1.The terms H II and H2 are both pronounced “H two.†What is thedifference in meaning of those two terms? Can there be such a thingas H III?
2. Why do nebulae near hot stars look red? Why do dust cloudsnear stars usually look blue?
3. Describe the characteristics of the various kinds ofinterstellar gas (HII regions, neutral hydrogen clouds, ultra-hotgas clouds, and molecular clouds).
4. Describe how the 21-cm line of hydrogen is formed. Why isthis line such an important tool for understanding the interstellarmedium?
5. Describe the properties of the dust grains found in the spacebetween stars.
6. Why do molecules, including H2 and more complex organicmolecules, only form inside dark clouds? Why don’t they fill allinterstellar space?
7. Why can’t we use visible light telescopes to study molecularclouds where stars and planets form? Why do infrared or radiotelescopes work better?
8. The 21-cm line can be used not just to find out wherehydrogen is located in the sky, but also to determine how fast itis moving toward or away from us. Describe how this might work.
9. Astronomers recently detected light emitted by a supernovathat was originally observed in 1572, just reaching Earth now. Thislight was reflected off a dust cloud; astronomers call such areflected light a “light echo†(just like reflected sound is calledan echo). How would you expect the spectrum of the light echo tocompare to that of the original supernova?
10. We can detect 21-cm emission from other galaxies as well asfrom our own Galaxy. However, 21-cm emission from our own Galaxyfills most of the sky, so we usually see both at once. How can wedistinguish the extragalactic 21-cm emission from that arising inour own Galaxy? (Hint: Other galaxies are generally moving relativeto the Milky Way.)
