Molecular Hydrogen, commonly written H₂, is a molecule formed by two hydrogen atoms that share their electrons.

H₂ is a molecule formed by two hydrogen atoms that share their electrons.

In astronomy, molecular hydrogen is notoriously difficult to detect. However, it has been shown that for every 10,000 hydrogen molecules in the interstellar medium there exists a carbon monoxide molecule (CO), and it is by searching for CO that astronomers are able to map out the H₂ distribution in the Galaxy.

This has revealed that molecular hydrogen is found primarily in molecular clouds. These are interstellar gas clouds which have been found to trace the spiral arms in the disks of spiral galaxies, and contain the material out of which new stars are born. In other words, molecular hydrogen is a vital ingredient in active star formation.