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.
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."
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