• Astronomers discovered Eos, the closest molecular cloud to Earth at 300 light-years away, using its hydrogen's fluorescent glow.
• Eos is a crescent-shaped cloud spanning 100 light-years, shaped by the North Polar Spur, challenging prior detection methods.
• The discovery of Eos provides new insights into star and planet formation and may lead to a NASA mission to study molecular hydrogen clouds.

Astronomers have recently discovered a massive molecular cloud named Eos, located approximately 300 light-years from Earth. This crescent-shaped cloud, primarily composed of hydrogen gas, spans about 100 light-years, making it one of the largest structures visible in the sky. The discovery is significant because Eos was detected through the fluorescent glow of hydrogen molecules, a novel approach that bypassed the typical reliance on carbon monoxide detection. Eos's proximity and size provide a unique opportunity to study the cosmic recycling of matter and gain insights into the processes of star and planet formation. The cloud's shape is influenced by its interaction with the North Polar Spur, a vast region of ionized gas.

• What: Discovery of Eos, a large, crescent-shaped molecular cloud composed primarily of hydrogen gas, located relatively close to Earth.
• Who: Blakesley Burkhart (Rutgers University) led the discovery; Thavisha Dharmawardena (New York University) is an author of the study. NASA is considering a mission concept.
• When: Discovery announced in a paper published April 28, 2025, in the journal Nature Astronomy. The data analyzed was 20 years old from a satellite launched in 2003.
• Where: Eos is located 300 light-years from Earth. It spans about 100 light-years, making it one of the largest structures in the sky.

The discovery of Eos marks a significant step forward in our understanding of molecular clouds and their role in star and planet formation. The fact that Eos eluded detection for so long due to its low carbon monoxide content highlights the limitations of relying solely on CO signatures to identify such clouds. The new method of detection, based on the fluorescent glow of hydrogen, opens up possibilities for discovering many more hidden clouds throughout the galaxy. The interaction between Eos and the North Polar Spur underscores the dynamic forces that shape these cosmic structures. The proposed Eos space telescope mission could revolutionize our understanding of molecular hydrogen and its distribution in the Milky Way.

"If you were to see this cloud on the sky, it’s enormous," said Dr. Burkhart, who announced the discovery with colleagues in the journal Nature Astronomy. And "it is literally glowing in the dark," she added.

"It's huge, and it's been hidden for this whole time," Blakesley Burkhart, an associate professor in the Department of Physics and Astronomy at Rutgers University in New Jersey who led the discovery, told Live Science.

"We are, for the first time, seeing this previously hidden reservoir of hydrogen that can form stars," said Thavisha Dharmawardena, an astronomer at New York University who is an author of the study. After Eos, she said, astronomers are "hoping to find many more" such hydrogen-heavy clouds.

The discovery of Eos represents a significant advancement in astronomical research, challenging existing assumptions about molecular cloud detection and providing new insights into star and planet formation. The cloud's proximity to Earth offers a unique opportunity for detailed study, and the proposed Eos space telescope mission holds the potential to further revolutionize our understanding of molecular hydrogen in the Milky Way. While many questions remain unanswered, the discovery of Eos marks a new chapter in the exploration of our galaxy.