1. What evidence indicates that a supermassive black hole is located at the center of our Galaxy? 2. Approximately how many times has the solar system orbited the center of the Galaxy since the Sun...

1. What evidence indicates that a supermassive black hole is located at the center of our Galaxy?

2. Approximately how many times has the solar system orbited the center of the Galaxy since the Sun and planets were formed?

3. Why do astronomers not detect 21-cm radiation from the hydrogen in giant molecular clouds?