1. Astronomers say that black holes could be the power source in X-ray binaries, quasars, and the cores of active galaxies. If nothing can escape from inside a black hole, where are the observed X-rays coming from?
2. Discuss the difficulties in measuring extragalactic distances. What phenomenon confused scientists for several decades, causing them to overestimate distances? What kinds of distance indicators can used? Describe some potential sources of error with the methods.