Gas clouds in the Milky Way's Galactic Center (GC) contain copious amounts of star-forming gas. But for some reason, few massive stars form there, even though similar gas clouds elsewhere in the galaxy easily form massive stars. The clouds also form fewer stars overall. Are they a new type of molecular cloud?
The star-forming regions are called Giant Hii regions, meaning the hydrogen is ionized by UV radiation from nearby stars. These clouds of gas can span hundreds of light-years while typical star forming clouds are only dozens of light years across. These giants hold enough gas to spawn thousands of stars. Some have a luminosity equivalent to millions of Suns and in other parts of the galaxy they're powered by star clusters with hundreds of hot, massive stars.
However, in the GC, these Giant Hii regions struggle to form massive stars. New research in The Astrophysical Journal examined three of these regions in the GC, also called the Central Molecular Zone (CMZ).
The research is titled "Surveying the Giant H ii Regions of the Milky Way with SOFIA. VII. Galactic Center Regions Sgr B1, Sgr B2, and Sgr C." The lead author is James M. De Buizer from the Carl Sagan Center for Research at the SETI Institute.
"Giant H ii (GH ii) regions are home to extremely massive young and forming OB stellar clusters," the authors write. "They contain a significant fraction of the most massive stars in a galaxy and therefore can dominate a galaxy's thermal emission." Studying these regions can help astronomers understand how to interpret observations of other galaxies and how massive stars form and evolve in the regions.
"Understanding how massive stars form and evolve can shed light on the initial mass function and star formation theories," the researchers explain.
The study is focused on 77 massive young stellar objects (MYSO) found in the three GH ii regions in the CMZ. During their formation, they're extremely influential objects because they can dramatically reshape their surroundings by emitting powerful UV radiation. Their powerful outflows can either trigger or stifle star formation in nearby regions. They can excavate cavities in their molecular clouds and disperse star-forming material. Observing them helps astronomers understand their energetic processes and how they influence galactic evolution. Unfortunately, they're buried inside dense clouds and are difficult to observe. This research used SOFIA observations to probe them inside their obscuring clouds.
The researchers found that there are similarities between all three GH ii regions, and also some significant differences.
This figure from the research shows Sgr B1 and B2. MYSOs are marked in letters and numbers. Letters represent MYSOs identified as radio continuum sources and numbers represent MYSOs identified as infrared sources. Image Credit: De Buizer et al. 2025. The Astrophysical Journal.
When they compared the three GH ii regions in the GC, they found that the star formation rate is below average. They also found that despite the dense clouds of gas, they struggle to form massive stars and may not have enough material to form more than one generation of stars. This sets the regions apart from other star-forming regions elsewhere in the Milky Way.
"Recent studies have concluded that star formation is likely depressed near the Galactic Center, and even that there may be no present star formation occurring there," said lead author De Buizer in a press release. "Since presently-forming massive stars are brightest at long infrared wavelengths, we obtained the highest resolution infrared images of our Galaxy's central-most star-forming regions. The data show that, contrarily, massive stars are presently forming there, but confirm at a relatively low rate."
This figure shows Sgr C and its three sub-regions. The numbers indicate MYSO candidates identified by SOFIA. Image Credit: De Buizer et al. 2025. The Astrophysical Journal.*
Sgr B2 seems to host the youngest of the MYSOs and also the most massive ones. The largest MYSO in Sgr B2 is about 64 solar masses, which is "... consistent with several of the other GH ii regions we have studied farther out in the Galactic plane," the authors write. However, the largest MYSOs in Sgr B1 and Sgr C are only half as massive at about 32 solar masses.
Sgr C stands apart for different reasons. "Sgr C has fewer confirmed MYSOs, and it seems to have a higher fraction of low-mass young stellar objects and contamination from more evolved interloper/foreground stars," the authors write. "Furthermore, unlike typical GH ii regions, Sgr B1 and Sgr C are significantly ionized by evolved interloper stars, which likely did not form within these regions," the researchers explain.
&t=32sOur Solar System is 26,000 light-years from the GC, but the three HII regions in the study orbit the GC at about 300 light years. This puts them in extremely close proximity to the energetic chaos that defines the region. There are intense tidal forces from the supermassive black hole that resides there, powerful stellar winds from massive stars, extreme temperatures, and powerful magnetic fields. This all disrupts the star formation process.
"These Galactic Center star-forming regions are in many ways very similar to the massive star-forming regions in the relatively calm backwaters of our galaxy," said study co-author Wanggi Lim from IPAC at Caltech. "However, the most massive stars we are finding in these Galactic Center regions, though still remarkably large, fall short in both size and quantity compared to those found in similar regions elsewhere in our Galaxy. Furthermore, such star-forming regions typically hang on to large reservoirs of star-forming material and continue to produce multiple epochs of stars, but that appears to not be the case for these Galactic Center regions."
One of the three regions is different, though. Sgr B2 has a low star formation rate (SFR) like the other H ii regions, but it appears to have held onto its reservoir of star forming material. Potentially, this means another future star cluster could be formed there. The authors explain that the nearby Sgr B1 could be more evolved and has "had time to disperse much of its molecular material."
Even though there are distinct differences between the three GH ii regions, they all have similar MYSO stellar densities. They're all within the range of GH ii regions elsewhere in the galaxy, but are all below the average. "This result is consistent with the perception that CMZ star-forming regions do not appear to be as prolific as one might expect, but they are indeed presently producing MYSOs at a rate consistent with GH ii regions farther out in the Galactic plane, albeit at a rate below the average," the authors write. But what's surprising is that MYSO stellar densities are comparable for all three GH ii regions, even though Sgr B2 is a much more prolific star-forming environment.
Astronomers think that typical GH ii regions host massive star clusters that are still embedded in their gas clouds. But two of the regions in this study, Sgr B1 and Sgr C, don't fit into this definition. They may be a new type of region.
"In these ways, Sgr B1 and Sgr C deviate from classical GH ii region behavior, thus potentially representing a new category of GH ii region or challenging their classification as GH ii regions," the authors write.
Alternatively, they may not properly be called GH ii regions at all. "On the other hand, since the naive assumption is that GH ii regions are the result of clusters of massive stars forming from, and still residing within, their natal giant molecular clouds, one could argue that, instead of being a new category of GH ii region, Sgr B1 and Sgr C are not legitimate GH ii regions at all," the authors conclude.
