Eos is located at the edge of the Local Bubble, an irregularly shaped region of hot, rarefied gas within the Orion Arm that envelops the solar system. Scientists estimate that the cloud is enormous in size when projected onto the sky — about 40 moons — and has a mass about 3,400 times that of the Sun. Models suggest that Eos will evaporate in about 6 million years.
Eos poses no danger to Earth, the researchers say. But because it is relatively close, the gas cloud offers a unique opportunity to study the properties of its structure and how gas and dust will begin to transform into stars. And the far-ultraviolet fluorescence emission technique used to detect Eos could rewrite our understanding of the interstellar medium, revealing hidden clouds throughout our galaxy and beyond.
Molecular clouds are also called stellar nurseries. They are made of gas and dust, the most common molecule of which is hydrogen, a fundamental building block needed to form stars and exoplanets. But they also contain other molecules, such as carbon monoxide (CO). Our Sun was also formed by the gravitational collapse of a molecular cloud.
Astronomers typically find molecular clouds using radio and infrared observations, which easily pick up the chemical signature of carbon monoxide. But the new cloud was discovered using a different method — by directly looking for the ultraviolet light emitted by hydrogen molecules. To do this, the astronomers used the FIMS-SPEAR far-ultraviolet spectrograph, which was operating on the Korean satellite STSAT-1.
Map showing the position of the solar system, the Local Bubble, and the newly discovered Eos molecular cloud. Thomas Müller (HdA/MPIA) and Thavisha Dharmawardena (NYU)
The giant cloud, named Eos after the Greek goddess of the dawn, contains gas equivalent to about 3,400 solar masses. The unexpected discovery of a huge cloud of molecular gas – the stuff that forms stars – just 300 light-years away opens up new ways to study the conditions that help stars form.
Stars form from collapsing clouds of molecular gas. We see this in the Orion Nebula, which is powered by the hot ultraviolet radiation of the young stars born within. However, finding molecular clouds before they start producing stars is more difficult.
Such clouds are mostly made of molecular hydrogen, which, when not receiving energy from starlight, is very faint — almost invisible. Atomic hydrogen, on the other hand, is easily detected by radio telescopes. Astronomers commonly use radio telescopes to detect carbon monoxide, which is present in much smaller quantities in molecular clouds, as a proxy.
Molecular cloud Barnard 68. ESO
Astronomers led by Blakeslee Burkhart of Rutgers University-New Brunswick in New Jersey and Tavisha Dharmawardena of New York University have discovered a completely new way to see the invisible. Using far-ultraviolet data from Korea’s STSAT-1 satellite, they directly detected fluorescent hydrogen molecules, they report April 28, 2025, in the journal Nature Astronomy.
“This is the first molecular cloud discovered by directly searching for far-ultraviolet emission from molecular hydrogen,” Burkhart said in a statement. “This cloud literally glows in the dark.”
The cloud is crescent-shaped and sits at the edge of the Local Bubble, a volume of space where the interstellar medium is thinner than its surroundings, perhaps devastated by shock waves from hundreds of ancient supernovae. The Sun and our solar system are passing through the Local Bubble, and have been doing so for about five million years.
The cloud, named Eos after the Greek goddess who symbolized the dawn, contains about 3,400 solar masses of gas. It is also depleted in carbon monoxide, so it has not been detected by conventional means.
An artist’s impression of what the Eos cloud would look like in the night sky if it were visible to the naked eye. It is so huge that at a distance of 300 light-years, it would be 40 lunar diameters across. NatureLifePhoto/Flickr (New York City Skyline), Burkhart et al. 2025
Eos is predicted to dissipate, or photodissociate, as a result of the impact of background photons on the cloud molecules in about 5.7 million years. This is too early to start forming stars unless there is some other trigger that speeds things up, such as the gravitational perturbation of another passing cloud. Interestingly, the average rate of star formation in the vicinity of our Sun has been calculated to be 200 solar masses per million years.
Eos is losing mass into the wider interstellar medium at a rate of 600 solar masses per million years, three times the rate at which molecular gas is turning into stars. So this dispersion of molecular clouds, a result of photodissociation of light emitted by nearby stars, appears to act as a feedback mechanism to regulate the rate of star formation, Burkhart’s team believes. This is useful information that will tell us more about the conditions needed for star formation in other, more distant clouds.
“When we look through our telescopes, we see entire solar systems in the process of forming, but we don’t know in detail how this happens,” Burkhart said. “Our discovery of Eos is exciting because we can now directly measure how molecular clouds form and disintegrate, and how the galaxy begins to transform interstellar gas and dust into stars and planets.”
And the discovery of other such clouds may be just around the corner.
“Using the far-ultraviolet fluorescence emission method could redefine our understanding of the interstellar medium by revealing hidden clouds throughout the galaxy and even to the farthest detectable reaches of the cosmic dawn,” Dharmawardena said.
Eos may not see the dawn of new stars, but its existence is a testament to a greater dawn, dating back to the very beginning of the universe, when stars brought daylight to dark space.
