New Brunswick, New Jersey — Astronomers have uncovered the nearest known molecular cloud to Earth, offering a unique glimpse into the cosmic cycle of matter that fuels the birth of stars and planets. Named "Eos," after the Greek goddess of dawn, this massive, crescent-shaped hydrogen cloud is situated a mere 300 light-years away. Spanning about 100 light-years, Eos is roughly equivalent to the length of 40 Earth moons aligned in a row, positioning it as one of the most significant celestial structures observed.
Despite its impressive dimensions, Eos had previously avoided detection, primarily due to its minimal carbon monoxide content—a key indicator scientists typically rely on to spot molecular clouds. Instead, researchers identified Eos by observing the fluorescent glow emitted by hydrogen molecules, marking a breakthrough method that could potentially reveal many other hidden clouds across the galaxy. "There are definitely more CO-dark clouds out there waiting to be discovered," said Blakesley Burkhart, an associate professor in the Department of Physics and Astronomy at Rutgers University and the leading investigator in this discovery.
Hydrogen, the most ubiquitous element in the universe, makes clouds like Eos critical for understanding material availability for star and planet formation. By examining such hydrogen-rich clouds, astronomers may uncover previously unrecognized hydrogen reservoirs that can refine their estimates of cosmic matter distribution. The team’s findings were published in a recent study in Nature Astronomy.
The discovery was made while re-evaluating two decades’ worth of data from a spectrograph aboard the Korean Science and Technology Satellite-1, which was launched in 2003 to map hot gases in the Milky Way. The spectrograph enabled scientists to analyze emissions from various molecules by breaking down far-ultraviolet light into different wavelengths. Within what appeared to be an empty patch of sky, Burkhart’s analysis unveiled Eos, which she described as "literally glowing in the dark."
Eos’s striking crescent shape was influenced by its interaction with a vast nearby feature known as the North Polar Spur, a region filled with ionized gas extending from the Milky Way’s plane to the northern celestial pole. Burkhart explained that Eos’s alignment with this immense structure suggests that the energy and radiation from the Spur, possibly originating from past supernovae or stellar winds, have played a vital role in shaping the surrounding gas.
Simulations conducted by the research team predict that Eos may evaporate over the next 6 million years, as incoming photons and high-energy cosmic rays disrupt its molecular hydrogen reservoir. A subsequent investigation utilizing data from the now-retired Gaia space telescope explored the possibility of recent star formation within Eos. Though the findings remain under review, they indicate the cloud has yet to witness significant bursts of star formation, leaving open the question of whether it will initiate star creation before it dissipates.
Moreover, Burkhart and her colleagues are conceptualizing a NASA mission that would use a spacecraft named after the new molecular cloud. This proposed Eos space telescope would investigate far-ultraviolet wavelengths to measure molecular hydrogen content across the Milky Way, including in Eos itself, facilitating a detailed understanding of the cycles of formation and destruction of molecular hydrogen gas.
"There’s a lot still to explore," Burkhart noted. "We’re just beginning to uncover the mysteries of Eos and its cosmic significance."