A neutron star is as dense as an atomic nucleus . Indeed, we can consider it a gigantic nucleus in its own right. It's downright puny on astronomical scales, measuring only about 10 miles across. Although that's just the size of a large city on Earth, the star contains more mass than our Sun. One teaspoonful of this matter weighs more than 3 billion tons. That's like stuffing a herd of 50 million elephants into a thimble. If we dropped a small piece of neutron star onto the ground, it would slice through Earth like a bullet through cotton and come out the other side.

Most neutron stars enter the universe like whirling dervishes, thanks to nature's laws of motion. Even if the large parent star spun slowly at the end of its life, the conservation of angular momentum dictates that the tiny neutron star must spin more than a million times faster. Such stars usually have intense magnetic fields that drive particles outward along two narrow jets. If the jets happen to point in Earth's direction, we see the neutron star emit blips of radiation each time it rotates. Those flashes, like the pulses from a rotating lantern in a lighthouse, prompted astronomers to call the extreme stars pulsars .

The British radio astronomers Jocelyn Bell and Antony Hewish discovered the first pulsar by accident in 1967. Bell spotted regular fluctuations in the radio signal from an unknown celestial object. Upon further analysis, the astronomers were stunned to find that the source blipped once every 1.33731109 seconds, the steadiest pulsations ever seen. Their first code name for the object was LGM-1, for "Little Green Men," since they half-jokingly proposed that it might emanate from an extraterrestrial civilization. But other pulsars turned up within a few months, including one at the center of the Crab Nebula . That object, which flashes 30 times per second in the middle of an expanding cloud of debris from a supernova, verified Bell and Hewish's suspicion that the pulsars were spinning neutron stars. Since then astronomers have found more than 1,000 other pulsars , almost all of them in our Milky Way galaxy. Most whirl between one and 100 times each second. The fastest yet seen spins an incredible 642 times per second. At that pace a point on its equator moves at one-seventh the speed of light.

The motions of pulsars are so steady that we can time them to within one part in a million billion. That's 15 decimal places, a rotational accuracy that rivals the best atomic clocks on Earth. Occasionally, we see tiny glitches in their rotation speeds--sudden increases that may make the pulsar spin a billionth of a second faster than before.

If we dropped a small piece of neutron star onto the ground, it would slice through Earth like a bullet through cotton and come out the other side.

These flaws probably arise when "starquakes" fracture the pulsar's brittle crust, briefly changing its moment of inertia . But unlike earthquakes on our planet, which crack Earth's crust a few yards at a time, a pulsar quake might break its surface by less than one one-thousandth of an inch. That tiny movement creates enough of a change in a pulsar's spin that we can detect it from thousands of light-years away.

A pulsar comes as close as any object in the universe to a perpetual motion machine. However, its spin must wind down as surely as that of any gyroscope or top on Earth. The slowdown stems from a pulsar's intense magnetic field , which exerts a gradual but persistent drag on the spinning object by interacting with nearby material in space. After tens of millions of years of this steady braking, most pulsars spin just once every few seconds. We can no longer see those old pulsars because they spin too slowly to create beams of radiation. As time goes on, they spin more slowly still, although it may take many billions of years for them to stop. Dead and invisible to us, these barely spinning pulsars are the most nearly perfect spheres in the cosmos. No rotational stresses make them bulge, and their fierce gravitational fields flatten any bumps on the surface more than a few atoms high.

The realm of pulsars seems exotic indeed. But for the ultimate in compact-matter weirdness, black holes win the prize. If the core of the collapsing star is more massive than about three times the mass of our Sun, not even neutron degeneracy can sustain its structure. Quarks , the foundation of nuclear matter, crush down upon themselves. In gravity's final triumph, what's left of the star collapses without limit as the fabric of space and time folds in on itself. Only mass, rotation, and electric charge remain where subatomic particles once existed.

Unlike other kinds of objects, black holes do not merely wrinkle the fabric of space-time. They rip it, permanently. Everything going into a black hole falls through a hole in the space-time cloth. According to Einstein's general theory of relativity , we can represent space as a rubbery sheet. Objects make dimples in the sheet and stretch space-time toward their centers. A black hole is aptly named, for it behaves like an infinitely deep dimple. If something falls into it--including a beam of light--it won't come back.

The edge of the black hole, the point of no return, is called the "event horizon ." A black hole's mass is the primary factor that determines the size of its event horizon . (continued)