View of the Coalsack nebula

A dark Galactic cloud similar to the Coalsack nebula (central black blob above) has been seen for the first time in the act of generating molecular hydrogen. Credit: ESO/Digitized Sky Survey 2/Davide De Martin

Astronomy and astrophysics

Dark space cloud caught donning halo of hydrogen molecules

For the first time, a Galactic cloud is seen producing an ingredient that is fundamental in star formation.

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.