A multi-year study of the Whirlpool galaxy (M51) has shaken up  astronomers’ views of the properties of giant molecular clouds. The new  study, which mapped 1,500 such clouds, shows that, instead, they are  embedded in a kind of fog of molecular hydrogen much more dense than  anyone expected, which permeates the whole of the galactic disc.  Pressure exerted by this fog is crucial in determining whether or not  new stars will form within the clouds. The study, led by Eva Schinnerer  from the Max Planck Institute for Astronomy, made extensive use of the  millimeter telescopes of IRAM, the Institut de Radioastronomie  Millimétrique.

  Most of a galaxy’s stars are born within giant molecular clouds -  accumulations of hydrogen molecules with total masses between a thousand  and several million times that of our Sun. As a region within such a  cloud collapses under its own gravity, it contracts until pressure and  temperature are high enough for nuclear fusion to set in: a new star is  born.

  Now, a new study challenges astronomers’ traditional views about  these stellar birthplaces. Study leader Eva Schinnerer (Max Planck  Institute for Astronomy) explains: “Over the past four years, we have  created the most complete map yet of giant molecular clouds in another  spiral galaxy similar to our own Milky Way, reconstructing the amounts  of hydrogen molecules and correlating them with the presence of new or  older stars. The picture that is emerging is quite different from what  astronomers thought these clouds should be like.” The survey, known as  PAWS, targeted the Whirlpool galaxy, also known as M51, at a distance of  about 23 million light-years in the constellation Canes Venatici  (“Hunting dogs”).

 Molecular hydrogen in the Whirlpool Galaxy M51. The blueish  features show the distribution of hydrogen molecules in M51, the raw  material for forming new stars. The PAWS team has used this data to  create a catalogue of more then 1,500 molecular clouds. The reddish  structures show the distribution of hydrogen atoms. The background is a  color image of M51 by the Hubble Space Telescope. Credit: PAWS  team/IRAM/NASA HST/T. A. Rector (University of Alaska Anchorage)

For a little more insight into the project see this：http://www.iram-institute.org/EN/news-astronomers/2013/85.html