Why are Forbidden lines seen in interstellar nebula? What conditions make these lines possible?
Describe the appearance of a dark dust cloud. How can we detect these clouds if they are dark and absorb light?
Since the density of dark dust clouds is much lower than the density of Earth's atmosphere, how can they block light?
What causes 21-cm radiation? What causes these atoms to be excited? How is the excited state different from the excitation discussed previously?
Why is 21-cm radiation important for studying the Galaxy?
Describe the density and composition of a typical molecular cloud. What fraction of the cloud is composed of complex molecules (NOT hydrogen gas)?
List 5 of the molecules detected in interstellar space?