Black holes grow in mass by capturing nearby material. Anything that enters the event horizon cannot escape the black hole's gravity. So objects that do not keep a safe distance get swallowed.

Despite their reputation, black holes will not actually suck in objects from large distances. A black hole can only capture objects that come very close to it. They're more like Venus' Flytraps than cosmic vacuum cleaners. For example, imagine replacing the Sun by a black hole of the same mass. Permanent darkness would fall on Earth, but the planets would continue to revolve around the black hole at the same distance and speed as they do now. None of the planets would be sucked into the black hole. Our Earth would be in danger only if it came within some 10 miles of the black hole, much less than the actual distance of Earth from the Sun (a comforting 93 million miles).

The diet of known black holes consists mostly of gas and dust, which fill the otherwise empty space throughout the Universe. Black holes can also consume material torn from nearby stars. In fact, the most massive black holes can swallow stars whole. Black holes can also grow by colliding and merging with other black holes. This growth process is what can reveal the presence of a black hole. As gas falls toward a black hole, it is heated to high temperatures, generating powerful radio waves and X-rays that can be studied by astronomers.

The following pages showcase the supermassive black holes in the centers of two distant galaxies that reveal their presence through their very powerful radio waves and X-rays: