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Anatomy of a dying star

Image: Simulated death of white dwarf
A simulation of a star's final hours may help scientists uncover what triggers its death. The program simulated the death of a white dwarf (pictured), which is a compact remnant of a star similar to our sun.WFPC2/HST/NASA
/ Source: Discovery Channel

A computer program that simulates the final hours of a star's life has been developed by researchers at the Lawrence Berkeley National Laboratory in California and Stony Brook University in New York.

The scientists designed the simulation hoping to uncover what actually triggers a star's death.

For decades, scientists have relied on supernovae to serve as mile markers on the highways of space. These exploded stars can be measured for brightness, which provides an estimate of their physical distance.

Supernovae have been used to compute the expansion of the universe and shed light on a mysterious and still unknown force called dark energy, which seems to be pressing the universe's expansion pedal more firmly in recent times than in the past.

Now for the first time, scientists have a tool to begin understanding just how "standard" these celestial mile markers really are.

"The thing that we're trying to find out is where and how the ignition happens," said researcher Ann Almgren, with Berkeley Lab's Computational Research Division.

That's important because where a star ignites plays a key role in how it explodes, which in turn can impact its brightness.

Scientists modeled a white dwarf, which is a compact remnant of a star similar to our sun.

If the white dwarf reaches a critical size, known as the Chandrasekhar mass, enough heat and pressure builds up that it begins simmering, a process that can last for centuries.

During its final two hours, convection can't carry heat from the center of the star away fast enough. It grows hotter and hotter. On one side, fluid moves inward toward the center of the star. Jets blast out the other.

The fluid flow grows stronger and more turbulent until the star reaches a temperature of about 1.8 billion degrees Fahrenheit and ignites. In seconds, the star explodes.

"We were able to model this for longer than people had before, in full 3-D, and get a realistic picture of what the convective pattern looked like and evolve it to the point where you see that first hotspot form," Stony Brook University astrophysicist Michael Zingale told Discovery News.

More work remains to determine if a supernova ignites at a single spot or many, and where the triggers are located.

"All these things are relevant because if you have a hotspot and it is carried away quickly toward the outside of the star, that will affect what sort of flame might propagate or whether it goes out. There's a whole lot of science in what the flows are doing," said Almgren.

The computer program breaks down the model star into 50 million grids and calculates changing heat, density, pressure and other characteristics over time. The researcher devised a new method for creating the simulation so that it could run within available time on a supercomputer at the Oak Ridge Leadership Computing Facility in Tennessee.

"We operate at the edge of the computational resources that we have," Almgren said.

The team's work is being published in the October issue of Astrophysical Journal.