The first stars of the universe, known as Population III stars, were the pioneers of stellar evolution. These stars, which formed shortly after the Big Bang, were unlike any that exist today. They were made entirely of hydrogen and helium, the only elements available in the early universe, and they were thought to have grown to massive sizes, hundreds of times larger than our Sun.
How Did the First Stars Form?
In the early universe, there were no metals (elements heavier than helium). The first generation of stars, Population III stars, formed entirely from primordial hydrogen and helium. This is in stark contrast to modern stars, which contain elements like carbon, oxygen, and iron, affecting their structure and evolution.
Population III stars are believed to have formed within giant clouds of hydrogen gas. Under the pull of gravity, these clouds collapsed, forming dense protostars that began to grow by accreting more material from their surroundings.
Why Were They So Big?
Since there were no heavy elements to cool the gas clouds, the temperature remained high. This led to larger gas fragments, which in turn formed much larger stars than those we see today. Many of these stars are believed to have grown to hundreds of solar masses, shining with an intensity we cannot observe in modern times.
However, a key question remained: Why didn’t these stars continue growing indefinitely? If gravity kept pulling in more material, what prevented them from becoming even larger?
How Radiation Stops Star Growth
For years, scientists believed that radiative feedback was the primary limiting factor in star growth. As a young star accretes mass, it begins to heat up and emit intense radiation. This radiation pushes against the surrounding gas, preventing more material from falling in.
This effect is significant, especially for massive stars. The more massive a star becomes, the stronger its radiation, and the more it pushes away the gas around it. This creates an upper limit on how big a star can grow.
How Magnetic Fields Work in Star Formation
Magnetic fields have long been known to influence modern star formation, but their role in Population III stars was not well understood—until now.
According to the new study, magnetic fields played a key role in limiting the growth of the first stars by opposing gravity. These fields acted as a barrier, slowing down the infall of gas and preventing excessive accretion.
Here’s how it works:
- As a protostar forms, it begins to rotate rapidly.
- This rotation generates a magnetic field, which then interacts with the surrounding gas.
- The magnetic field exerts pressure against the infalling material, slowing down accretion and limiting the star’s growth.
This mechanism was previously overlooked in early star formation models.
Simulation Findings: The Impact of Magnetic Fields
To test this theory, Sharda et al. (2025) conducted advanced radiation magnetohydrodynamic (RMHD) simulations. These simulations incorporated magnetic fields, something previous models often ignored.
The results were striking:
- In simulations without magnetic fields, Population III stars grew to around 120 solar masses.
- In simulations with magnetic fields, the stars only reached about 65 solar masses.
This means that magnetic fields essentially halved the maximum size of the first stars. This finding fundamentally changes our understanding of how the first generation of stars evolved.
Why This Matters: The Cosmic Implications
1. It Redefines the First Stars’ Role in the Universe
If Population III stars were smaller than previously thought, this affects how we model the early universe’s chemistry. These stars were responsible for producing the first heavy elements through nuclear fusion, and their final mass determines how much material they expelled into space.
Smaller stars would have produced fewer heavy elements, meaning that the chemical enrichment of the universe might have been slower than previously estimated.
2. It Affects the Formation of Early Black Holes
Many Population III stars are thought to have ended their lives as black holes. If magnetic fields limited their growth, it means that the first black holes may have started smaller than we previously believed.
This could have implications for:
- The formation of supermassive black holes at the centers of galaxies.
- The rate of black hole mergers detected by gravitational wave observatories.
- The overall evolution of galaxies and cosmic structures.
3. It Changes How We Search for Population III Stars
Since no Population III stars have been directly observed (they existed billions of years ago), astronomers rely on indirect methods to study them. If they were smaller than previously assumed, this affects how we model their:
- Light signatures (which telescopes like the James Webb Space Telescope are searching for).
- Supernova explosions, which leave behind specific chemical fingerprints in ancient galaxies.
What’s Next: Future Research and Discoveries
1. Observations with the James Webb Space Telescope
The James Webb Space Telescope (JWST) is currently searching for the earliest galaxies and stars. This new research could help refine JWST’s search parameters, improving our chances of finding indirect evidence of Population III stars.
2. More Advanced Simulations
This study was a breakthrough, but more simulations are needed to fully understand the role of magnetic fields. Future research will incorporate:
- More detailed physics models of early star formation.
- The effects of cosmic environment variations on different star-forming regions.
- Interactions between magnetic fields and other forces, such as turbulence and cosmic radiation.
3. Direct Evidence from Metal-poor Stars
While Population III stars themselves no longer exist, some of their chemical signatures might be preserved in ancient metal-poor stars that still exist today. Studying these stars can give us more clues about how their predecessors formed.
Conclusion: A New Chapter in Understanding the Universe’s First Stars
This discovery marks a significant turning point in our understanding of the early universe. It challenges the long-held assumption that radiative feedback was the primary force limiting star growth, revealing that magnetic fields played a far more important role than previously thought.
Reference:
Piyush Sharda et al, Magnetic fields limit the mass of Population III stars even before the onset of protostellar radiation feedback, arXiv (2025). DOI: 10.48550/arxiv.2501.12734
