Chandra Astounds Astronomers

NASA's Chandra X-ray Observatory, peering at the wreck of a star that exploded 945 years ago, has beamed back spectacular images that may provide the missing link needed to explain how the doomed star's core energizes vast gulfs of space like some cosmic dynamo.

Scientists aimed the $1.6 billion X-ray telescope at the famous Crab Nebula, 6,000 light years away in the constellation Taurus, primarily to help calibrate the recently-launched observatory's sensitive detectors. But the images they got back took everyone by surprise.

"The Crab is the gift that keeps on giving," marveled Jeff Hester, an Arizona State University astronomer who has studied the Crab Nebula with the Hubble Space Telescope. "It's a truly remarkable object."

The ghostly X-ray images show a brilliant, slightly tilted ring of high energy radiation circling the spinning neutron star, or pulsar, at the core of the Crab Nebula. The never-before-seen ring is about two light years across.

Shooting out in opposite directions perpendicular to the ring are two titanic jets. Hints of the jets have been seen before, but the Chandra images are the first that show them extending all the way to the poles of the pulsar.

Elated scientists said the features may be the missing link needed to explain how magnetized, spinning neutron stars like the one at the heart of the Crab Nebula radiate such enormous amounts of energy into the local environment.

"One of the difficulties we have in understanding the system is sort of analogous to watching an ice skater do twirls," said Martin Weisskopf, project scientist with the Chandra X-ray Observatory program.

"Imagine you're watching an ice skater do her twirls and she slows down, so she's losing energy," he said. "And all of the sudden the entire audience lights up because somehow the energy was transferred from her to the audience."

Said Hester in a statement: "It's like finding the transmission lines between the power plant and the light bulb."

Stars maintain their equilibrium by balancing the inward pull of gravity with the outward pressure of energy produced by nuclear fusion in the core. When a massive star finally runs out of fuel, fusion stops, gravity causes the star's core to collapse and the star explodes.

What happens after that depends on the mass of the original star. If the star is massive enough, the core collapses and vanishes from normal space, becoming a black hole.

In the case of the star that exploded to form the Crab Nebula, not enough mass was present in the core to generate the gravity needed to overcome quantum mechanical effects opposing the collapse.

The result was a magnetized pulsar about a dozen mies across that is 50 trillion times denser than lead and spinning at 33 times per second. Astronomers believe there are about 100 million such pulsars in Earth's Milky Way Galaxy - one for every 1,000 stars - which act like cosmic dynamos, generating quadrillions of volts of electricity.

Some of the energy radiates away with the pulsar's magnetic field. Energy also is carried away by electrons and their antiparticles, positrons, which are accelerated to enormous velocities.

A third possibility is that electrically charged atoms, or ions, are somehow carried away at close to the speed of light.

"The remarkable Chandra pictures may give definitive clues as to what the nature of that beam is and therefore tell us [how a pulsar] loses power and deposits it in the surrounding environment," said Malvin Ruderman of Columbia University.

By William Harwood