Παρασκευή, 24 Σεπτεμβρίου 2010

3-D Supernova Simulation Aims to Solve Astronomy Puzzles

Most stars end their lives in a whimper - our own sun will almost certainly be one of them - but the most massive stars go out with an impressive bang. When that happens, creating what's known as a Type II supernova, the associated blast of energy is so brilliant that it can briefly outshine an entire galaxy, give birth to ultra-dense neutron stars or black holes, and forge atoms so heavy that even the Big Bang wasn't powerful enough to create them. If supernovas didn't exist, neither would gold, silver, platinum or uranium. The last time a supernova went off close enough to earth to be visible without a telescope, back in 1987, it made the cover of TIME.

Given the Type II supernovas' cosmic importance, you might think astronomers would have figured out how they work - and in a general way, they have. But when it comes to the most critical few moments of the detonation process, says Princeton theorist Adam Burrows, you'd be wrong. "We've been working on this for about 50 years," he explains, "but every time we think we've nailed it, the answer turns out to be ambiguous or wrong." (There's an entirely different kind of a supernova by the way, called a Type I, which astronomers don't fully understand either, but that's a different story.)

Thanks to a new, powerful supercomputer simulation, though, reported in the current Astrophysical Journal, Burrows and a group of colleagues at Princeton and Lawrence Berkeley National Laboratory, in California, are convinced they're getting closer. "We're not there yet," he says, "but victory is in sight." (See pictures of earth from space.)

To understand what Burrows, lead author Jason Nordhaus and the others have done, you first have to understand the most basic fact about a star, which is that it's essentially a thermonuclear reaction - an H-bomb - held in place by its own powerful gravity, which goes on for many billions of years. The nuclear furnace in a star's core welds atoms together, transforming hydrogen into slightly heavier helium. In very massive stars, the helium is forged, in turn, into carbon and oxygen and on up the periodic table until the star's core has been transformed into iron.

That's the end of the road. Nuclear fusion stops, and without the enormous energy generated by that process, the core caves in on itself. "It's as if the earth had suddenly collapsed to the size of New York City," Burrows told TIME in 1987. "At this point the rest of the star is oblivious. It doesn't know the core has collapsed and that it's doomed." (See the top 50 space moments since Sputnik.)

But the rest of the star soon learns. Like Wile E. Coyote standing in thin air above a deep canyon, it pauses - then plummets. When the outer layers slam into the collapsed core, the impact generates a massive shock wave of matter, blasting outward. And here's where astrophysicists' ignorance sets in. Powerful as it is, this shock wave alone isn't energetic enough to create the blinding flash of a supernova. Something must be supplying the shock wave with extra power. And that something, theorists have long believed, comes from a blast of subatomic neutrinos, generated in the heat and pressure of the core collapse. The neutrinos slam into the shock wave and that provides the turbocharge. (Comment on this story.)

Here's the problem: neutrinos are so ethereal that only they pass right through the shock wave without sufficiently perturbing it. And in the most sophisticated computer simulations to date, which render supernovas in two dimensions (that is, using a flat circle to represent a spherical star) the amount they transferred simply wasn't sufficient.

But the new simulation by Nordhaus, Burrows and the others renders a star in 3-D. That makes things look different. "Over the past decade and a half," says Burrows, "we've learned that [shock waves] have all sorts of instabilities." In other words, they churn, and this simulation lets the scientists examine the effect of that churning in detail. It appears that the instabilities give neutrinos more of a chance to mingle with the matter in the shock wave and transfer enough of their considerable energy to create the signature flash that can be seen halfway across the universe. (See pictures of Saturn.)

This still doesn't explain the process fully, Burrows notes. "We still need to model the neutrino physics better," he says - a step so complicated that it will require tens to hundreds of times more computer power than the scientists currently have available. Once they get there, theorists could finally end up explaining just about everything supernova-related, from the birth of neutron stars and black holes to the creation of heavy elements, in detail.

"We'll do the physics better," says Burrows. "But that won't change this effect." What it will change is our knowledge base, which will finally include an explanation of what may be the most extraordinary phenomenon known to science.

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