Putting Black Holes On A Diet

Science fiction, and science journalism, have tended to portray black holes as constantly devouring everything that comes close. That's basically accurate. Astronomers have known for a while, however, that the black holes at the centers of some galaxies were "starved" of material. Now, there's a theory to explain how.

Supernova explosions are regular events in most galaxies that have black holes at the center. Not only are they bright enough to ourshine their parent galaxy for a time, but such explosions also set up huge shock waves that behave akin to tsunamis in Earth's oceans. A tsunami moving through deep water does little harm, but when that energy builds up in shallow water, huge, powerful waves result. Similarly, in massive galaxies, gravity overwhelms supernova shock waves, the waves do not drive gas and dust away from the center of the galaxy, and that material continues to fall into the black hole. In less massive galaxies, however, the gravity is not as strong, and continual shock waves can sweep gas and dust out of the reach of black holes.

Of course, the situation is temporary. At some point, the rate of supernova explosions within a galaxy will fall off, the effectiveness of the shock wave mechanism will decrease, and the black hole will still be there. Waiting.