To the greatest possible extent, answers should be in your own words and should demonstrate an understanding of the concepts involved. (More than just a glossary definition of the terminology is expected).
1. How is the 21-cm line of radiation generated by hydrogen atoms?  What can we learn about the Galaxy from observations of 21-cm radiation?
2. Why do we not observe O and B stars in globular clusters?
3. What observations led Harlow Shapley to conclude that we are not at the center of the Galaxy?
4. Briefly describe how the theory of self-propagating star formation accounts for the existence of spiral arms in some spiral galaxies.
5. What is a standard candle, and why is it important to measuring the Hubble constant?