Supermassive Black Holes. Assume that a black hole with 10^15 Msun exists somewhere in the universe.

a. If the mass of the black hole were made up by typical main sequence stars of a common mass at the same density, what would the average separation between the stars be (in AU)?

b. Would an observer be able to see this black hole from a great distance? Describe what happens to the view of the black hole as an observer approaches it, and as (s)he crosses the event horizon.