Black holes often look very different from each other. But this is because of variety in what happens in their surroundings. The black holes themselves are all
identical, except for three characteristic properties: the mass of the black hole (how much stuff it is made of), its spin (whether and how fast it rotates around
an axis), and its electric charge. Amazingly, black holes completely erase all of the other complex properties of the objects that they swallow.
Astronomers can measure the mass of black holes by studying the material that orbits around them. So far, we have found two types of black holes:
stellar-mass (just a few times heavier than our Sun) or supermassive (about as heavy as a small galaxy). But black holes might exist in other mass ranges as well.
For example, recent observations suggest there may be black holes with masses between stellar-mass and supermassive black holes.
Black holes can spin around an axis, although the rotation speed cannot exceed some limit. Astronomers think that many black hole in the Universe probably do
spin, because the objects from which black holes form (stars for example) generally rotate as well. Observations are starting to shed some light on this issue, but
no consensus has so far emerged. Black holes could also be electrically charged. However, they would then rapidly neutralize that charge by attracting and
swallowing material of opposite polarity. So astronomers believe that all black holes in the Universe are uncharged.