Introduction

For decades, the prevailing picture of the universe’s first stars—known as Population III—was that they formed in isolation and were overwhelmingly massive. In this view, each of these pristine stars, forged from hydrogen and helium alone, might have weighed in at tens to hundreds of times the mass of the Sun, shining briefly and explosively before seeding the cosmos with the first heavy elements. While that scenario captured essential physics, a growing body of research suggests a more nuanced origin story: the first stars may not have been as uniformly massive as once thought. Instead, they likely formed across a broader range of masses, often in multiples or small clusters, with complex feedback shaping their growth and fate.

The traditional view: Giants born in a quiet cosmos

Early theoretical work painted a simple picture. In a young universe lacking metals (elements heavier than helium), gas could not cool efficiently. With higher gas temperatures came larger “clumps” of star-forming material and, as a result, a top-heavy population of very massive stars. These giants would:

• Live briefly—only a few million years—before dying in spectacular supernovae.
• Flood their surroundings with ultraviolet light, contributing to cosmic reionization.
• Provide the first heavy elements, enabling more efficient cooling and the formation of later generations of stars.

This narrative predicted a relatively narrow distribution skewed toward high masses. In particular, pair-instability supernovae—cataclysmic explosions expected from stars roughly 140 to 260 times the mass of the Sun—were anticipated to leave strong chemical fingerprints. Yet, as evidence accumulated, this simple picture began to fray.

Why the first stars may not have been uniformly massive

Advances in computational cosmology, higher-resolution simulations, and clues from stellar archaeology hint that the first stars formed under messier, more varied conditions than originally envisioned. Several key physical processes point toward a broader initial mass function (IMF) for Population III stars:

• Disk fragmentation around newborn protostars: As a massive protostar forms, it often develops a rotating disk that can fragment into multiple smaller protostars. This process can create binaries or small clusters, distributing mass among several stars rather than one giant.
• Radiative feedback limits growth: Intense radiation from a forming massive star heats and pushes away surrounding gas, throttling accretion. This feedback caps stellar masses and can prevent all stars from reaching extreme sizes.
• Turbulence and streaming velocities: Supersonic “streaming” between dark matter and baryons in the early universe can stir turbulence and reduce the gas available to a single star, fostering fragmentation and variety in stellar masses.
• Magnetic field amplification: Even tiny magnetic seeds can be amplified by turbulent dynamos, influencing how gas collapses and helping regulate the flow of material onto forming stars.
• Environmental diversity: The first star-forming halos differed in mass, merger histories, and background radiation fields (especially the Lyman–Werner background that destroys molecular hydrogen). These differences likely produced a range of characteristic stellar masses.

Taken together, these effects challenge the idea of a uniformly top-heavy population. They support scenarios where Pop III stars spanned a wider range—from tens of solar masses down to perhaps a few solar masses—though still typically heavier than most stars forming today.

What’s the evidence?

Directly observing Population III stars remains a formidable challenge; they formed more than 13 billion years ago and lived short lives. Instead, astronomers build the case from multiple lines of indirect evidence and modeling.

1) High-resolution simulations

State-of-the-art simulations now resolve the formation of individual protostars within “minihalos” of dark matter and gas. Many find that the collapse of metal-free gas can fragment, creating multiple protostars whose growth is checked by radiation and dynamics. Rather than producing only isolated supergiants, these models regularly yield systems with mixed masses and binaries—an outcome similar in spirit to star formation in today’s universe, though under more extreme conditions.

2) Chemical fingerprints in ancient stars

The oldest, most metal-poor stars in the Milky Way’s halo preserve a record of the first supernovae. Their elemental abundance patterns often point to explosions from progenitors with tens of solar masses, while the unmistakable signatures expected from very frequent pair-instability supernovae appear rare. This suggests that the earliest IMF was not dominated by only the most massive stars.

3) Early galaxy observations

Observations of galaxies at extreme redshifts, including candidates from the first few hundred million years after the Big Bang, reveal complex star formation histories earlier than many models anticipated. While no Population III star has been unambiguously detected, searches for spectral features such as strong helium emission with minimal metal lines continue. The emerging picture is consistent with a mixture of stellar populations and a range of stellar masses rather than a uniform army of giants.

Why a broader mass range matters

If the first stars covered a wider spectrum of masses, the consequences ripple across cosmic history.

• Reionization: The number and mass of early stars determine the supply of ionizing photons that reionized the intergalactic medium. A more varied IMF changes the timing and patchiness of this transformation, affecting how quickly the universe became transparent to ultraviolet light.
• Chemical enrichment: Different masses produce different yields of elements. A broader IMF introduces a richer mix of heavy elements and isotopes into early galaxies, setting the stage for rapid cooling and the birth of later stellar generations.
• Black hole seeds: Massive stars collapse into black holes that may grow into the supermassive black holes seen less than a billion years after the Big Bang. The initial mass spread influences how many seeds form and how quickly they can grow. Fewer ultra-massive stars could mean fewer pair-instability events and more remnant black holes in the tens of solar masses range.
• Gravitational waves and binaries: If the first stars often formed in binaries, their remnants—black holes and neutron stars—could merge, producing gravitational waves. Some observed black hole mergers may trace back to metal-poor or even primordial-like environments, though multiple channels remain possible.
• Survivors in the present day? If the Pop III IMF extended to relatively low masses (below about one solar mass), some pristine stars might survive today. So far, surveys have not found definitive metal-free survivors, suggesting the lowest-mass end may have been suppressed—or that surface pollution complicates the search.

Open questions

Despite progress, key uncertainties remain:

• What was the true shape of the Pop III initial mass function, and did it vary with environment and time?
• How common were binaries and small clusters among the first stars?
• How strong and widespread was radiative and mechanical feedback from the first stars, and how quickly did it alter subsequent star formation?
• To what extent did magnetic fields and streaming velocities limit the formation of the most massive stars?
• When and how did the first trace amounts of metals and dust appear, and how rapidly did they trigger fragmentation into lower-mass stars?

What will settle the debate?

Progress will come from a blend of deeper observations and sharper simulations:

• High-redshift spectroscopy: Next-generation space telescopes and large ground-based observatories aim to detect the spectral fingerprints of metal-free or extremely metal-poor stellar populations, including strong helium emission lines accompanied by weak or absent metal lines.
• Stellar archaeology: Expanding surveys of the Milky Way’s most metal-poor stars will refine constraints on the first supernovae, revealing which progenitor masses were most common and how enrichment propagated through early galaxies.
• 21-centimeter cosmology: Mapping the neutral hydrogen signal across the cosmic dawn will help chart the timing and topology of heating and reionization, indirectly constraining the number and type of early stars.
• Gravitational-wave catalogs: As detections accumulate, the mass distribution and redshift evolution of merging black holes may shed light on how often massive, metal-poor binaries formed.
• Physics-rich simulations: Continued improvements in resolution and physical realism—including radiation transport, chemistry, magnetic fields, and feedback—will test how robust a broad Pop III mass spectrum is under diverse conditions.

The bottom line

The image of the first stars as uniformly titanic is giving way to a more dynamic, diverse origin story. The early universe likely produced stars across a spectrum of masses, often in pairs or small clusters, with radiation, turbulence, and environment shaping their growth. This richer portrait helps explain observed chemical patterns in ancient stars, informs how the first galaxies assembled, and reframes the rise of black holes and the reionization of the cosmos.

While direct proof remains elusive, the convergence of simulations, stellar fossils, and high-redshift observations strongly suggests that variety—not uniformity—was the hallmark of the universe’s first lights. As new instruments push further into cosmic dawn, we may soon witness the signatures that reveal exactly how the first stars were born.