January 19, 2007

Researchers to peer into stellar blasts

The University’s Center for Astrophysical Thermonuclear Flashes and the U.S. Department of Energy (DOE) announced last week that they would collaborate on a project to conduct the most advanced simulations of exploding supernovas in history.

As part of the DOE’s Innovative and Novel Computation Impact on Theory and Experiment (INCITE) program, the Department has awarded the Flash Center roughly 2.5 million hours of supercomputer time to run the complex calculations, which are expected to improve the understanding of dark energy, nuclear explosions, and the ability to measure distances in the universe.

The team is studying a class of stars known as white dwarves, which have exhausted all their nuclear fuel and collapsed to become objects roughly the size of the Earth. When these white dwarves explode, they become what astrophysicists call “Type Ia supernovae.”

“Type Ia supernovae are among the most energetic explosions in the universe. A single supernova can outshine the light of an entire galaxy of stars,” said Dr. Robert Fisher, a research associate at the Flash Center, in an e-mail interview. These supernovae are often referred to as “the biggest powder kegs in the universe.”

Although the explosion’s entire process only takes about three seconds, the calculations call for the star to be divided into more than a billion “cells,” each of which must be looked at every 1/2000th of a second. It would take a single supercomputer processor about 285.4 years to run the entire simulation.

The agreement with the DOE, however, gives the Flash Center complete control over more than 6,000 processors working simultaneously at the Seaborg supercomputer at California’s Lawrence Berkeley National Laboratory. “The grant of 2.5 million supercomputer hours is equivalent to roughly 2.5 weeks running flat out on the full machine,” Fisher said.

In addition to providing information on how supernovas detonate, the simulations are expected to shed light on nuclear reactions. The DOE is interested because the explosion of a Type Ia supernova resembles the explosion of a nuclear weapon.

Since the U.S. is a signatory to the Comprehensive Nuclear Test Ban Treaty, it cannot detonate any of its weapons to test them. Instead, the DOE uses a combination of computer simulations and past test data to “test” the American nuclear stockpile; these new calculations could prove important to that program, said Don Lamb, professor in the Department of Astronomy and Astrophysics and director of the Flash Center.

In addition, the DOE and scientists worldwide hope that this research will help explain “dark energy,” a mysterious force in the universe. “Dark energy exerts a repulsive, rather than attractive, gravitational force, and is causing the expansion of our universe to accelerate,” Fisher said. “[T]he universe is dominated by [it].”

Because each Type Ia supernova has approximately the same luminosity, astronomers can use them to measure distances in the universe. Since scientists know how bright a supernova is when it detonates, they can determine how far away it is by measuring the intensity of its light on earth. The dimmer the explosion, the further away the supernova.

By understanding how these supernovas explode, scientists can make more accurate distance measurements and learn more about dark energy.

“Understanding the nature of dark energy is widely considered to be the most important problem in all of physics and astronomy,” Lamb said.