5 mins read 03 Feb 2021

Stellar Simulation Reveals the Turbulent Nature of Star Birth

Led by a researcher from the Australian National University, a team of astronomers have quantified the so-called ‘sonic scale’ that determines how interstellar gas clouds ultimately form stars.

An image of the giant molecular cloud complex, Mon R2. Stars are formed in the dense regions of diffuse molecular clouds like this one, but the exact process is still an area of active research. Credit: Adam Block/Mt. Lemmon SkyCenter/U. Arizona

With perhaps 100 billion stars in the Milky Way galaxy, you could be forgiven for assuming that the process that leads to the birth of new stars was quite efficient and resulted in rapid stellar formation. But at present, only about one new star is born into the Milky Way every year, with turbulence in the gas clouds playing a major role in controlling the formation rate. To see how this plays out on large scales, researchers have used supercomputers to run high-resolution simulations of turbulence.

Turbulence is something that all of us are familiar with. We recognise it from the jolts that we experience when flying in an aircraft, or in the way that smoke billows out from a chimney. It is a result of the chaotic motion of a fluid (that is, a liquid or a gas) due to changes in its pressure and flow velocity, a consequence of excessive energy in the fluid overcoming the damping effect of its viscosity.

But it is not just an earthly phenomenon. The giant interstellar gas clouds that spawn new stars exhibit turbulent flow as well, something that we have observed using powerful Earth-based telescopes. Astronomers quantify turbulence by measuring the Mach number of the gas, that is, its motions compared to the local speed of sound.

Remember that the speed of sound is simply the speed that a disturbance propagates through a substance. The sound part just refers to the sensation in our brains in response to the sensory inputs from our ears. We wouldn’t necessarily hear sound waves propagating through an interstellar gas cloud, but that does not stop us from observing and measuring the speed of the waves.

Turning Clouds into Stars

This image shows a slice through the gas density contrast and is zoomed-in to a dense region with multiple shock interactions. The shocks are a result of supersonic turbulence that prevents immediate gravitational collapse of the cloud. Credit: Federrath et al. 2021

Gravity is one of the most important drivers of star formation. Huge concentrations of gas are drawn together under the force of gravity until gas densities are high enough that nuclear fusion can begin. But given the vast amounts of high-density gas currently in the Milky Way, it seems as though star formation should be at least 10-100 times faster than it actually is.

Although it took astronomers a long time to figure it out, it is the kinetic energy carried by a cloud’s turbulence that counters its rapid gravitational collapse. If there is enough energy, the turbulence is supersonic, and gravity alone is not enough to ignite new stars.

But they also realised that turbulence was helping to get star formation happening in the first place. In supersonic turbulence, pressure waves overtake each other causing shock waves and local compressions that change the temperature and density of the gas almost instantaneously. This seeds the gravitational collapse that condenses part of the cloud such that star formation can begin. But for that to occur the turbulent motion needs to be subsonic.

Finding the Sonic Scale

The key finding of this research was to understand where the transition between supersonic and subsonic turbulence occurs, a property known as the sonic scale. Lead researcher Assoc Prof Christoph Federrath from the Australian National University explained the importance of this metric in advancing our understanding of star formation rates.

“The sonic scale is a key ingredient for our understanding of star formation. Basically, the turbulence on scales larger than the sonic scale is so strong that it helps prevent the interstellar gas clouds from collapsing under their own weight. That is important because otherwise stars would form about 100 times faster than actually observed in our Milky Way.”

“On the other hand, on scales below the sonic scale, the turbulence is not strong enough to keep gravity from contracting the dense shocked gas, and thus, below the sonic scale, star formation can proceed.”

“Our measurement of the sonic scale provides a quantification of the scales and gas densities of interstellar gas clouds that ultimately become dense enough to form stars.”

The research was carried out using FLASH, compressible hydrodynamics computer code that can be used for simulating various astrophysical processes. Assoc Prof Federrath and his team required a supercomputer with 65,536 compute cores to run their simulation which consumed approximately 50 million core hours. It produced 91 output files, each with a size of about 20 terabytes.

This was the world’s highest-resolution simulation of turbulence to date, but the team are already looking to expand their study to include magnetic fields, chemistry and cooling to learn even 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,” said Assoc Prof Federrath.

Clearly, research like this depends on the availability of powerful supercomputers that can crunch the numbers. But with the right tools, astronomers are managing to solve long-standing mysteries of the Universe and to reveal the origins of life itself. After all, we are all made of star-stuff.

Video Credit: Federrath 2015

The paper appears in the journal Nature