Shelest already had the ISM dust map. The plan was to combine that map with a map of the ultraviolet radiation that heats the ISM. By chance, Bialy located one such map while striking up a conversation at a conference with astrophysicist Troy Porter of Stanford University, who had made a map but had not yet published it. So Shelest, Bialy and their colleagues set out to combine the 200 gigabytes of data in the two maps. At each point in space, they compared the density of the dust with the intensity of the radiation to understand where the ISM was hot and where it was cold. Finally, they were able to combine the dust and radiation information into a three-dimensional grid. Then Shelest set out to see what the map could tell him. The biggest surprise came when he investigated the overall temperature of the ISM: Shelest found that 27% of the ISM in our galactic neighborhood was cold, 32% was warm, and the remaining 41% was directly in the forbidden zone as an unstable medium. “I thought I was wrong,” Shelest said. “How do you go to your scientific advisor and tell him that you have so many unstable media?” Jonathan Shelest, a student at the Technion, discovered that the interstellar medium was more unstable than expected. But the map didn’t seem to be broken. Shelest saw no evidence of artifacts or random statistical noise in the 3D grid. Instead, the unstable ISM formed massive, coherent blankets that completely enveloped the cold clouds. All that midrange, neither hot nor cold, implies that the older theory is incomplete, said Adam Ginsburg, an astronomer at the University of Florida. “Something is taking material that, according to static theory, should be in one of these other two states, and shaking it up so that it is forced to change states.” Desika Narayanan, an astrophysicist at the University of Florida, uses an organic metaphor. “Just like blood in the body contains metabolic waste or oxygen, and moves different things to different places,” he said, “the ISM does the same thing, but with energy and is in a constant state of flux.” A measured approach On the day in July 2026 that the group published the paper, Shelest’s inbox was filled with messages from excited astronomers and requests for collaboration. Still, researchers recommend caution before rewriting textbooks. The model could have overestimated the amount of unstable ISM if the cold and warm sections were too close together for the model to distinguish them, McKee said. Alternatively, the method the team used to calculate the overall distribution of gas and dust could have led them astray, said Philipp Frank, an astrophysicist at Stanford’s Kavli Institute for Particle Astrophysics and Cosmology. The researchers used an “ensemble-averaged mean,” a calculation that naturally tends to smooth out stark contrasts. If a cold cloud abruptly transitioned to a warm one, this method would artificially smooth its steep drop in density into a more gradual slope, inflating the amount of unstable ISM. Shelest and Bialy (whose analysis is currently under peer review) acknowledge that limited resolution and map smoothing are both problems, but maintain that mathematical confusion alone cannot explain most of the unstable ISM. When the authors tested their model on a completely different dust map with lower resolution, the unstable fraction shifted by only about 5%, they said. Galaxies in motion of creation are notoriously inefficient at taking raw material (gas and dust) and turning it into stars. For decades, simulations have struggled to explain why. The new map suggests one answer: star formation is suppressed by the constant cycle of heating and cooling that churns up the ISM. “The reason star formation is so inefficient is that the feedback heats and pushes around the surrounding gas,” said Anna Rosen, an astrophysicist at San Diego State University. That turmoil could explain why classical models, which assume that the ISM is in a calm equilibrium, do not seem to match reality. Rosen said the models give us “physically based snapshots” (immutable images of hot and cold ISM patches) “but between those snapshots, a lot is still happening.”
