The Cygnus X star-forming region is about 4,600 light-years away. It contains a huge number of massive protostars, and one of the most massive star-forming molecular clouds known. With all of this activity, it’s not surprising that it also hosts some objects that have puzzled astronomers.

One of them is the so-named “Diamond Ring,” an usual bubble blown in space by a massive star. While other stellar bubbles are spherical and expand rapidly, the Diamond Ring is flat and is expanding slowly. An international team of researchers have figured out what’s going on.

Their results are in research published in Astronomy and Astrophysics titled “The Diamond Ring in Cygnus X: Advanced stage of an expanding bubble of ionised carbon.” The lead author is Simon Dannhauer from the Institute of Physics at the University of Cologne in Germany.

“The radiation and stellar winds emitted by OB stars inject substantial radiative, thermal, and mechanical energy into the interstellar medium (ISM),” the authors write. This stellar feedback is important and can reduce star formation in its vicinity. Recently, astronomers have developed a method of quantifying this stellar feedback by using the fine structure of ionized carbon (CII). These CII emissions do a great job of tracing the boundary and interface between ionized regions and neutral gas. This is exactly where stellar winds and radiation interact with the ISM.

“The ‘Diamond Ring’ within Cygnus X, south-west of the DR21 ridge, stands out as a prominent, nearly circular structure in infrared (IR) and far-infrared (FIR) emission, spanning approximately 6 pc in diameter,” the researchers write. It contains an HII region, which is hydrogen ionized by powerful UV radiation coming from massive young stars. The researchers say that the HII region is very similar to other regions associated with expanding stellar bubbles, however those regions are routinely seen in ionized carbon (CII).

This new research is based on observations from SOFIA, the Stratospheric Observatory for Infrared Astronomy, which ended operations in September, 2022. SOFIA’s CII observations show that the Diamond Ring is tilted, that is has a mass of about 103 M⊙, or about 1,000 solar masses. It’s also expanding at a rate of about 1.3 km s−1, or about 4,680 km/h. While that is incredibly fast in our Earthly frame of reference, it’s a relatively sedate pace for an expanding stellar bubble.

The bubble, and the HII region, were created by a B0.5e star. All B-type stars are hot, but B0.5 stars are among the hottest. The ‘e’ indicates emission lines, which means that the star is rotating very rapidly and ejecting material from its equator, which forms a disk of material. The disk is where the emissions come from.

This image is an overview of the northern part of the Cygnus X star formation region. The black star labelled #227 is the star that carved out the bubble. The part of the bubble that appears as a diamond on the ring is actually not part of the structure. It's a cluster of young stars a few hundred light-years closer. Image Credit: Dannhauer et al. 2025. A&A *This image is an overview of the northern part of the Cygnus X star formation region. The black star labelled #227 is the star that carved out the bubble. The part of the bubble that appears as a diamond on the ring is actually not part of the structure. It’s a cluster of young stars a few hundred light-years closer. Image Credit: Dannhauer et al. 2025. A&A*

“This observation marks the first instance where we observe only a slowly expanding ring of [C II] emission and not an expanding 3D shell,” the authors write. “We suggest that the H II region (along with its associated [C II] bubble), initially formed by a massive star, expanded outwards from a flat slab of molecular gas nearly in the plane of the sky.”

The slowly expanding ring represents the final stage in the expanding bubble.

“For the first time, we observed the final stage of such a gas bubble in a distinctly flat cloud structure,” explained lead author Dannhauer in a press release. “The bubble has ‘burst’, because gases were able to escape into the thinner areas around it. All that remained was the particular flat shape.”

These are some of the CII scientific images of the Diamond Ring from the study. Each panel shows the star that carved the bubble with a yellow star, while magenta crosses represent other protostars in the region. The images show "the emission distribution and velocity structure of the [C II] and CO lines in the Diamond Ring. All figures reveal a mostly coherent ring-like structure in [C II] at the DR21 bulk velocity, spanning from −3 km s−1 to 1 km s−1" the authors explain. Image Credit: Dannhauer et al. 2025. A&A *These are some of the CII scientific images of the Diamond Ring from the study. Each panel shows the star that carved the bubble with a yellow star, while magenta crosses represent other protostars in the region. The images show “the emission distribution and velocity structure of the [C II] and CO lines in the Diamond Ring. All figures reveal a mostly coherent ring-like structure in [C II] at the DR21 bulk velocity, spanning from −3 km s−1 to 1 km s−1” the authors explain. Image Credit: Dannhauer et al. 2025. A&A*

The bubble initially expanded in all directions, like a regular bubble. Now, the ring is confined by gas swept up into the flat slab of molecular gas. The portions of the bubble that were moving perpendicular to the slab have dissipated, and that’s caused a decrease in the expansion of the portion of the ring that remains. The team ran simulations that trace how the CII bubble evolves, and they support this explanaion.

According to this work, the ring itself is only about 400,000 years old, young for a cosmic structure. The observations also show that the diamond-like feature isn’t actually part of the bubble. “The [C II] data revealed that the ‘Diamond’ of the Diamond Ring is an unrelated, dense gas clump,” the authors write.

Stellar feedback plays a powerful role by shaping the surroundings of young stars. It regulates the conversion of gas into stars by heating nearby gas. Galaxies wouldn’t be the same without it, so it plays a role in shaping entire galaxies. A single massive star can have a defining effect on the gas cloud that spawned it, not only negatively, but positively, too. Some of the filaments, bubbes, and shells created by stellar feedback can trigger star formation rather than inhibit it.

“The ‘Diamond Ring’ is a prime example of how enormous the influence of individual stars can be on entire cloud complexes,” said Dr Nicola Schneider, co-author of the study. “Such processes are crucial for understanding the formation of stars in our Milky Way,” added Dr Robert Simon, another of the study’s co-authors.

The researchers explain in their article that the Diamond Ring is the terminal phase of an expanding bubble oc CII, and that it’s driven both by thermal pressure and stellar winds.”This study offers insights into how stellar feedback of a massive star impacts the dynamics of the gas in slab-like molecular environments, which are likely to be more common in star-forming regions than the idealised spherical expansion scenarios that are typically modelled,” the authors conclude.