A dense cloud containing H₂, He, and many other molecules. Molecular clouds are typically 100-, 10-20 K, size of pc, and have 10-10⁶ molecules cm^-3. The free-fall time for such clouds is y. However, magnetic fields support the cloud and support the collapse, regulating star formation. Molecular clouds with masses in the range 10³- are sometimes called giant molecular clouds.

HI Region, HII Region