Orbiting observatory finds ring around collapsed star; Pictures of Crab Nebula show huge energy burst


WASHINGTON -- Imagine trying out new spectacles in your living room and discovering furniture you'd never seen before.

Astronomers calibrating the new, orbiting Chandra X-ray Observatory have aimed it at the well-studied Crab Nebula -- a crab-shaped cloud of glowing dust and gas in the winter constellation Taurus -- and discovered a brilliant ring around the collapsed star at its core.

The ring is providing scientists with a new understanding of the stellar explosion observed 945 years ago that created the nebula and clues to how the star's spinning cinder keeps the gas clouds in the outer nebula glowing in a torrent of atomic particles.

"It has never been seen before," said Arizona State University physicist Jeff Hester, who has studied the Crab Nebula using the Hubble Space Telescope.

"It's like finding the transmission lines between the power plant and the light bulb."

The 5-ton Chandra X-Ray Observatory was launched in July into a looping orbit 6,000 to 86,000 miles high. Engineers have been working ever since to switch on its systems and calibrate its instruments for a planned five-year exploration of the hottest, most violent regions of the universe.

Problems with the performance of some of Chandra's X-ray detectors are believed to have been caused by the spacecraft's regular passages through the Earth's radiation belts, said Martin C. Weisskopf, Chandra's project scientist.

The detectors are now shielded during the belt-crossings. Except for some lost ability to measure the precise frequency of X-rays, he said, "we can continue with our science mission unimpeded."

The Crab Nebula was chosen as an early target for Chandra because it is relatively close (7,000 light years from Earth) and well-known to astronomers.

The nebula was born in the explosion of a massive star. Light from the blast, or "supernova," reached Earth in July 1054. Chinese astronomers spotted it, recorded its position and wrote that it was as brilliant as the full moon. It could be seen in the daytime for a month, and at night for a year.

Had the star been within 50 light years of Earth, the radiation it released would have wiped out all living things.

After the star blew up, much of its mass was hurled into space in a cloud of gas and dust that is still expanding. Its glowing arcs reminded 19th-century astronomers of a crab.

The rest of the star collapsed into a sphere just 12 miles in diameter, composed mostly of neutrons. It is incredibly dense. One teaspoon would weigh a billion tons on Earth.

As it collapsed, the star also began to spin faster -- 33 revolutions per second -- like a twirling skater who has pulled in her arms. As it spins, it sends out regular pulses of energy, like a rotating lighthouse beacon. Astronomers have found hundreds of these "pulsars" in our Milky Way galaxy.

And because the spinning neutron star has a powerful magnetic field -- 1 billion times stronger than Earth's -- the magnetic field is spinning with it, like a colossal generator.

That creates enormous electric currents around the star and makes the region so hot and energized that is actually creating matter -- pairs of electrons and positrons -- out of pure energy, Hester said, and hurling them into space at nearly the speed of light.

"The remarkable Chandra pictures may give us clues to how a neutron star loses power and deposits it in the surrounding environment," said Malvin Ruderman, a Columbia University physicist and an expert on neutron stars.

The Crab Nebula's central neutron star is far too small to be visible in the Chandra X-ray photo released yesterday at NASA headquarters in Washington. But the impact of the particles being thrown off the spinning star seems to be apparent in the bright ring around the star's equatorial plane.

"I'm not going to say we really understand it," said Hester, "but this thing stands out like a sore thumb."

As the particles race farther away from the star, they crash into the gas and dust clouds blown off by the original supernova explosion. That sets the gas glowing in wavelengths all across the spectrum -- radio waves, infrared, visible light, ultraviolet, X-rays and gamma rays.

If the neutron star ever stops spinning, Ruderman said, "the nebula would fade away."

Pub Date: 9/29/99

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