gravitational potential energy into superenergetic bursts of light. Their strategy is ruthless but simple: just rip all incoming matter to shreds.

Think back to James Joule 's water-paddle experiment and his reasoning that some of the potential energy of water plunging over Niagara Falls should convert to heat because of the resistance of the water itself. In the case of a supermassive black hole, entire stars play the role of the falling water. Stars that wander too close to the black hole feel its tidal forces and are pulled apart. The spiraling gas forms a thick disk that whirls ever closer to the hole. The gas cannot just plummet into the black hole because the material in the disk is too dense. The disk fiercely resists free-falling motion, like the water within the falls. Therefore, the gravitational potential energy of the gas converts almost entirely to heat, and the disk shimmers with a temperature of millions of degrees. The infalling gas prevents gamma rays and x-rays from escaping out the sides of the disk. Instead, the energy blasts into space above and below the disk in colossal jets , carrying along matter at more than 99 percent the speed of light. The surrounding gas glows with visible light and radio waves that we can see to the edge of the observable universe.

This is the leading model for the incredible energies produced by quasars. No one quite knows how the gargantuan black holes form in the first place. But once they begin adding entire stars to their menu, their gravitational hunger only increases. In vigorous quasars the black holes eat as many as 10 stars a year. Less active galaxies can maintain their luminosities on more meager diets.

Most of this action in distant quasars took place long ago. We see none in our neck of the universe today. One perfectly reasonable explanation is that the nuclear regions of nearby galaxies have run out of stars to feed their engines. Perhaps the black holes already devoured all stars whose orbits came too close. No more food, no more explosive regurgitations.

There's also a more fascinating explanation. As a black hole's mass grows, its event horizon --the point of no return for matter and for light plunging into the hole--also expands. This has the counterintuitive effect of decreasing the black hole's tides. The critical factor in tidal forces is not the total gravity felt by a falling object. Rather, it is the difference in gravitational force from one end of the object to the other. The gravitational field of a small low-mass black hole exerts fantastic tidal forces near its event horizon because the field declines in strength quickly as you move away from the hole. On the other hand, an enormous black hole extends powerful gravitational tendrils far into space. The gravity remains strong over long distances, and so the tides above the event horizon are not nearly as pronounced.

Consequently, some black holes are so bloated that they gulp entire stars without tearing them to atoms. Each star's gravitational potential energy converts entirely to speed--like the glass falling from the dinner table just before it hits the floor. None converts to heat and radiation. With no outpouring of energy, such black holes would be invisible. Calculations show that this intriguing shutoff valve may kick in when a black hole becomes about a billion times more massive than the Sun.

Under these circumstances, quasars are just early chapters in the lives of the cores of ordinary galaxies. As the central black hole ages and grows larger, the quasar becomes less hyperactive--and the galaxy around it looks more and more like a normal quiet galaxy. If that's true, supermassive black holes should be common, whether or not the cores of the galaxies are energetic. A growing set of data supports this notion: The list of nearby galaxies that appear to host dormant black holes has grown to more than two dozen. Our own Milky Way is among this fraternity. Astronomers believe that our galaxy's core probably contains a black hole a few million times more massive than our Sun. The giveaway is the vigorous speed that stars reach as they orbit near (but not too close to) the sleeping beast. Without such detailed observations, it would be hard to imagine that our deceptively peaceful galaxy harbors a heart of darkness.