Stars and planets are born inside swirling clouds of cosmic gas and dust that are brimming with hydrogen and other molecular ingredients. On Monday, astronomers revealed the discovery of the closest known cloud to Earth, a colossal, crescent-shaped blob of star-forming potential.

Named Eos, after the Greek goddess of the dawn, the cloud was found lurking some 300 light-years from our solar system and is as wide as 40 of Earth’s moon lined up across the sky. According to Blakesley Burkhart, an astrophysicist at Rutgers University, it is the first molecular cloud to be detected using the fluorescent nature of hydrogen.

“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.

Identifying and studying clouds like Eos, particularly based on their hydrogen content, could reshape astronomers’ understanding of how much material in our galaxy is available to produce planets and stars. It will also help them measure the creation and destruction rates of the fuel that can drive such formations.

“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.

Molecular hydrogen, which consists of two hydrogen atoms bound together, is the most abundant material in the universe. Stellar nurseries are chock-full of it. But it is difficult to detect the molecule from the ground because it glows in far-ultraviolet wavelengths that are readily absorbed by the Earth’s atmosphere.