Astronomers have built the first three-dimensional map of gas temperature in the space surrounding our solar system, revealing a far messier and more dynamic picture than current models predict. The study was submitted to the arXiv preprint server on July 16.
A tale of two temperatures
The interstellar medium, the thin gas filling the space between stars, isn’t just empty space. It is the raw material for new stars and planets to eventually form. Whether that gas stays warm and diffuses or cools into dense clouds comes down to a balance between heating from starlight and cooling through radiation.
Classic theory predicts this gas should settle into one of two stable states: cold, dense clouds around 200 Kelvin or warm, thin gas around 7,000 Kelvin. In between sits a supposedly rare, short-lived unstable zone called the unstable neutral medium. But radio observations have long hinted that this in-between gas is far more common than theory allows, suggesting something keeps stirring the pot—or, as the researchers put it, “pointing to an ISM continuously stirred by turbulence on timescales of order of the thermal relaxation time.”
The problem is that most of what we know about this gas comes from studying it along single lines of sight, essentially looking in one direction and averaging everything along the way. That method cannot reveal how different gas types are arranged in three dimensions or how they connect to their surroundings.
Mapping the gas
Led by Jonathan Shelest of the Technion—Israel Institute of Technology, the team built a tool called 𝒫3D to fix this. It combines a three-dimensional map of interstellar dust, a new three-dimensional map of ultraviolet starlight from nearby massive stars and a model that calculates whether gas at any given point should be cold, warm or unstable. This resulted in a full three-dimensional “thermal weather map” spanning a 1-kiloparsec-wide region centered on the sun, roughly 3,260 light-years across.
The map reveals cold clouds wrapped in shells of unstable gas, all floating within a much larger pool of warm material. According to the paper, the structure makes physical sense: The dense regions cool faster but also have a lot of dust piled up that blocks incoming starlight. At the boundary of a cloud, the gas is no longer dense enough to cool as efficiently as the core, but it is not yet exposed to enough UV radiation to be fully warm, so it is unstable.
As the team puts it, the unstable gas acts as “the spatial transition from dense cold clouds to the diffuse warm surroundings.”
A ‘multiphase’ architecture
What’s more surprising is just how much gas sits in that unstable state. Near the galactic plane, the team finds that roughly 41% of the gas mass is unstable, far more than classical theory would allow. This unstable gas fraction, compared with 27% cold gas and 32% warm gas, implies it must be “cycling” rapidly between warm and cold states on a timescale of just 3 to 6 million years. This suggests that the interstellar medium is constantly being stirred, not settling into just two categories.
Further from the galactic plane, gas density drops off quickly with height, and warm gas becomes the largest mass component.
Perhaps the most unexpected result concerns the cold clouds themselves. Despite being surrounded by turbulent, unstable gas, the clouds’ internal density largely remains constant. This points to surprisingly mild turbulence inside them.
This result is significant because many star formation models treat cold gas as an isolated, highly turbulent system. This new map suggests that the cold clouds are relatively quiet and stable internally but constantly exchange material with a much larger, restless multiphase system around them.
The team says future models of star formation should account for this bigger picture, treating cold gas not as a standalone turbulent reservoir but as one component of a constantly cycling, three-part system.
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Publication details
Jonathan Shelest et al, The first three-dimensional map of thermal phases in the local interstellar medium, arXiv (2026). DOI: 10.48550/arxiv.2607.15352
Journal information:
arXiv
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First 3D map reveals hidden thermal structure of nearby interstellar medium (2026, July 29)
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