A vast star-forming cloud of molecular hydrogen that is one of the largest single structures in the sky has been discovered hidden near the solar system.

The crescent-shaped cloud—which its discoverers named “Eos,” after the ancient Greek goddess of the dawn—has long been invisible to us, despite being just 300 light-years away.

However, a new approach using ultraviolet emissions has made the cloud visible to us.

“This opens up new possibilities for studying the molecular universe,” said study lead and astrophysicist professor Blakesley Burkhart of Rutgers University–New Brunswick in a statement.

Eos’ proximity to the solar system poses no danger—but, the researchers explain, it does provide a unique opportunity to study the evolution of structures in the interstellar medium.

“When we look through our telescopes, we catch whole solar systems in the act of forming, but we don’t know in detail how that happens,” Burkhart explained.

“Our discovery of Eos is exciting because we can now directly measure how molecular clouds are forming and dissociating—and how a galaxy begins to transform interstellar gas and dust into stars and planets.”

Eos is located just on the edge of the so-called Local Bubble—a large, low-density void in the interstellar medium that encompasses the solar system and is thought to have been formed as a result of past supernovae explosions.

The molecular cloud has an estimated mass equal to that of 3,400 suns and, if it could be seen with the naked eye, would appear some 40 times wider than the moon in the sky. Spotting the cloud in reality, however, was a more challenging proposition.

Despite typically being dominated by hydrogen, molecular clouds are typically detected by looking for the chemical signature for another common component, carbon monoxide.

Eos, however, contained very little carbon monoxide, explaining how it was able to stay invisible to use for so long. The cloud was instead revealed by looking for hydrogen, in a novel approach, based on data collected by the far-ultraviolet instrument on STSAT-1, a South Korean satellite.

“This is the first-ever molecular cloud discovered by looking for far ultraviolet emission of molecular hydrogen directly,” said Burkhart.

“The data showed glowing hydrogen molecules detected via fluorescence in the far ultraviolet. This cloud is literally glowing in the dark.”

The discovery, the researchers noted, came as something of a surprise.

“When I was in graduate school, we were told that you can’t easily directly observe molecular hydrogen,” explained paper co-author and New York University (NYU) astrophysicist Thavisha Dharmawardena in a statement.

“It’s kind of wild that we can see this cloud in data that we didn’t think we would see.”

The researcher’s modeling suggests that Eos will evaporate in around 6 million years’ time.

The full findings of the study are published in the journal Nature Astronomy.

With this investigation complete, the team is resuming their search for other molecular hydrogen clouds—both those near and very far away.

“The use of the far ultraviolet fluorescence emission technique could rewrite our understanding of the interstellar medium, uncovering hidden clouds across the galaxy and even out to the furthest detectable limits of cosmic dawn,” said Dharmawardena.

In fact, the team believe they may have also detected the most distant molecular cloud found to date, in data collected by the James Webb Space Telescope (JWST). The findings of this study, which have not yet been peer reviewed, can be read on the arXiv preprint server.

“Using JWST, we may have found the very furthest hydrogen molecules from the sun,” said Burkart.

She added: “So, we may have found both some of the closest and furthest using far-ultraviolet emission.”

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References

Burkhart, B., et al. (2025). A nearby dark molecular cloud in the Local Bubble revealed via H2 fluorescence. Nature Astronomy. https://doi.org/10.1038/s41550-025-02541-7

Johnson, M., Burkhart, B., D’Eugenio, F., Bourlot, J. L., Bialy, S., Tacchella, S., Maiolino, R., Roueff, E., Petit, F. L., Bron, E., Abgrall, H., Nelson, E., Menon, S., & Orr, M. E. (2025). A Tentative Detection of Molecular Hydrogen (H$_2$) Emission Lines at Cosmic Dawn. arXiv. https://doi.org/10.48550/arXiv.2502.19484