Molecular clouds, where stars are born, are surrounded by a fog of hydrogen that interacts with the clouds. These interactions could affect whether stars form.

In the Plateau de Bure Interferometer Arcsecond Whirlpool Survey, astronomers led by Eva Schinnerer of the Max Planck Institute for Astronomy in Heidelberg, Germany, mapped 1,500 giant molecular clouds in the Whirlpool galaxy, about 7 million parsecs (23 million light years) away. They report in a series of papers that the clouds are not isolated as previously thought, but are embedded in a molecular fog that is much denser than expected and contains 50% of the galaxy's hydrogen.

Clouds that are surrounded by the fog experience decreased pressure when the fog moves relative to larger galactic structures, such as the arms of the spiral galaxy. This reduced pressure lowers the chances of star formation.

Astrophys. J. 779, 42; 43; 44; 45; 46 (2013)