One topic that people always ask astronomers about is black holes. These seemingly mysterious and bizarre objects are known to gobble up everything around them, even light, and physics as we know it cannot accurately describe what goes on inside.
From the outside, however, we consider black holes to be pretty simple objects, described completely by their mass, or size, spin and charge. This is often called the “no-hair” theorem of black holes.
For the most part, something that massive is almost certainly neutral, so astronomers really only care about how big a black hole is and how fast it is spinning.
As you might imagine, actually measuring these quantities can be a bit tricky.