more dense than a typical molecular cloud. The cloud is known for its complex structure, where the gas density varies widely along with the temperature. Probing a complex region like this is exactly why the JWST was built.
“Webb’s powerful infrared instruments provide detail we’ve never been able to see before, which will help us to understand some of the still-elusive mysteries of massive star formation and why Sagittarius B2 is so much more active than the rest of the galactic center,” said astronomer Adam Ginsburg of the University of Florida, co-author of new research presenting the JWST’s observation.
The new research is titled “JWST’s first view of the most vigorously star-forming cloud in the Galactic center – Sagittarius B2.” The lead author is Nazar Budaiev from the Department of Astronomy at the University of Florida. The research is available at arxiv.org.
“We report JWST NIRCAM and MIRI observations of Sgr B2, the most active site of star formation in the Galaxy,” the authors write in their research. The new observations reveal the cloud’s multilayered, highly-structure nature, along with two populations of massive stars. One is a revealed, low-extinction population, and the other is a hidden, high-extinction population. In this context, low extinction means not much of the star’s light is blocked by dust, while high extinction means much of the light is blocked.
Sgr B2 is in the Milky Way’s Central Molecular Zone (CMZ), a gas-rich region that hosts an estimated 60 million solar masses of star-forming gas, all within a complex of gas clouds. The gas in the CMZ is much denser than elsewhere in the galaxy.
This Spitzer Space Telescope gives an overview of the Milky Way’s Central Molecular Zone. Each of the white rectangles is the footprint of completed JWST observation programs. The SMBH Sgr A is highlighted for context. Image Credit: Budaiev et al. 2025*
Despite holding all of that gas, the CMZ’s star formation rate is not as high as expected. “Despite containing around 80 percent of the Galaxy’s dense molecular gas, the CMZ only forms around 10 percent of the Galaxy’s stars, more than an order of magnitude lower from what we expect according to the typical dense gas relations,” the researchers explain.
This discrepancy is one of the reasons astronomers observed Sgr B2 with the JWST. They hope to understand how things work in this extreme environment. Sgr B2’s SFR is similar to the most active period of star formation around z = 2, during the Universe’s Cosmic Noon. Researchers are especially interested in it’s high star formation rate (SFR), which sets it apart from the CMZ. “Sagittarius B2 (Sgr B2), is a powerful laboratory for studying star formation and evolution in conditions similar to the most active period of cosmic star formation (z ≈ 2),” the authors write.
“Despite the great sensitivity of these observations, no extended population of YSOs has been detected, placing a limit on their minimum extinction; this result hints that star formation has only just begun in the cloud,” the authors write.
Though the images of Sgr B2 show multitudes of stars, another element of the images stands out, especially in images from the JWST’s Mid-Infrared Instrument. Some parts of the cloud are extremely dark. Not because there’s nothing there, but because the gas and dust is so dense that not even the powerful JWST can see inside them. The thick clouds are the material out of which stars form, and there are very likely YSOs inside them that haven’t yet made themselves visible.
“Together, these results suggest that, despite already holding the crown for most actively star-forming cloud, we have underestimated the total star formation in Sgr B2,” the authors explain.
Webb’s MIRI (Mid-Infrared Instrument) shows the Sagittarius B2 (Sgr B2) region in mid-infrared light, with warm dust glowing brightly. The darkest areas of the image are not empty space but areas where dust and gas are so dense that even MIRI can’t see the stars inside the darkness. One clump of clouds captured astronomers’ attention. It’s on the right side of the image, and it’s redder than the other clouds. Observations with other telescopes show that it’s one of the most most molecularly rich regions known. This region could hold clues that explain why Sgr B2 is so much more productive in making stars than the rest of the galactic centre. Image Credit: NASA, ESA, CSA, STScI, A. Ginsburg (University of Florida), N. Budaiev (University of Florida), T. Yoo (University of Florida). Image processing: A. Pagan (STScI)
The particularly red cloud in the MIRI image is attracting a lot of attention because observations with other telescopes revealed that it’s very molecularly rich. But these new JWST images are the first time that astronomers have seen it so clearly. It’s bright because it’s highly ionized by massive young stars.
This image is a NIRCam tricolor figure overlaid on top of MeerKAT 1.2 GHz continuum with the main parts of the cloud and the observed features labeled. The blue color represents low extinction, green shows regions with higher extinction, and red shows warm dust. Image Credit: Budaiev et al. 2025
The central question about Sgr B2 and the CMZ is why star formation is so high in the former, yet puzzlingly low in the latter. While these observations can’t exactly explain why, the detailed data gathered by the JWST and its powerful MIRI and NIRCam instruments may lead to an eventual answer. “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 authors write.
By discovering a dual population of revealed low-extinction and hidden high-extinction massive stars, the JWST has shown that other telescopes have been missing a significant fraction of star formation in Sgr B2 that was obscured by dust. Beyond that, the telescope found more than one dozen HII regions that were previously unknown. These regions contain ionized hydrogen that tells astronomers that hot young stars have recently formed and ionized the hydrogen with powerful UV radiation.
Humans have a long history of gazing at the cosmos in wonder, identifying things that need an explanation, and then working towards a solution. We’re wonderers and intellectual nomads, and powerful technological tools like the JWST have amplified our sense of wonder.
“Humans have been studying the stars for thousands of years, and there is still a lot to understand,” said Nazar Budaiev, a graduate student at the University of Florida and the co-principal investigator of the study. “For everything new Webb is showing us, there are also new mysteries to explore, and it’s exciting to be a part of that ongoing discovery.”
For interested readers, an interactive view of the JWST observations of the Galactic center can be viewed at: https://starformation.astro.ufl.edu/avm images/jwst cmz hips.