The Universe is full of fascinating structures, and some of the most striking take shape inside the giant clouds where stars are born. There, streams of gas appear to converge from all directions toward a dense central hub, like spokes meeting at the center of a wheel.

Now, researchers from Kyushu University and Nagoya University have used 3D computer simulations to reveal the physics behind these elegant structures. The study was published in March 2026 in The Astrophysical Journal Letters. (May 29, 2026 Press Release)

Figure 1: Observed hub-filament system compared with simulation results. The left panel shows a hub-filament system observed in an actual star-forming region; the right shows the structure produced by this study's 3D simulation. Both show multiple elongated filaments of gas radiating toward a dense central hub. The study shows that this characteristic pattern can emerge when a fast interstellar shockwave strikes a molecular cloud with a curved magnetic field.
Download: [Obeservation and Simulation, PNG (562 KB)][Observation, PNG (218 KB)][Simulation, PNG (1.2 MB)]

“Stars are born inside molecular clouds—vast, cold clouds of gas that drift through space,” says Shingo Nozaki, a doctoral student at Kyushu University's Graduate School of Sciences, and a JSPS Research Fellow. “But they only form in the coldest and densest parts of those stellar nurseries, where gas can collapse under its own gravity. In some of these star-forming regions, gas is organized into characteristic hub-and-spoke patterns known as Hub-Filament Systems (HFS).”

How this radial pattern forms, however, has long remained unclear. At a workshop at Kyushu University last summer, Nozaki and Shu-ichiro Inutsuka of Nagoya University began exploring one possible explanation: what happens when an external shock hits a gas cloud with a pinched magnetic field shape?

Using ATERUI III, an astronomy-dedicated supercomputer operated by the National Astronomical Observatory of Japan (NAOJ), they conducted a 3D magnetohydrodynamic simulation, a computational method that models how gas and magnetic fields evolve together over time.

Figure 2: Schematic illustration of the simulation performed in this study. (a) An interstellar shock wave (orange) collides with a molecular cloud possessing an hourglass-shaped magnetic field structure (magenta). The blue solid arrow indicates the propagation direction of the interstellar shock wave. (b) After the shock wave passes through the molecular cloud, the molecular cloud gas is compressed into elongated structures along the magnetic field lines, and gas flows toward the center of the constricted region, as indicated by the blue dashed arrows. The blue dashed arrows represent the gas flow in the reference frame of the shock wave. (Credit: Shingo Nozaki (Kyushu University))
Download: [PNG (133.8 KB)]

As a rough analogy, Nozaki pictures the initial molecular cloud as a dorayaki—a Japanese pancake that's thick in the middle and thin at the edges. A vertical magnetic field runs through the cloud, while gravity pulls the field inward at the center, bending it into an hourglass shape. The team then introduced a cosmic disturbance into the cloud, mimicking the kind of disturbance triggered by a supernova remnant or by expanding gas around massive stars.

The results show several elongated structures that develop toward a dense central region, closely resembling observed HFSs. As the magnetic field lines curve inward, the shock wave strikes different parts of the cloud at different angles, creating what physicists call oblique shocks. These shocks strengthen parts of the magnetic field, forming invisible channels that guide compressed gas into long, narrow filaments converging toward the center.

The simulation also revealed that gas does not flow uniformly into the hub. Dense gas within the filaments moves steadily inward, accelerating as it approaches the center, while the low-density gas between filaments stays mostly still. This indicates that the main carriers of mass to the central region are the shock-produced dense filaments, not the cloud as a whole, offering insights into why star formation efficiency remains limited to a few percent.

The team notes that this study focused on a geometrically regular type of HFS, while many observed systems are more asymmetric and complex. Next, they plan to systematically vary the shock direction and strength; the cloud’s density structure; and the magnetic field geometry. This will help them connect different cloud environments to the formation of various massive stars and clusters, and more broadly, to understand how star formation proceeds across galaxies.

“There are two main sources of these shock waves: radiation-driven ‘bubbles’ from newly formed massive stars, and expanding supernova remnants formed when a massive star reaches the end of its life,” adds Nozaki. “There is something almost like a life-cycle in this. What a star leaves behind can go on to shape the next cradle of stars.”

Research Paper

Title: "An Origin of Radially Aligned Filaments in Hub-filament Systems"
Authors: Shingo Nozaki (Kyushu University) and Shuichiro Inutsuka (Nagoya University)
Journal: The Astrophysical Journal Letters
DOI: 10.3847/2041-8213/ae4c84

Supercomputer Used in This Study

In this study, simulations were carried out using “ATERUI III,” the astronomy-dedicated supercomputer operated by NAOJ. ATERUI III has a total theoretical performance of 1.99 petaflops and consists of two systems: “System M,” which provides high memory bandwidth, and “System P,” which offers large memory capacity. In this study, the large-memory capability of System P was utilized to perform large-scale simulations. (Credit: NAOJ)

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