Adam Burrows of the University of Arizona and colleagues have developed computer models that simulate the full second or more of a giant star's death, running from the dynamics of core collapse through the supernova explosion. Although the model covers only two dimensions, he said it allows for the fact that supernovas are not spherical or symmetrical events.

"Our simulations show that the inner core starts to execute pulsations," Burrows said, �and they allow us to follow the development to explosion for a longer time than other models do. They show that after about 500 milliseconds, the inner core begins to vibrate wildly, and after 600, 700 or 800 milliseconds, this oscillation becomes so vigorous that it sends out sound waves. In these computer runs, it's these sound waves that actually cause the star to explode, not the neutrinos."

A supernova is a massive star that has burned for 10 million to 20 million years and developed a hot, dense white-dwarf star about the size of Earth at its core. When the white dwarf reaches a critical mass - or about 1.5 times the mass of the Sun - it collapses and creates a spherical shock wave, all within less than half a second before the star explodes in a supernova, one of the largest blasts of energy in the universe.

Researchers have been using computer models since the 1960s to test ideas about what causes stars to explode, but up to now the models have not explained the inner workings of supernovas satisfactorily.

In particular, neutrinos � the nearly massless subatomic particles that had been thought to power supernova explosions - don't seem to be energetic enough to do the job. Recent simulations that include convective motion work a bit better, but not well enough, because even in the best recent simulations, the shock wave stalls. So theorists recently have focused their work on what might revive the shock wave into becoming a supernova explosion.

Burrows said his team seems to have solved the problem by running the simulation longer. Their model involves about 1 million steps, or about five times more than typical models, which calculate only the first few hundred milliseconds of supernovae events. The new simulations also characterize the natural motion of a supernova core, something that other detailed models have not done.

"We were quite sure when we started seeing this phenomenon that we were seeing sound waves, but it was so unexpected that we kept rechecking and retesting our results," Burrows said.

The simulations show how collapsing material falls lopsidedly onto the inner core and soon excites oscillations at specific frequencies. Within hundreds of milliseconds, the inner core vibrations become so intense that they actually generate sound waves. Burrows said typical sound frequencies are about 200 to 400 hertz - in the audible range bracketing middle C.

"Sound also generates pressure, which pushes the exciting streams of infalling matter to the opposite side of the core, further driving the core oscillations in a runaway process," he explained. "The sound waves reinforce the shock wave (created by the collapsed star) until it finally explodes aspherically."

Burrows said others who study supernova explosions in computer experiments will be skeptical of his team's results -- and should be. "This is such a break from 40 years of traditional thinking that one should be cautious trumpeting it," he said. "Nevertheless, this is provocative and interesting. It would open up many new possibilities and perhaps solve a long-standing problem of what triggers supernovae explosions."

To develop the model, the team used computer clusters in the UA astronomy department, and at the University of California, Berkeley's, supercomputer center and elsewhere, and they have been checking the analysis for the past year. The research is scheduled to be published in detail in an upcoming edition of the Astrophysical Journal.

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