Thanks, DoctorWho984! As an amateur astronomer, I am fond of observing open clusters. I haven't found comprehensive answers to this question before -- I learned a lot. And thanks for the Star Formation link.

Most stars are created in open clusters. Open clusters are created from the collapse of molecular clouds. But what creates molecular clouds? Why are molecular clouds often hanging around ready for collapse? Are they just relatively dense parts of the interstellar medium? If so, what makes those parts dense?