This can be a simulation of the interstellar medium flowing by way of the Milky Method like smoke

How do stars form?

We know that they are formed from massive structures called molecular clouds that form themselves from the Interstellar Medium (ISM). But how and why do certain types of stars form? Why, in some situations, does a star like our sun form against a red dwarf or a blue giant?

That is one of the central questions in astronomy. It’s also very complex.

The ISM is matter and energy between solar systems in a galaxy. Star formation begins when the ISM breaks up into giant gas clouds known as molecular clouds, which are the precursors of the stars. Scientists have questions about what role turbulence plays in this fragmentation and how it affects the types of stars that eventually form.

The ISM has a complex relationship with stars. After stars have formed, they finally return the material to the ISM via supernovae, planetary nebulae and stellar winds. This back and forth between stars and the ISM determines the star formation rate of a galaxy and its lifespan for star formation.

Turbulence plays a central role here. A new study presents a gas simulation of the ISM and how it forms molecular clouds. The authors of the new study wanted to understand it better and carried out the highest-resolution supercomputer simulations of this turbulence to date.

Your article is titled “The Sound Scale Determined by the World’s Largest Supersonic Turbulence Simulation.” The first author is Christoph Federrath, professor at the Institute for Theoretical Astrophysics (ITA) at the Center for Astronomy at Heidelberg University. The study is published in Nature Astronomy.

The turbulence in the ISM not only determines the star formation rates; it determines the types of stars that form. In that sense, it also affects planet formation and whether these planets can be something like Earth. So the study of turbulence is not an esoteric tangent in astronomy. It is directly related to planets, even life.

The ISM is not evenly distributed in the space between the stars. It spreads like smoke due to turbulence rising and falling and flowing. According to the study’s authors, turbulence is key to understanding how the gas fragments.

The figure shows a section through the cube of the turbulence simulation. The colors show the density contrast relative to the mean density of the gas. Its turbulent structure is clearly visible. In particular, the numerous shock fronts arise, recognizable by the strong changes in density from high density (light orange) to low density (dark purple). This is particularly evident in the enlarged section. (Source: C. Federrath) Photo credits: Federrath et al., 2021.

There are similarities between the turbulence in the ISM and the turbulence in clouds of smoke. Large-scale turbulence in both cases tends to be smaller-scale turbulence. However, the comparison is not perfect: the ISM is extremely thin with only 1 to 100 particles per square centimeter of volume. Obviously, smoke is much denser.

In the thin ISM, the turbulent energy drops on a smaller scale than in the smoke, not only because of the thinness, but also because the ISM has a very low viscosity. Ultimately, this cascading reduces the speed of turbulent motion above a threshold from supersonic to the speed of sound.

When the turbulence exceeds this threshold, the gas cloud changes from turbulence-dominated to gravity-dominated. When and how this happens determines the size of the dense nuclei of molecular clouds. And it is the dense nuclei that lead to star formation.

The area shown in this image is known as Polaris Flare, a region of dust and gas in the Ursa Minor constellation, 490 light years from Earth. It was captured by ESA’s Herschel Infrared Space Observatory and displayed as a composite color. It shows several tangled interstellar filaments stretching across space for ten light years. Denser patches of material are embedded in the filaments that could become stars in the future. Federrath and his colleagues compared the properties of these filaments and those in other molecular cloud regions with their simulations and found very good agreement. (Source: ESA and the SPIRE & PACS consortia, Ph. André (CEA Saclay) for the Goulds Belt Survey Key Program Consortium and A. Abergel (IAS Orsay) for the development of the Interstellar Dust Key Program Consortium)

This transition from turbulence-dominated to gravity-dominated is a physical location in the cloud, and despite theoretical predictions, the location, shape and width of the transition zone were unknown. It’s because of the complexity.

“The physical processes are so enormously complex that their interaction can only be examined with the help of computer simulations,” said study co-author Professor Rafl Klessen from Heidelberg University.

Klessen heads a research group at the university and used the equipment at the Leibniz Supercomputing Center to carry out the simulations.

Federrath and his colleagues modeled the turbulence on both sides of the supersonic and sonic scale. The dynamics of these turbulences in the gas clouds are extremely complex and require extreme computing power for simulation. “For our special simulation, in which we want to follow both the supersonic and subsonic turbulence cascades with the sound scale in between, we have to resolve spatial expansion of at least four orders of magnitude,” said Federrath in a press release.

This is a screenshot of the team’s simulation video. Click to show. Photo credit: Federrath et al. 2021.

According to the research team, their simulations were a complete success and confirmed theoretical predictions. They were able to determine the position of the transition zone between the supersonic and sonic scale and quantify its width and shape. They also found that the transition is not sharply delineated but occurs on a large scale.

They also compared the results of their simulation with observations of a gas cloud in the Milky Way. These observations confirmed their results.

“Theoretically, this transition zone defines the frequency with which dense nuclei can be found in interstellar gas clouds,” explained Prof. Klessen. “We therefore compared our predictions with observations of the gas cloud IC5146 in the Milky Way and achieved a very good agreement. This is an encouraging result, ”he added.

The broader astronomical research community has taken note of the team’s work. Christopher McKee of the Department of Astronomy at UC Berkeley and James Stone of the Institute for Advanced Studies in Princeton, New Jersey, wrote an article on nature and astronomy discussing the importance of this research.

“Star formation is central to astrophysics,” they explain. “It not only leads to the diversity of the stars observed in the universe, but also (indirectly) to the formation of planets and black holes, to the generation of heavy elements, to the excitation of the interstellar medium and the circumgalactic medium through feedback from radiation, winds and supernovae and even the evolution of galaxies. “

A Hubble panoramic image of the Carina Nebula showing the turbulent effects of stellar winds and ionizing radiation from massive stars on the molecular cloud from which the stars were born. Photo credit: NASA / ESA / N. Smith (University of California, Berkeley) / Hubble Heritage Team (STScI / AURA).

Due to the time scale for the formation of molecular clouds and stars, this cannot be examined observationally. It can only be approached with simulations, and the results of those simulations can then be compared to observations, as in this new study. “The complex and nonlinear structure of supersonic turbulence makes numerical experiments crucial to understanding the physics of star formation,” the couple wrote in their article.

And Federrath and his colleagues have carried out the strictest and most detailed simulation to date. Rapid advances in computing power have made these types of simulations possible, and as McKee and Stone point out, the supercomputer used in that simulation has already outperformed it.

For scientists studying the problem, the development of powerful computers and equally powerful software push the boundaries of understanding. “Both in the USA and in Europe plans have been announced to make so-called exascale systems (with 1018 floating-point calculations per second, about ten times faster than current supercomputers) available,” write McKee and Stone. “While developing scientific software that can take full advantage of such systems will be a major challenge, the future of computational approaches to studying a wide variety of problems in astrophysics, including star formation, remains very promising.”

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