Plenty of gas, too few stars: the turbulence choking star birth in Stephan’s Quintet

The textbook says galaxy collisions ignite bursts of star formation by slamming clouds of gas together. Stephan’s Quintet, a famous cosmic pileup about 290 million light-years away, tells a stranger story. In several of its regions there is plenty of raw gas, yet almost no stars are being born. A new study by a team at Osaka Metropolitan University, led by Misaki Yamamoto, mapped the molecular gas across the entire group and points to an unexpected culprit: turbulence. The results appear in The Astrophysical Journal.

Mapping the fuel of an entire group

Stephan’s Quintet, catalogued as Hickson Compact Group 92, is one of the James Webb Space Telescope’s most celebrated portraits: five galaxies, four of them tangled in a slow-motion collision while the fifth is merely a foreground interloper, with one intruder galaxy plowing through and driving a giant intergalactic shock. Using the Atacama Compact Array in Chile, the team produced the first detailed map of the group’s molecular gas, the cold reservoir from which stars form, traced through the glow of carbon monoxide across the whole system.

Violent gas makes fewer stars

Here is the key result, and it is a measurement rather than a guess. Across the group, the star formation efficiency, meaning how quickly gas is converted into stars, varies enormously, by more than a hundredfold from place to place. That efficiency tracks one particular quantity: the velocity dispersion of the gas, essentially how chaotically it is churning. The more violently the gas moves, the fewer stars it forms, even where gas is abundant. Turbulence and fertility run in opposite directions.

Why chaos starves star birth

Now the interpretation, clearly labeled as such. Stars are born when a clump of cold gas grows dense enough to collapse under its own gravity. The authors argue that the turbulence stirred up by the galaxies’ collisions keeps the gas agitated and spread out, so it never settles, concentrates and falls in on itself. One possibility they raise is that this large-scale churning prevents giant molecular clouds, the nurseries of massive stars, from assembling in the first place. It is worth stressing that a strong correlation between turbulence and low efficiency does not by itself prove cause; the team weighs several explanations, with turbulence-suppressed collapse as the leading one.

The other half of the story

This reframes a simple idea. Galaxy encounters are usually cast as star-formation factories, and they can be, since the same interactions that stir gas can also compress it into new stars elsewhere in the group. Stephan’s Quintet shows both faces at once: a single collision that kindles star birth in some places and smothers it in others, depending on how turbulent the local gas happens to be. That duality matters far beyond this one group. In the early universe, galaxy collisions were far more common, so knowing when a crash builds stars and when it stalls them gives astronomers a sharper tool for tracing how galaxies grew across cosmic time.

© 2026 Homer Dávila Gutiérrez, FRAS — SKYCR.ORG. All rights reserved. Publication: Misaki Yamamoto et al., Molecular Gas Structure and Star Formation Diversity in Stephan’s Quintet Revealed by ACA CO(1–0) Mapping, The Astrophysical Journal (2026). DOI: 10.3847/1538-4357/ae7b30