Worlds First 3D Computer Simulation of White Dwarf Supernova

A team of astrophysicists and mathematicians has created the first full-star simulation of the hours leading up to a Type Ia supernova. Until now, the exact conditions inside the largest thermonuclear explosions in the universe have remainednunclear.

"We're trying to understand something very fundamental, which is how these stars blow up, but it has implications for the fate of the universe," said Ann Almgren, of Berkeley Lab's Computational Research Division.

Astrophysicists are interested in type Ia supernovae because they are all believed to be startlingly alike to each other, leading to their use as "standard candles" which scientists use to measure the expansion of the universe. Based on observations of these massive stellar explosions a single supernova is as bright as an entire galaxy, scientists believe our universe is expanding at an accelerating rate. But what if Type Ia supernovae have not always exploded in the same way? What if they aren't standard?

But astrophysicists still do not know precisely how a star of this type explodes, several simulations have been tried over the years to answer this, however, traditional methods and available supercomputing power have proved insufficient.

"Few have tackled this problem before because it was considered intractable,"said Almgren. "We needed to simulate the conditions for hours, not just a few seconds. We are now doing calculations that weren't possible before."

MAESTRO on Cray XT4

Over the past three years, Almgren, Bell and Nonaka, along with other collaborators from the U.S. Department of Energy's (DOE) Lawrence Berkeley National Laboratory, have been developing a simulation code they call MAESTRO. Simulating the flow of mass and heat throughout the star over time, it requires supercomputers to model the entire star. It's unique in as much as it is designed for processes which take place much slower than the speed of sound; this lets the simulation produce results using much less supercomputing time than traditional codes. MAESTRO's approach removes sound waves, allowing the code to run much more efficiently than traditional methods.

The team ran the simulations on Jaguar, a Cray XT4 supercomputer at the Oak Ridge Leadership Computing Facility in Tennessee, using an allocation under DOE's Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program. "The INCITE allocation on Jaguar was crucial in enabling the successful runs leading to these groundbreaking results," said Woosley, leader of the SciDAC supernova project. "And the continuing support of the Department of Energy Office of Science is critical to advancing our research."

The simulation provided a valuable glimpse into the end of a process that started several billion years ago. A Type Ia supernova starts as a white dwarf, the compact remnant of a low-mass star that never got hot enough to fuse its carbon and oxygen. But if another star is near enough, the white dwarf may start taking on mass ("accreting") from its neighbor until it reaches a critical limit, known as the Chandrasekhar mass. Eventually, enough heat and pressure build up and the star begins to simmer, a process that lasts several centuries.

1.8 Billion Degrees Fahrenheit

During this simmering phase, fluid close to the center of the star gets hotter, more buoyant, and the buoyancy-driven convection "floats" the heat away from the center. During the final few hours, the convection can't move the heat away from the center fast enough, and the star gets hotter more rapidly. The fluid flow becomes stronger and more turbulent, but even so, at some point or points in the star, the temperature finally reaches about 1,000,000,000 degrees Kelvin ( about 1.8 billion degrees F), and ignites. A burning front then moves through the star, slowly at first, but gaining speed as it goes. From ignition to explosion is only a matter of seconds.

At the early stages, the simulations show, the motion of the fluid looks like random swirls. But as the heating in the center of the star increases, the convective flow clearly moves into the star's core on one side and out the other, a pattern known as a dipole. The flow also becomes increasingly turbulent, with the orientation of the dipole bouncing around inside the star. While others have also seen this dipole pattern, the simulations using MAESTRO are the first to have captured the full star in three dimensions.