A black hole is just an object so dense that, as you approach it, even light can’t escape. And since nothing travels faster than the speed of light that means absolutely nothing can escape. While that might sound a bit scary, the first thing you should know is that there is a black hole sitting at the center of the galaxy we live in. It is named Sagittarius A, and it is actually not so terribly big … for a black hole of its kind.
Black holes are divided into two groups that are very different in size. First, there are stellar-mass black holes, which are about as heavy as a couple of suns. These are formed when giant stars die. Then, there are supermassive black holes, like Sagittarius A. Scientists don’t know for sure how supermassive black holes form or why they shrink, but ours is actually pretty small.