A Pulsing Heart At the core of the billowing gas and dust of the Crab Nebula (right) is a remarkably metronomic powerhouse known as the Crab Pulsar. First detected as radio sources in 1967, pulsars have proven to be a varied lot, radiating at almost every wavelength of light, including very-high-energy gamma rays. As explained on the next page, these complex objects are believed to be neutron stars, the collapsed and tightly compressed remains of massive suns that have exploded as supernovas. Spinning at the rate of 30 times per second, the Crab Pulsar is what's left of a star whose blazing death throes were witnessed nearly a thousand years ago.

In the sequence of photos above, the pulsar at the center of the Crab Nebula winks off and on and off again at visible wavelengths of light detected by a specially developed instrument called the Pulsar Hunter. The blue glow in the inner portion of the nebula (top) is light emitted by energetic electrons as they spiral through the pulsar's strong magnetic field.

(A rapidly spinning black hole has a smaller event horizon than a stationary black hole of the some mass.) That dimension is called the Schwarzschild radius, for the German astrophysicist Karl Schwarzschild. For example, a collapsing stellar core with three times the mass of our Sun would form a black hole with a Schwarzschild radius of about 5 miles. A black hole with the mass of Earth would have a Schwarzschild radius of less than half an inch. Its density would be 1.5 trillion times greater than that of a neutron star. A bathtub full of these gumball-sized black holes would outweigh all of the matter in our solar system.

One does not need to cross the event horizon of a black hole for bizarre things to start happening. Imagine a wayward astronaut with the misfortune of drifting headfirst toward a three-solar-mass black hole. By itself the hole's gravity is no deadlier to the astronaut than if he were diving into a swimming pool on Earth. After all, one is always weightless in free fall. The black hole's tides are a different story. As our wanderer falls farther toward the center of the hole's gravity well, he feels a vastly stronger pull on his head than on his feet. Within 10 miles or so of the black hole, the tides tear his body to pieces. The forces extrude him through space like toothpaste being squeezed from a tube. When he reaches the event horizon, his former body is no more than an extended string of atoms.

The best evidence for black holes with a few times the mass of the Sun comes from binary star systems, in which one star orbits an invisible but hungry companion. Gas drawn off from the visible star spirals into the companion. This disk of material accelerates to fantastic velocities as it slides down the black hole's space-time vortex. Friction within the disk heats up the gas to hundreds of thousands of degrees. The gas glows blue-hot and emits floods of ultraviolet radiation and x-rays. Although the black hole itself eludes our view, we can deduce its presence because a gaseous speedway encircles it, ablaze with high-energy radiation. Astronomers also have found strong signs that supermassive black holes--monsters with millions or even billions of times the mass of our Sun--lurk at the cores of most galaxies. These objects probably produce some of the most energetic outbursts in the cosmos (page 146).

On the other end of the size spectrum, some cosmologists believe the universe may contain swarms of "mini" black holes. Conditions might have been just right during the first moments of the Big Bang to compress small clumps of matter into (continued)