Astrophysicists simulate a star’s final hours

Researchers have created the first full-star simulation of the hours preceding the largest thermonuclear explosions in the universe.

Type Ia supernovae are of particular interest to astrophysicists as they are all believed to be surprisingly similar to each other. Based on observations of these massive stellar explosions, scientists believe our universe is expanding at an accelerating rate.

“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,” Ann Almgren of Berkeley Lab’s Computational Research Division said.  “Few have tackled this problem before because it was considered intractable. We needed to simulate the conditions for hours, not just a few seconds. We are now doing calculations that weren’t possible before.”
Almgren’s team developed a simulation code, Maestro, which simulates the flow of mass and heat throughout the entire star over time. It’s unique in that it is intended for processes that occur at speeds much lower than the speed of sound, allowing detailed results using much less supercomputing time. The sound waves have been stripped out, allowing the code to run more efficiently.

A Type Ia supernova begins as a white dwarf, the remnant of a low-mass star that never got hot enough to fuse its carbon and oxygen. But if another star is near enough, it may start taking on mass from its neighbor until it reaches a critical limit, known as the Chandrasekhar mass.

Eventually, the star begins to simmer: fluid near its center becomes hotter and more buoyant, and convection “floats” the heat away from the center. During the final few hours, the convection can’t move the heat fast enough, and the star gets hotter, faster. Eventually, the temperature reaches about 1,000,000,000 degrees Kelvin, and it ignites. A burning front then moves through the star and it explodes.

The team’s simulations show that at first the motion of the fluid appears as random swirls. But as the heat in the star’s centerincreases, the convective flow moves into the star’s core on one side and out the other, a pattern known as a dipole. But 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 Maestro simulations  are the first to have captured the full star in three dimensions. This, according to the the team, could be a critical piece in understanding how the final explosion happens.

The research appears in Astrophysical Journal.