1. How do hydrogen atoms generate 21-cm radiation? What do astronomers learn about our Galaxy from observations of that radiation? 2. Why do astronomers believe that vast quantities of dark matter...

1. How do hydrogen atoms generate 21-cm radiation? What do astronomers learn about our Galaxy from observations of that radiation?

2. Why do astronomers believe that vast quantities of dark matter surround our Galaxy?

3. Why are there no massive O and B stars in globular clusters?