Astrophysicist Jonathan Shelest already possessed a detailed map of the dust in the interstellar medium (ISM). His next step was to combine this with a map of ultraviolet radiation, which heats the interstellar gas.
By a stroke of serendipity, his colleague Bialy discovered the exact radiation map needed, encountering astrophysicist Troy Porter of Stanford University at a conference. Porter had compiled such a map but had not yet published it.
With this crucial data, Shelest, Bialy, and their team merged the 200 gigabytes of information from both maps. By cross-referencing dust density with radiation intensity at every point in space, they mapped out the temperature variations of the ISM, successfully integrating the datasets into a comprehensive three-dimensional grid.
When Shelest analyzed the resulting map, he encountered a major surprise. In our galactic neighborhood, 27% of the interstellar medium was cold, 32% was warm, and a staggering 41% fell into a “forbidden zone” of unstable medium. “I thought I was wrong,” Shelest recalled. “How do you go to your science adviser and tell him that you get so much unstable media?”
Jonathan Shelest, a student at the Technion, found the interstellar medium to be more unstable than expected.
The map, however, does not appear to be flawed. Shelest detected no signs of artificial anomalies or random statistical noise within the three-dimensional grid. Instead, the unstable interstellar medium formed vast, cohesive blankets that completely enveloped the cold molecular clouds.
The presence of this substantial midrange medium—neither hot nor cold—indicates that older theoretical models are incomplete, according to Adam Ginsburg, an astronomer at the University of Florida. “Something is taking material that should, by 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,” Ginsburg explained.
Desika Narayanan, an astrophysicist at the University of Florida, offered a biological analogy to describe this dynamic process. “Just like your blood in your body is going to have metabolic waste or oxygen in it, and it’s moving different things to different places,” he said, “the ISM is doing the same thing, but with energy, and it’s constantly in a state of flux.”
A Measured Approach
Following the release of the paper in July 2026, Shelest’s inbox was flooded with enthusiastic messages from astronomers eager to collaborate. Despite the excitement, researchers caution against rushing to rewrite astronomical textbooks just yet.
Some scientists have raised potential methodological concerns. For instance, the model might have overestimated the amount of unstable interstellar medium if the cold and warm regions were positioned too closely together for the resolution to distinguish between them, noted McKee. Additionally, Philipp Frank, an astrophysicist at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford, suggested that the team’s calculation method could have skewed the results. The researchers utilized an “ensemble-averaged mean,” a statistical technique that naturally smooths out sharp transitions. If a cold cloud transitions abruptly to a warm one, this averaging method would artificially soften the steep density drop into a gentler slope, potentially inflating the apparent volume of unstable gas.
Shelest and Bialy—whose analysis is currently undergoing peer review—concede that limited resolution and map smoothing present challenges. However, they argue that mathematical blurring alone cannot explain the vast majority of the unstable interstellar medium. When the team tested their model on an entirely separate, lower-resolution dust map, the proportion of unstable medium shifted by only about 5%, they noted.
The Churn of Creation
Galaxies are remarkably inefficient at converting raw materials—gas and dust—into stars, a mystery that has puzzled astrophysicists for decades. This new map offers a compelling explanation: the constant churning of heating and cooling cycles within the interstellar medium actively suppresses star formation.
“The reason why star formation is very inefficient is that feedback heats up and pushes on the surrounding gas,” explained Anna Rosen, an astrophysicist at San Diego State University. This dynamic churning might clarify why traditional models, which assume a tranquil equilibrium for the interstellar medium, fail to align perfectly with observational data. Rosen noted that existing models provide “physically grounded snapshots”—static views of hot and cold patches—”but in between those snapshots, there’s still a lot going on.”
