A new three-dimensional map suggests the interstellar medium is more dynamic and unstable than long assumed, with researchers finding 41% of the ISM in the galactic neighbourhood in an intermediate state that contradicts leading theoretical models taught in textbooks.
Jonathan Shelest, an undergraduate at the Technion in Haifa, Israel, found that much of the interstellar medium (ISM) is neither cold nor hot but sits in a forbidden mid-range, challenging a model first proposed in 1969.
For decades, astronomers treated the ISM, the diffuse fog of hydrogen and other atoms between stars, as a static backdrop. The classic model, developed by George Field, Donald Goldsmith and Harm Habing, predicted that gas in the ISM exists in two stable states: cold clouds at about 100 kelvins (minus 173 degrees Celsius) and warm diffuse clouds at about 8,000 kelvins, with almost no unstable material in between. In 1977, Chris McKee and Jeremiah Ostriker added a third phase of hot plasma at half a million kelvins, but even then, atomic gas outside blast zones was assumed to be calm and pressure-balanced.
To test the theory, Shelest, his adviser Shmuel Bialy, and colleagues combined two large datasets: a 2024 dust-density map covering a sphere more than 4,000 light-years wide around the Sun, and a map of ultraviolet radiation from Troy Porter of Stanford University. They compared the density of dust with the intensity of radiation at points on a 3D grid to infer where the ISM is hot and where it is cool.
The result: 27% of the ISM in our galactic neighbourhood was cold, 32% was warm, and the remaining 41% sat squarely in the unstable zone. “I thought I was wrong,” said Shelest. “How do you go to your science adviser and tell him that you get so much unstable media?”
The unstable ISM formed massive, coherent blankets that wrapped around cold clouds, suggesting something is stirring the gas. “Something is taking material that should, by the static theory, be sitting in one of these other two states, and it’s stirring it up so that it’s being forced to change states,” said Adam Ginsburg, an astronomer at the University of Florida. Desika Narayanan, an astrophysicist also at the University of Florida, described the ISM as “constantly in a state of flux.”
Caveats remain. McKee said the model could have overestimated unstable ISM if cold and warm sections were too close for the model to distinguish. Philipp Frank of the Kavli Institute for Particle Astrophysics and Cosmology noted that the team used an “ensemble-averaged mean,” a calculation that naturally smooths sharp contrasts and could inflate unstable amounts. Shelest and Bialy, whose analysis is undergoing peer review, acknowledge the issues but argue that blurring alone cannot account for the majority. When they tested a lower-resolution dust map, the unstable fraction shifted by only about 5%.
The finding may help explain why galaxies are so inefficient at turning gas into stars. “The reason why star formation is very inefficient is that feedback heats up and pushes on the surrounding gas,” said Anna Rosen, an astrophysicist at San Diego State University. The constant cycle of heating and cooling churns the ISM, suppressing star formation in ways that classic equilibrium models miss.
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Share of interstellar medium in the galactic neighbourhood, by state
percent
Analysis by Shelest, Bialy and colleagues; undergoing peer review. Source: Quanta Magazine
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Analysis
Why This Matters
The finding challenges a half-century-old textbook model of the interstellar medium, forcing a re-evaluation of how gas behaves between stars.
It may help resolve why star formation is so inefficient in galaxies, with implications for understanding galaxy evolution and the cosmic matter cycle.
The map opens a new 3D window onto the dynamic processes that shape the environment in which stars and planets form.
Background
The interstellar medium (ISM) is the diffuse gas and dust between stars, composed mostly of hydrogen and helium left over from the Big Bang, supplemented by material from dying stars. For decades, astronomers treated it as a calm, static backdrop because it changes so slowly. The classic two-phase model from 1969 predicted gas would settle into either cold dense clouds or warm diffuse clouds, with almost nothing in between. A third hot phase was added later for supernova blast zones. Only recently have new space telescopes and statistical methods allowed scientists to create 3D maps of dust and radiation over thousands of light-years, enabling a direct test of the theory.
Key Perspectives
Jonathan Shelest and Shmuel Bialy (Technion): The data show that 41% of the ISM in our region is in an unstable intermediate state, contradicting the classic theory. They argue that mathematical smoothing alone cannot explain the result, as tests with lower-resolution maps gave only a 5% difference.
Chris McKee (co-author of the three-phase model): The model could have overestimated unstable ISM if cold and warm regions were too close for the map to resolve.
Philipp Frank (Kavli Institute): The method used (ensemble-averaged mean) naturally softens sharp density contrasts, which could inflate the amount of unstable gas.
Adam Ginsburg and Desika Narayanan (University of Florida): The result is plausible and suggests the ISM is actively churned by feedback processes, keeping it out of equilibrium.
What to Watch
The team’s analysis is currently undergoing peer review; the outcome will determine how much weight the field gives to the result.
Future higher-resolution maps of the ISM, combined with more sophisticated statistical techniques, could confirm or refine the unstable fraction.
Astronomers will look for direct evidence of mechanisms (e.g., supernova shocks, stellar winds) that could be stirring the gas into intermediate states.
12 claims in this story checked against 1 source: 12 supported.
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SupportedThe new 3D map reveals that the interstellar medium is more unstable than expected.Quanta Magazine: “Instead, much of the ISM appears to be unstable — a finding that directly contradicts the leading theoretical models taught in textbooks.”
SupportedResearchers found 41% of the interstellar medium in the galactic neighbourhood in an intermediate state, challenging the established model.Quanta Magazine: “Shelest found that 27% of the ISM in our galactic neighborhood was cold, 32% was warm, and the remaining 41% sat squarely in the forbidden zone as unstable medium.”
SupportedShelest said, “I thought I was wrong. How do you go to your science adviser and tell him that you get so much unstable media?”Quanta Magazine: ““I thought I was wrong,” Shelest said. “How do you go to your science adviser and tell him that you get so much unstable media?””
SupportedGinsburg said material that should be in a stable state is being stirred and forced to change states.Quanta Magazine: ““Something is taking material that should, by the static theory, be sitting in one of these other two states, and it’s stirring it up so that it’s being forced to change states.””
SupportedNarayanan described the ISM as “constantly in a state of flux.”Quanta Magazine: ““the ISM is doing the same thing, but with energy, and it’s constantly in a state of flux.””
SupportedFrank said the team used an “ensemble-averaged mean,” a calculation that smooths sharp contrasts and could inflate the amount of unstable ISM.Quanta Magazine: “The researchers used an “ensemble-averaged mean,” a calculation that naturally tends to smooth over sharp contrasts.”
SupportedRosen said feedback heats and pushes on surrounding gas, explaining why star formation is inefficient.Quanta Magazine: ““The reason why star formation is very inefficient is that feedback heats up and pushes on the surrounding gas,” said Anna Rosen, an astrophysicist at San Diego State University.”
SupportedThe 1969 model proposed cold and warm stable states, and the 1977 framework added hot plasma at about half a million kelvins.Quanta Magazine: “In 1969, the researchers George Field, Donald Goldsmith, and Harm Habing came up with a model of how the ISM behaves. They predicted that it exists in two stable states: cold and warm.”
SupportedThe team combined a 2024 dust map covering a sphere more than 4,000 light-years wide around the Sun with a map of ultraviolet radiation, comparing dust density and radiation intensity on a 3D grid.Quanta Magazine: “In 2024, a separate team used these tools to create a continuous three-dimensional map of the density of the ISM’s dust inside a sphere more than 4,000 light-years wide around our sun.”
SupportedMcKee said the model might have overestimated unstable ISM if cold and warm sections were too close for it to distinguish.Quanta Magazine: “The model could have overestimated the amount of unstable ISM if cold and warm sections were too close together for the model to tell them apart, McKee said.”
SupportedShelest and Bialy acknowledge resolution and smoothing issues but argue blurring alone cannot account for most of the unstable ISM.Quanta Magazine: “Shelest and Bialy — whose analysis is currently undergoing peer review — acknowledge that limited resolution and map smoothing are both issues, but they argue that mathematical blurring alone cannot account for the majority of the unstable ISM.”
SupportedTesting a lower-resolution dust map shifted the unstable fraction by only about 5%.Quanta Magazine: “When the authors tested their modeling on an entirely different dust map with lower resolution, the unstable fraction shifted by only about 5%, they said.”
