Wow! Astronomers Explode a Virtual Star
By Jeanna Bryner
Staff Writer
posted: 22 March 2007
01:01 pm
ET
For years astronomers have tried in vain to blow up an Earth-size star using strings of computer code. Finally, mission accomplished. And the resulting 3-D simulation has revealed the step-by-step process that fuels such an explosion.


http://www.space.com/scienceastronomy/070322_stellar_explosion.html



Dubbed white dwarfs, stars about the size of Earth and weighing as much as the Sun end their lives with quite a show. When their core furnace begins to burn out, white dwarfs explode in so-called type-1a supernovas that astronomers say could be responsible for producing most of the iron in the universe.


Until now, a peek beneath the hood of such a white-dwarf explosion has been tricky.
Prior attempts to produce the simulated explosion required scientists manually tell the computer model to detonate the star, which meant the model was not quite right or it would have generated its own cataclysm. With more tweaking of models, University of Chicago scientists generated natural detonations of white dwarf stars in simplified, two-dimensional simulations.


“There were claims made that it wouldn’t work in 3-D,” said Don Lamb, director of the University of Chicago’s Center for Astrophysical Thermonuclear Flashes. With some extreme computing, the team produced a 3-D detonation.


The scientists demonstrated the incineration at the “Paths to Exploding Stars” conference today in Santa Barbara, Calif.


Crash code


The simulation confirmed what the team already suspected from previous tests: The stars detonate in a supersonic process resembling diesel-engine combustion [image].


Unlike a gasoline engine, in which a spark ignites the fuel, compression triggers ignition in a diesel engine. “You don’t want supersonic burning in a car engine, but the triggering is similar,” said team member Dean Townsley of the Joint Institute for Nuclear Astrophysics at Chicago.


Though the computer simulation took a total of 58,000 hours and more than 700 computer processors, the actual process from start to finish—when the star explodes—played out in just three seconds.


The “movie” unveiled a complex, yet orderly, series of events that concluded with a bang. Split seconds before the stellar finality, a virtual flame bubble spanning about 10 miles in diameter formed near the center of the white dwarf. Immediately the giant bubble jetted the roughly 1,200 miles to the star’s surface. One second later, at the opposite end of the star, this flame crashed into itself and triggered the detonation.


“It seems that the dynamics of the collision is what creates a localized compression region where the detonation will manifest,” Townsley said.


Lighting up darkness


The ability to carry out type-1a supernovas here on Earth could illuminate an enigmatic force responsible for the expansion—dark energy.


These type-1a supernovas seem to explode with about the same intensity, and astrophysicists have taken advantage of this uniformity. By calibrating the distance to each explosion, they can refine calculations of how fast the universe has been expanding throughout its lengthy history.


They used this method to come to the conclusion in the late 1990s that the expansion of the universe is accelerating. The finding left a looming question: What force could be pushing against gravity to cause the mushrooming? Astronomers dubbed the gravity challenger “dark energy.”


The new simulations could help scientists tweak their calibrations to account for the minor variations in intensity from one supernova to another.


Flash team member Robert Fisher said, “To make extremely precise statements about the nature of dark energy and cosmological expansion, you have to be able to understand the nature of that variation.”