John Heise, a lanky astrophysicist with a shock of white hair, gestured westward over the sunwashed Pacific as he tried to describe how this scene might change if we were, instead, hanging out on a neutron star.

"You'd start seeing past the horizon so that, in practice, the horizon lifts," he said. "The sky gets smaller. . . . Eventually we'd see Tokyo rising higher and higher in the sky."

That would be the effect of light bending (or space curving) in the grip of the star's powerful gravity to the point that, in theory, you could "see around corners." A pen dropped from table height would thunder with as much energy as a ton of high explosives. A rocket would have to blast off at half the speed of light (about 93,000 miles per second) in order to escape.

Neither Heise nor anyone else would be able to observe any such weird goings-on from a deck chair on the star's surface. The gravity would squash them to oblivion. But with ever better instruments on Earth and in space, he and other researchers have pried loose a mounting trove of information from these stingy targets just 10 or 15 miles across and hundreds or thousands of light-years away.

Heise was among several researchers who presented the latest mind-bending findings on the topic to several hundred scientists gathered earlier this month for a meeting of the High Energy Astrophysics Division of the American Astronomical Society.

A neutron star is the last category of gravitational collapse short of a black hole. It is born in a titanic stellar explosion known as a supernova, in which a massive star blows off its outer shell as its core implodes to a hot, spinning ember.

The collapse squeezes more than a sun's worth of mass into a ball with a diameter about the width of the District. At the subatomic level, compacted electrons and protons merge into neutrons. A sugar cube-size chunk of the stuff is famously estimated to weigh (on Earth) perhaps 1 billion tons.

"Neutron stars are really the most extreme physics lab we have to observe," said Tod Strohmayer of NASA's Goddard Space Flight Center in Greenbelt. "If you can study neutron stars, you can understand the physics of very dense matter" down to the most exotic particles.

Neutron stars harbor conditions that can never be duplicated in any Earth laboratory, he and others said, and may re-create a state of matter that existed for about one-millionth of a second after the moment of cosmic creation known as the Big Bang.

In the annals of extreme collapse, black holes have sucked up most of the public attention. But because no light can escape their powerful gravity, they are invisible. Neutron stars stop just short of "black," at about the density of an atomic nucleus. They bend light severely but do not devour it.

There are perhaps a billion of them scattered across the Milky Way galaxy, astronomers estimate, but most of them are old and therefore too dim and cool to be seen. But, at certain stages, surreal exhibitions of light and violence betray their presence.

First conceived in theories of the early 1930s, neutron stars were discovered as real objects in 1967, when Cambridge student Jocelyn Bell and her supervisor, Anthony Hewish, detected amazingly clock-like pulses at radio wavelengths coming from far out in space.

Scientists since have learned that neutron stars may spin almost 1,000 times per second (though 50 times a second is typical) at rates predictable to within a few parts per quadrillion--a precision that rivals the best atomic clocks.

With magnetic fields perhaps a trillion times that of Earth, these dervishes crackle with rippingly fierce voltages and throw off "lighthouse beams" of radiation that sweep Earth with every rotation. More than a thousand of these "pulsars" have been found.

Some neutron stars travel in complex, evolving relationships with companion stars whose substance they draw onto themselves--generating fireworks that include regular thermonuclear blasts. Strohmayer reported here on an extraordinary three-hour thermonuclear explosion on one such "binary" neutron star.

The cataclysm released about a trillion times the energy used by the United States in 1999. The members of his group, who at first thought something was wrong with their instrument, have speculated that the inferno may have been the product of a billion trillion pounds of carbon at billion-degree temperatures--a year or so worth of nuclear ash from the star's briefer, daily, helium-fueled explosions packed so tightly below the surface that it fused and blew. Some, questioning the carbon theory, are working on other explanations.

"Such a long burst--with a rich assortment of X-ray data--provides new insights into the physics of neutron stars and thermonuclear explosions--particularly about what is happening underneath the [star's] surface," Strohmayer said.

Heise created a stir here with the announcement that his group has used the Italian-Dutch Beppo-SAX space observatory to provide a potentially crucial link for future neutron star studies.

They observed bursts of X-rays from a key, well-studied pulsar whose rapid spin rate had been well documented. The trick was finding both phenomena in the same star--and determining that the two ran at similar frequencies.

If the findings are confirmed, Heise said, astronomers could determine the spin rate of hundreds of neutron stars that only become visible during X-ray bursts.

Then there is the amazing neutron "streaker." The closest neutron star ever seen, just 200-light years away, it is hurtling toward Earth at 240,000 miles per hour--like a gift presenting itself to observers. It should swing past our neighborhood in 300,000 years, at a safe distance of 170 light-years. (A light-year is the distance traveled by light in a year--about 6 trillion miles.)

Even better, the star is "naked"--that is, lacking a companion whose effects would obscure its surface. The star is glowing only with its own internal heat and with no detected spin or pulsing.

"It's just a dull, boring neutron star," said Frederick M. Walter of the State University of New York in Stony Brook, whose team has been tracking the object (first spotted in 1992 by the Roentgen Satellite) with the Hubble Space Telescope. But the discovery means "we can for the first time really directly measure the various properties of one of these things."

STRUCTURE AND BEHAVIOR OF NEUTRON STARS

Neutron stars are the imploded cores of dead stars, in the most extreme state of collapse short of a black hole. Powerful gravity pulls gas off any companion star orbiting nearby, below, triggering spectacular fireworks that include nuclear explosions.

A pulsar, right, is a neutron star whose signature is a rotation rate predictable to within a few parts per quadrillion. With a magnetic field perhaps a trillion times that of Earth, it emits a focused "lighthouse" beam from its magnetic pole that sweeps Earth each time it rotates, typically 50 times a second.

A neutron star squeezes more than the mass of the sun into a sphere with a diameter not much bigger than the District. Densities increase from the outer crust toward the center, where subatomic particles are squeezed together until little or no space separates them.

Outer Crust: Solid, superdense crystalline iron and nickel nuclei and electrons.

Inner Crust: Nuclei, electrons and superfluid neutrons or normal neutrons.

Core: Possibly superfluid neutrons (a fluid that has no resistance to flow), superconducting protons and electrons or other exotic particles.