A massive, previously undetected molecular cloud has been found just 300 light-years from Earth — making it the closest such cloud ever discovered and potentially reshaping our understanding of how stars and planets form. The cloud, named Eos after the Greek goddess of the dawn, is not only enormous in size and mass, but also scientifically unique, offering astronomers an unprecedented opportunity to observe the earliest stages of star formation.
Hidden Giant in Our Backyard
Spanning an area 40 times the width of the full moon and weighing in at an estimated 3,400 solar masses, Eos should have been easily noticeable. Yet, it remained hidden until recently. That’s because traditional methods used to detect molecular clouds rely on spotting carbon monoxide, a tracer gas that emits at radio wavelengths. Eos, however, contains very little carbon monoxide, making it essentially invisible to conventional instruments.
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“We normally look for carbon monoxide, and we’ve got lots of facilities that can spot that,” said astrophysicist Thomas Haworth of Queen Mary University of London. “This thing was pretty much in our cosmic backyard, and we’ve just missed it.”
New Way of Seeing
The breakthrough came when researchers turned to an unconventional method — analyzing far-ultraviolet emissions from hydrogen molecules. These emissions are difficult to detect and have never before been used to find molecular clouds. The data was obtained from FIMS-SPEAR, a spectrograph aboard the Korean satellite STSAT-1. The data set had only recently been made publicly available in 2023.
“This is the first-ever molecular cloud discovered by looking for far-ultraviolet emission of molecular hydrogen directly,” said lead author Blakesley Burkhart, an astrophysicist at Rutgers University. “This cloud is literally glowing in the dark.”
The far-ultraviolet spectrum revealed hydrogen molecules fluorescing faintly — a telltale sign of a molecular cloud in its earliest phase. The crescent-shaped cloud sits on the edge of the Local Bubble, a low-density cavity in space thought to have been carved out by ancient supernovae. The Sun and our solar system are currently passing through this region.
Why Eos Matters
Eos is not just a curiosity — it’s a rare astrophysical laboratory. Molecular clouds like Eos are the birthplaces of stars, but catching one before star formation begins is exceptionally rare. Scientists believe Eos has not yet begun forming stars and may never do so unless triggered by an external disturbance, such as a nearby shockwave or a passing cloud.
Models predict that Eos will gradually disperse over the next 5.7 to 6 million years due to photodissociation, a process in which ultraviolet light breaks down the cloud’s molecular bonds. Interestingly, Eos is losing mass to the interstellar medium at a rate of about 600 solar masses per million years — three times the average star formation rate in our solar neighborhood. This suggests a regulatory feedback mechanism where interstellar radiation limits how much gas is available to form stars. “The discovery of Eos lets us directly observe how clouds like this form and dissolve,” said Burkhart. “It helps us understand how galaxies convert raw material — gas and dust — into stars and planets.”
A Dawn for New Discoveries
Eos could be the first of many hidden molecular clouds to come to light. The success of using far-ultraviolet fluorescence as a detection tool may open the door to discoveries far beyond our solar system, even reaching back toward the era of cosmic dawn — the time when the first stars formed.
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Astronomers are now calling for more missions and instruments that can capture ultraviolet fluorescence, especially since such data may reveal other structures long hidden in plain sight. “This technique could rewrite our understanding of the interstellar medium,” said co-author Thavisha Dharmawardena of New York University. “We may soon uncover entire populations of molecular clouds that have so far escaped detection.”