Dusty Dark Clouds
There are thousands
of giant molecular clouds in the disk of our galaxy, each containing from about
100,000 to several million solar masses of hydrogen. These clouds must be cold enough for molecules to
form, since a variety of molecules are observed in them using radio telescopes.
For the molecules to
exist, it is probably necessary that the cloud have enough dust to shield the molecules from
UV radiation from other stars. Thus, they tend to be very dark because of the dust.