Eos molecular cloud discovered 300 light-years from Earth

Astronomers have discovered a massive molecular-hydrogen cloud named Eos located approximately 300 light-years from Earth. Despite its vast scale, the cloud remained undetected until 2025 because it is “CO-dark,” meaning it lacks the carbon monoxide typically used as a proxy to identify molecular hydrogen in conventional surveys.

Spanning an angular width of roughly 20 degrees—equivalent to about 40 full-Moon widths across the sky—Eos has an estimated mass of 5,500 solar masses. The discovery, published in Nature Astronomy, was made by detecting H2 through far-ultraviolet fluorescence rather than traditional radio signals.

The cloud’s nearest portion is situated about 94 parsecs (307 light-years) from the Sun, with three-dimensional dust maps tracing the structure out to approximately 424 light-years. The finding highlights significant gaps in current astronomical observation strategies, demonstrating how large, nearby interstellar structures can be overlooked when they do not emit strongly in standard detectable wavelengths.