Star-making motion

Quantum physicist Richard Feynman once called turbulence “the most important unsolved problem of classical physics”.

Even after centuries of research, scientists are still trying to understand the complex and unpredictable nature of turbulence down here on Earth, from weather patterns to blood flow in arteries.

Astrophysical turbulence is perhaps even more perplexing.

Now, in a study published in Nature Astronomy, Australian and German scientists have used the computing power of the Leibniz Supercomputing Centre (LRZ) in Germany to probe how turbulence shapes the interstellar medium, and thus helps form stars and planets.

“Turbulence is a key ingredient for star formation,” says co-lead author Cristoph Federrath, an astrophysicist from the Australian National University.

“It controls the pace of star formation, stirring up gas and slowing down the action of gravity, which – without turbulence – would make stars form a hundred times quicker than observed.

“The formation of stars powers the evolution of galaxies on large scales and sets the initial conditions for planet formation on small scales, and turbulence is playing a big role in all of this.”

Even though star formation is slow – only approximately one star the mass of our Sun forms each year in the Milky Way – the turbulence itself can take a range of speeds, from large-scale supersonic motions (faster than the speed of sound) to smaller subsonic scales.

This “sonic scale” is at the centre of all theoretical models of astrophysical turbulence. It strikes a balance between turbulence and gravity, setting the size of dense molecular clouds from which stars form.

Too much turbulence tends to lead to sparser star formation, while smaller, subsonic regions are dominated by gravity, causing more localised clusters of stars form.

To understand this transition from supersonic to subsonic, Federrath and team created the largest-ever simulation of supersonic turbulence. This involved using a massive amount of computing power to solve the complex equations of gas dynamics over a range of scales – modelling the large-scale phenomena happening faster than the speed of sound, as well as accurately capturing the details of the smaller, slower dynamics.

“With this simulation, we were able to resolve the sonic scale for the first time,” Federrath says.

This was even more difficult than the simulations performed by engineers and scientists seeking to understand terrestrial turbulence.

On Earth, most fluids are incompressible, so their density remains fairly constant. But in the interstellar medium, the highly compressible mix of gas and dust adds an extra twist to the problem.

The results of the simulation closely align with theoretical predictions, according to Federrath, but with subtle differences that will lead to further refinements of star-formation models.

“We ultimately hope that this simulation advances our understanding of the different types of turbulence on Earth and in space,” he says.

“Next we’d like to add magnetic fields, chemistry and cooling to a simulation of this size, in order to learn more about the processes taking place when stars form.

“This will be extremely challenging, as it would take even more memory, space, and computing power. Such a simulation would just fit on Australia’s new supercomputer ‘Gadi’, at the National Computational Infrastructure.”