Researchers from Samara University named and the Samara Branch of the Lebedev Physical Institute of the Russian Academy of Sciences have uncovered the origin of enigmatic plasma “phantoms”—massive, shape-shifting structures observed in the Orion Bar, a prominent region within the Orion constellation. Using advanced mathematical analysis and numerical simulations, they identified the physical mechanism behind these phenomena for the first time.
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These orderly-moving formations—filaments, spheres, and wave-like structures composed of optically thin plasma—were previously captured by both ground-based and space telescopes. While various theories attempted to explain their origin, Samara scientists have now provided the first rigorous mathematical proof: the structures arise from auto-wave acoustic disturbances triggered by thermal imbalance in the interstellar gas of the Orion Bar.
The study was supported by the Russian Ministry of Science and Higher Education under its state assignment program for educational and research institutions. The findings have been published in the prestigious international journal Astronomy & Astrophysics.
“Earlier, Russian scientists hypothesized that a specific type of substructure in the Orion Bar might stem from so-called isoentropic, or acoustic, instability,” explained Dmitry Zavershinsky, Head of the Department of Physics at Samara University and one of the study’s lead authors.
“We mathematically confirmed this hypothesis using systems of gas-dynamic equations, describing the most probable formation mechanism of these telescope-observed features. Through numerical modeling of a shock-wave pulse propagating through a medium with parameters matching the Orion Bar, we generated periodic structures whose characteristic size and spacing closely match those seen in actual astronomical observations. In other words, our simulation reproduced the very same patterns visible in the Orion Bar.”
Isoentropic (acoustic) instability occurs in environments where heating and cooling processes fall out of equilibrium. When thermal balance is disrupted, even a tiny perturbation can trigger a cascade of powerful shock waves—an echo that doesn’t fade but grows louder with each repetition. This phenomenon appears not only in space—in the solar corona or Earth’s ionosphere—but also in terrestrial laboratory plasmas.
“Isoentropic instability amplifies acoustic waves due to a positive feedback loop between the waves and nonequilibrium heat release,” Zavershinsky elaborated.
“It’s analogous to the familiar audio feedback when a microphone placed near a speaker picks up its own output, creating a piercing screech—the signal loops endlessly through the amplifier. In this cosmic scenario, the interstellar gas in the Orion Bar acts as both microphone and amplifier.”
The team determined that gas-dynamic disturbances in the Orion Bar transform into shock-wave pulses when the medium’s temperature exceeds 480 Kelvin (about 207°C). Denser pockets of ionized interstellar gas become hotter and, under higher pressure than their diffuse surroundings, begin moving through space at supersonic speeds—advancing in coordinated fronts and adopting varied morphologies. These colossal gas clumps, riding the leading edges of acoustic shock waves, are precisely the plasma “phantoms” detected by telescopes.
According to calculations, individual “phantoms” span roughly 120 billion kilometers in length or diameter, with spacing between wavefront groups reaching about 600 billion kilometers. Unfortunately, humanity cannot “hear” this rhythmic “music of the celestial spheres.” Not only is the Orion Bar located some 414 parsecs (over 12 quadrillion kilometers) from Earth, but the extremely low density of interstellar gas results in acoustic wavelengths far beyond the range of human hearing. Moreover, the full “composition” unfolds over millennia: simulations show it takes 5,000 to 40,000 years for the leading wave pulse to reach a steady amplitude—far longer than a human lifetime.
“Our research significantly advances understanding of shock-wave structure formation and evolution in photodissociation regions like the Orion Bar,” Zavershinsky concluded.
“By combining analytical and numerical methods, we modeled the nonlinear dynamics of these structures and compared our results with observational data from major telescopes—including ALMA, Herschel, Keck, and the James Webb Space Telescope. Our model aligns remarkably well with the dynamic features observed in the Orion Bar, offering deeper insight into the physical processes governing the interstellar medium across the Universe.”
