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Lawrence Berkeley National Laboratory
Gravity and buoyancy churn and warp the flame front in a star undergoing a supernova explosion. The front marks the boundary, as thick as a sheet of paper, where oxygen and carbon are being fused to heavier elements. Forty days and 40 nights on a supercomputer were required to produced this image, a patch about half a yard across.

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Life-or-Death Question: How Supernovas Happen


Published: November 9, 2004

Once a second or so, somewhere in the universe, a star blows itself to smithereens, blossoming momentarily to a brilliance greater than a billion suns.

Nobody understands how these events, among the most violent in nature, actually happen. But, until recently, that didn't much matter unless you were a practitioner of the arcane and messy branch of science known as nuclear astrophysics.

Lately, however, supernovas have become signal events in the life of the cosmos, as told by modern science.

Using a particular species of supernova, Type 1a, as cosmic distance markers, astronomers have concluded that a mysterious "dark energy" is wrenching space apart, a discovery that has thrown physics and cosmology into an uproar.

As a result, the fate of the universe - or at least our knowledge of it - is at stake, and understanding supernovas has become essential.

Astronomers are busy on many fronts trying to figure out the details of these explosions - scanning the skies to harvest more of them in the act, peering at the remains of ancient supernovas to seek a clue to their demise, harnessing networks of supercomputers to calculate moment by moment reactions in the heart of hell.

This has resulted recently in a kind of two-steps-forward, one-step-back progress, encouraging astronomers that they are on the right track, generally, with their theories, but at the same time underscoring complexities and baffling puzzles when it comes to pinning down the details of what happens in the explosions.

Last month members of an international team of astronomers led by Dr. Pilar Ruiz-Lapuente of the University of Barcelona announced that they had found a star speeding away from the site of a supernova blast seen in 1572 by the astronomer Tycho Brahe. This supernova, which appeared as a "new star" in the constellation Cassiopeia, was one of the earliest studied by astronomers, and helped shatter the Aristotelian notion that the heavens above the Moon were immutable.

The newly discovered star, presumably the companion of the star that exploded, supports a long-held notion that such explosions happen in double star systems when one star accumulating matter from the other reaches a critical mass and goes off like a bomb.

Meanwhile, members of a group of astrophysicists using a network of powerful supercomputers to simulate supernova explosions say they have succeeded for the first time in showing how such a star could blow up.

Over the course of 300 hours of calculation at the University of Chicago's Center for Astrophysical Thermonuclear Flashes, otherwise known as the Flash center, they watched bubbles of thermonuclear fury rise from the depths of the star like a deadly jellyfish and then sweep around the surface and collide in an apocalyptic detonation that Dr. Donald Lamb, a Chicago astrophysicist, called "totally bizarre and novel."

If true, the Chicago results could help explain not only how stars explode, but why the explosions are almost but not exactly alike, allowing astronomers to better calibrate their measurements of dark energy.

Many supernova experts said, however, that such computer simulations were more of a good start than a final answer. Dr. J. Craig Wheeler of the University of Texas called the Flash center work "a courageous calculation," but added that many details needed to be filled in. "I don't think this is the end of the story," he said. The story of Type 1a supernovas, experts have long agreed, begins with a dense cinder known as a white dwarf, composed of carbon and oxygen, which is how moderate-size stars like the Sun, having exhausted their thermonuclear fuels of hydrogen and helium, end their lives.

If it happens to be part of a double star system, the white dwarf can accumulate matter from its companion until it approaches a limit, known as the Chandrasekhar mass - about 1.4 times the mass of the Sun.

At that point, so the story goes, the pressure and density in the previously dead star will be great enough to reignite the star and thermonuclear reactions will ripple upward, transmuting the carbon and oxygen into heavier and heavier elements, ripping the white dwarf apart while its companion goes flying off.

Until recently, however, there was little evidence of this. Two white dwarfs could collide, for example, and blow up. In that case there would be no survivor.

Tycho Brahe's supernova has now offered new evidence for the former model, of the white dwarf bomb.

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