Extremely massive stars may have altered the chemistry of the universe

Based on a new astrophysics model, scientists are making a bold claim. A handful of extremely massive stars, each more than 1,000 times the mass of the Sun, may have shaped the chemistry of the universe’s oldest star clusters.

These colossal stars could explain the unusual elemental patterns astronomers see in ancient stellar systems.

The team’s framework connects how these clusters formed to the unusual mix of elements we measure in their surviving stars, a link astronomers have chased for decades.

These objects are globular clusters – spherical, densely packed swarms of ancient stars that likely formed within the universe’s first few billion years.

The clusters orbit most big galaxies, including the Milky Way, and their stars carry chemical fingerprints that do not match ordinary stellar birth.

Why old star clusters look odd

The work was led by Mark Gieles, an expert at the University of Barcelona’s Institute of Cosmos Sciences (ICCUB) and the Institute of Space Studies of Catalonia (IEEC). His research focuses on how star clusters form, change, and survive over cosmic time.

Globulars host multiple populations, groups of stars whose light elements vary in tandem in ways that standard single-episode star formation cannot produce. A classic signature is the oxygen-sodium anticorrelation, where oxygen falls as sodium rises. 

Another pattern that matters is the spread in helium across cluster stars. That spread is usually modest, and it tends to be larger in more massive clusters, a fact any viable theory must explain.

Chemical imprint of extremely massive stars

The model’s key players are extremely massive stars, stellar giants in the 1,000 to 10,000 solar mass range that live briefly and lose mass ferociously.

Their stellar winds, fast outflows of processed material from their surfaces, are rich in the products of very hot hydrogen burning.

Those winds mix with leftover, pristine gas inside the growing cluster. New, low-mass stars then form from this blend, locking in a distinct elemental pattern without needing separate generations of star formation.

“Our model shows that just a few extremely massive stars can leave a lasting chemical imprint on an entire cluster,” said Gieles.

How did these massive stars emerge?

To explain how such giants arise, the team scales up the inertial-inflow model (IIM), a star formation scenario where turbulence funnels gas inward and builds the most massive stars.

This idea does not require a single, huge core to collapse, it relies on converging flows in a chaotic cloud. 

In this picture, the larger the turbulent gas reservoir, the higher the upper mass a star can reach before winds push back and balance further growth.

The same flows also feed the cluster itself, keeping gas around long enough for wind material to mix with fresh inflow.

The approach naturally yields the small helium spreads seen in most clusters. Winds add processed material, yet ongoing accretion of clean gas keeps helium from running away, fitting the measured trend with cluster mass.

Why it matters for early galaxies

The model does more than solve a puzzle in the Milky Way’s backyard. If extremely massive stars were common in young clusters, they could explain nitrogen-rich galaxies that the James Webb Space Telescope is spotting at extreme distances, including GN-z11. 

That distant system shows unusually strong nitrogen lines, a clear chemical signature suggesting that its stars formed in a dense, fast-evolving environment where early clusters enriched their surroundings in record time.

Those stellar giants likely collapsed into intermediate-mass black holes – objects heavier than ordinary stellar remnants but lighter than the supermassive ones that anchor galaxies.

Their masses fall above the so-called pair-instability gap, a predicted desert in the black hole mass spectrum. Future gravitational-wave detections could uncover their hidden population.

Extremely massive stars shed material

The chemistry map offers testable predictions. More massive or metal-poor clusters should show stronger aluminum enrichment. At the extreme end, they may also display magnesium depletion. Both are signs of the higher core temperatures inside the largest stars.

Another prediction concerns timing. The winds that matter most should emerge very soon after cluster birth, before supernovae explode, so the enriched stars keep iron steady while only the light elements shift together.

This framework removes many of the assumptions that have long complicated the field. It doesn’t require a second round of star formation inside a small cluster, nor does it depend on clusters holding on to large amounts of gas after their massive stars have already died.

Instead, it casts cluster formation and chemical imprinting as one process. Turbulent inflows build a compact cradle, extremely massive stars shed hot-processed material, and low-mass stars quietly record the blend as they form.

Super-sized stars with a lasting legacy

If clusters formed this way, their present-day stars preserve an early account of how gas flowed, mixed, and cooled in the first galaxies.

That record, read through oxygen, sodium, magnesium, aluminum, nitrogen, and helium, can anchor models of galaxy assembly.

The research also hints that the universe’s first stellar ecosystems were efficient at both making and recycling what they created. A few super-sized stars may have been enough to steer the chemistry of entire clusters.

The study is published in Monthly Notices of the Royal Astronomical Society.

Image: NASA/ Hubble

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