A chilly cloud of molecular gas in the Milky Way is giving astronomers a rare look at one of the earliest steps in star formation.

The smallest, most fundamental molecules in the Universe are created when two hydrogen atoms bond to form hydrogen molecules (H₂). This process usually takes place in cold, dark clouds. But the molecule’s formation is rarely observed, because it’s hard to distinguish atomic and molecular hydrogen from other types of molecules and from each other.

Pei Zuo and Di Li at the National Astronomical Observatories of the Chinese Academy of Sciences in Beijing and their colleagues used the Arecibo radio telescope in Puerto Rico to observe dark clouds. The researchers found that one cloud had an outer ‘shell’ of atomic hydrogen that was being converted into molecular hydrogen — the first such detection of a dark cloud’s birth.

Further analysis of the rate of H₂ formation suggested that the cloud is roughly six million years old. This finding could help to constrain models of star, planet and galaxy formation, the authors write.