Review

#11

Light travels at a finite speed, but the escape speed from a black hole gets arbitrarily large at smaller and smaller distances. So there is a point at which even light is traveling too slow to get away from a black hole (which is to say, too slow to break out of orbit).

#15

The location where the escape speed from the black hole equals the speed of light.

#17

The mass of the unseen companion is too large to be a normal star, a white dwarf, or a neutron star. Moreover, the large X-ray brightness is expected from gas falling into a black hole.