How do astronomers study black holes when they can see them?
We “see” them by their gravitational effects on nearby stars, which they are often eating or causing to move at very high velocities just before the stars enter the black hole. The core of a galaxy that “weighs” 1 billion times the mass of the Sun but has only a few million visible stars in it in a small volume of space is a prime candidate for having a black hole. Also, a binary star system where the total mass is, say, 20 times that of the Sun but the only visible star has a mass of 8 solar masses, means it has a dark companion with 12 times the mass of the Sun. No normal stars are that massive and invisible, so, in other words, you have a black hole. Also, as shown in Figure 8, most models of real black hole environments imply very complicated mechanisms that produce high energy radiation and particles very unlike what you get in any other astronomical body. These emissions can also provide us with the smoking gun evidence to figure out whether a black hole lurks there or not.
