Argonne supercomputer to simulate extreme physics of exploding stars(Forwarded)

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May 1, 2008

Argonne supercomputer to simulate extreme physics of exploding stars

Robert Fisher and Cal Jordan are among a team of scientists who will
expend 22 million computational hours during the next year on one of the
world's most powerful supercomputers, simulating an event that takes less
than five seconds.

Fisher and Jordan require such resources in their field of extreme
science. Their work at the University of Chicago's Center for
Astrophysical Thermonuclear Flashes explores how the laws of nature unfold
in natural phenomena at unimaginably extreme temperatures and pressures.
The Blue Gene/P supercomputer at Argonne National Laboratory will serve as
one of their primary tools for studying exploding stars.

"The Argonne Blue Gene/P supercomputer is one of the largest and fastest
supercomputers in the world," said Fisher, a Flash Center Research
Scientist. "It has massive computational resources that are not available
on smaller platforms elsewhere." Desktop computers typically contain only
one or two processors; Blue Gene/P has more than 160,000 processors. What
a desktop computer could accomplish in a thousand years, the Blue Gene/P
supercomputer can perform in three days. "It's a different scale of
computation. It's computation at the cutting edge of science," Fisher
said.

Access to Blue Gene/P, housed at the Argonne Advanced Leadership Computing
Facility, was made possible by a time allocation from the U.S. Department
of Energy's Innovative and Novel Computational Impact on Theory and
Experiment program. The Flash Center was founded in 1997 with a grant from
the National Nuclear Security Administration's Office of Advanced
Simulation and Computing. The NNSA's Academic Strategic Alliance Program
has sustained the Flash Center with funding and computing resources
throughout its history. The support stems from the DOE's interest in the
physics that take place at extremes of concentrated energy, including
exploding stars called supernovas. The Flash Center will devote its
computer allocation to studying Type Ia supernovas, in which temperatures
reach billions of degrees.

A better understanding of Type Ia supernovas is critical to solving the
mystery of dark energy, one of the grandest challenges facing today's
cosmologists. Dark energy is somehow causing the universe to expand at an
accelerating rate.

Cosmologists discovered dark energy by using Type Ia supernovas as cosmic
measuring devices. All Type Ia supernovas display approximately the same
brightness, so scientists could assess the distance of the exploding
stars' home galaxies accordingly. Nevertheless, these supernovas display a
variation of approximately 15 percent. "To really understand dark energy,
you have to nail this variation to about 1 percent," said Jordan, a Flash
Center Research Associate.

The density of white dwarf stars, from which Type Ia supernovas evolve, is
equally extreme. When stars the size of the sun reach the ends of their
lives, they have shed most of their mass and leave behind an inert core
about the size of the moon. "If one were able to scoop out a cubic
centimeter -- roughly a teaspoon -- of material from that white dwarf, it
would weigh a thousand metric tons," Fisher explained. "These are
incredibly dense objects."

Type Ia supernovas are believed to only occur in binary star systems,
those in which two stars orbit one another. When a binary white dwarf has
gravitationally pulled enough matter off its companion star, an explosion
ensues. "This takes place over hundreds of millions of years," Jordan
said. "As the white dwarf becomes more and more dense with matter
compressing on top of it, an ignition takes place in its core. This
ignition burns through the star and eventually leads to a huge explosion."

The Flash team conducts whole-star simulations on a supercomputer at
Lawrence Berkeley National Laboratory in California. At Argonne, the team
will perform a related set of simulations. "You can think of them as a
nuclear flame in a box' in a small chunk of the full white dwarf," Fisher
said.

In the simulations at Argonne, the team will analyze how burning occurs in
four possible scenarios that lead to Type Ia supernovas. Burning in a
white dwarf can occur as a deflagration or as a detonation.

"Imagine a pool of gasoline and throw a match on it. That kind of burning
across the pool of gasoline is a deflagration," Jordan said. "A detonation
is simply if you were to light a stick of dynamite and allow it to
explode."

In the Flash Center scenario, deflagration starts off-center of the star's
core. The burning creates a hot bubble of less dense ash that pops out the
side due to buoyancy, like a piece of Styrofoam submerged in water. But
gravity holds the ash close to the surface of the white dwarf. "This
fast-moving ash stays confined to the surface, flows around the white
dwarf and collides on the opposite side of breakout," Jordan said. The
collision triggers a detonation that incinerates the star. There are,
however, three other scenarios to consider. "To understand how the
simulations relate to the actual supernovae, we have to do more than a
thousand different simulations this year to vary the parameters within the
models to see how the parameters affect the supernovae," Jordan said.

Related Video:

[Video 1:
http://www.ci.uchicago.edu/flashviz/...2_itemId=4252]
This animation shows a thermonuclear flame burning its way through a white
dwarf star. The flame produces hot ash, which buoyantly rises as the flame
burns. The ash breaks out of but remains gravitationally bound to the
surface of the star and collides at a point on the opposite side of the
star from the breakout location. The collision region is compressed and
heated from the converging ash flows and when the conditions there become
extreme enough a detonation is triggered. In the animation, green
represents the approximate surface of the star and the colors mark regions
of high temperature in the billions of degrees Kelvin. As the ash rises
and breaks out of the star, it decompresses and cools; however, the as the
ash flows over the surface the animation shows the collision region
attaining very high temperatures.

[Video 2:
http://www.ci.uchicago.edu/flashviz/...2_itemId=4827]
This animation shows a thermonuclear flame burning its way through a white
dwarf star. The flame produces hot ash, which buoyantly rises as the flame
burns. The ash breaks out of but remains gravitationally bound to the
surface of the star and collides at a point on the opposite side of the
star from the breakout location. The blue shows the approximate surface of
the star and the orange shows the interface between the star and the hot
ash produced by the flame.

Credit: DOE NNSA ASC/Alliance Flash Center at the University of Chicago.

IMAGE CAPTIONS:

[Image 1:
http://news.uchicago.edu/images/asse...image1-375.jpg (11KB)]
A snapshot of a three-dimensional simulation of a Type Ia supernova,
shortly after the nuclear flame bubble that initiates the Ia event is
ignited slightly off-center from the progenitor white dwarf star (shown
here as a light blue surface).

Credit: DOE NNSA ASC/Alliance Flash Center at the University of Chicago.

[Image 2:
http://news.uchicago.edu/images/asse...image2-375.jpg (28KB)]
A close-up, high-resolution image of the nuclear flame bubble shown in
image 1. This image depicts the vorticity of the flow, demonstrating the
complex, turbulent hydrodynamical processes that govern the nuclear flame
bubble burning during this stage, close to bubble breakout.

Credit: DOE NNSA ASC/Alliance Flash Center at the University of Chicago.

[Image 3:
http://news.uchicago.edu/images/asse...image3-375.jpg (13KB)]
A snapshot of a Type Ia supernova simulation taken very shortly after the
moment of detonation. The energy released during the detonation is
equivalent to 1,027 hydrogen bombs, each equivalent to 100 megatons of
TNT.)

Credit: DOE NNSA ASC/Alliance Flash Center at the University of Chicago.