Fast-moving gas jets and gravitational forces equal to millions or billions of Suns point to the existence of huge black holes in the center of some galaxies. There might even be one in the center of our own Milky Way galaxy. (In the summer, when you look in the direction of the constellation Sagittarius, you are looking toward the center of our galaxy.) These supermassive black holes might be the result of the collapse of a dense cluster of stars.

The event horizon is an imaginary sphere around the black hole where the escape velocity is greater than the speed of light. Once anything crosses the event horizon, nothing, not even light, can escape. When material crosses the event horizon, the mass of the black hole increases, which boosts the black hole's gravitational force and expands its event horizon — the point of no return. The result? The black hole can now grab material that was too far before.

This is how astronomers think the supermassive black holes grow. As material gets closer and closer to the event horizon, the material heats up and swirls around the hole. This "accretion disk" is visible to us. By measuring its motion, we can infer the mass of the black hole.