On an airplane, motions of the air on both small and large scales contribute to turbulence, which may result in a bumpy flight. Turbulence on a much larger scale is important to how stars form in giant molecular clouds that permeate the Milky Way.

In a new NASA-funded study in the journal Science Advances, scientists created simulations to explore how turbulence interacts with the density of the cloud. Lumps, or pockets of density, are the places where new stars will be born. Our Sun, for example, formed 4.6 billion years ago in a lumpy portion of a cloud that collapsed.

“We know that the main process that determines when and how quickly stars are made is turbulence, because it gives rise to the structures that create stars,” said Evan Scannapieco, professor of astrophysics at Arizona State University and lead author of the study. “Our study uncovers how those structures are formed.”

Giant molecular clouds are full of random, turbulent motions, which are caused by gravity, stirring by the galactic arms and winds, jets, and explosions from young stars. This turbulence is so strong that it creates shocks that drive the density changes in the cloud.

The simulations used dots called tracer particles to traverse a molecular cloud and travel along with the material. As the particles travel, they record the density of the part of the cloud they encounter, building up a history of how pockets of density change over time. The researchers, who also included Liubin Pan from Sun Yat Sen University in China, Marcus Brüggen from the University of Hamburg in Germany, and Ed Buie II from Vassar College in Poughkeepsie, New York, simulated eight scenarios, each with a different set of realistic cloud properties.  

The team found that the speeding up and slowing down of shocks plays an essential role in the path of the particles.  Shocks slow down as they go into high-density gas and speed up as they go into low-density gas. This is akin to how an ocean wave strengthens when it hits shallow water by the shore.   

When a particle hits a shock, the area around it becomes more dense. But because shocks slow down in dense regions, once lumps become dense enough, the turbulent motions can’t make them any denser.  These lumpiest high-density regions are where stars are most likely to form.

While other studies have explored molecular cloud density structures, this simulation allows scientists to see how those structures form over time. This informs scientists’ understanding of how and where stars are likely to be born.

“Now we can understand better why those structures look the way they do because we’re able to track their histories,” said Scannapieco.

NASA’s James Webb Space Telescope is exploring the structure of molecular clouds. It is also exploring the chemistry of molecular clouds, which depends on the history of the gas modeled in the simulations. New measurements like these will inform our understanding of star formation.