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Inside the Crab: A Detailed Look at Supernova Remnants - 2002-09-21

Astronomers have gotten the closest, most detailed look ever inside the remnants of a supernova, that's an aged star that explodes and then collapses into itself. The new images will help scientists study what occurs after a supernova, as they formulate theories about the life cycles of stars and how supernovas help recycle the matter in the universe.

Only in astronomy can something that has exploded and collapsed still be highly energetic and dynamic. Scientists have now learned that this is the case with the remains of supernovas, which occur when stars use up their nuclear fuel and blow up.

A supernova is one of nature's most spectacular displays. One can outshine a galaxy of billions of stars when it explodes. It quickly contracts into a rapidly spinning, neutron star so dense that a thimbleful would be too heavy to lift. In contrast, the aftermath, long after the blast, appears quieter, judging by observations of most of the 231 known supernova remnants in our Milky Way galaxy.

But astronomers using the orbiting U.S. Chandra X-Ray Observatory and the Hubble Space Telescope have found something remarkably different in a supernova remnant known as the Crab Nebula. This is gas surrounding the remains of a supernova recorded in 1054 by Chinese astronomers, one so brilliant it was apparent even during daytime. Now revealed for the first time, the nebula roils with high-energy jets and glowing ripples of expanding energy.

Physicist Victoria Kaspi of McGill University in Montreal, Canada, says researchers must now take a closer look at other such nebulas to see if they are so dynamic. "Because all of the others have appeared blobby [shapeless] and static, theoretical models that have been developed to understand the physics of these objects have assumed that they are static and roughly spherical," she says. "From the Chandra data now, it's quite clear that those theorists need to incorporate these new data into their models."

Astronomers from Pennsylvania State and Arizona State universities have combined still X-ray images of the Crab Nebula by the Chandra Observatory and visible light photos from the Hubble Telescope into motion picture sequences. The movie reveals a churning nebula.

Bright wisps of electrons and anti-electrons move outward at half the speed of light to form an expanding ring, like a ripple in a lake. The wisps appear to originate from a shock wave that shows up as an inner X-ray ring. This ring is illuminated by material flung out by the spinning, pulsating neutron star, called a pulsar. The ring consists of about two dozen knots that brighten, fade, flutter, and occasionally undergo outbursts that give rise to expanding clouds of particles.

Arizona State University physicist Jeffrey Hester says the movie has brought the Crab Nebula to life. "The remarkable thing here is, we are getting to see before our eyes what's happening in a truly extraordinary environment, an environment that's nothing like we know in our everyday lives, but that is an environment that you find around pulsars, you find around black holes, you find in the cores of quasars, a unique look at the physics that happens in that kind of a circumstance," he says.

The nebula may seem exotic, but Harvard University scientist Robert Kirshner says understanding this process is important to learning how the universe works. It is central to knowing how star death distributes material that eventually coalesces into new stars, elements from which we are made. "With the new data, we're able to take a good close look right in the neighborhood of the neutron star to see how that thing affects the rest of the nebula, how it helps push out the debris from the supernova that is going to mix to become the enriched material for the next generation of stars," he says.

Although these celestial events sound like science fiction, we humans are not so far removed from them. After all, we are made from the same stuff as the Crab Nebula. "In a certain sense, the Crab Nebula and we are cousins," he says.

"The elements of which we are made were born in supernova events like this, just as much as the pulsar in the Crab Nebula was born in a supernova event," says Mr. Hester.