Astronomers find the strongest evidence yet for the universe’s first stars

For decades, astronomers were only able to study the universe’s very first stars using theoretical models. Now, observations from the James Webb Space Telescope (JWST) have revealed what may be the most compelling evidence to date for these ancient “Population III” stars, finding them clustered around a small companion object that formed just 400 million years after the Big Bang.

The discovery has been reported in two companion studies, published as preprints on the arXiv server: one led by Roberto Maiolino at the University of Cambridge, and the other by Elka Rusta at the University of Florence. If confirmed, it could provide a direct observational window into the conditions of the early universe, and help to explain how the very first generations of stars shaped everything that came after.

The first generation

In contrast to today’s stars, Population III stars formed from clouds of almost pure hydrogen and helium, before heavier elements like carbon, oxygen, and iron had ever been forged in stellar interiors. Astronomers believe these stars were extremely massive and hot, burning through their fuel in just a few million years: a small blip on cosmological timescales. Afterwards, they would have exploded in colossal supernovae, seeding the next generation of stars with heavier elements.

In 2024, Maiolino and his colleagues spotted an unusual signal in the halo of GN-z11—one of the brightest known galaxies in the early universe. Using NIRSpec-IFU, a near-infrared spectroscopy instrument on board JWST, they detected a faint emission line from a small companion object named Hebe, located just three kiloparsecs from the host galaxy.

The line matched the signature of doubly ionized helium: a feat which requires extraordinarily energetic radiation. Combined with the lack of detectable metals in the spectrum, the team suggested that Population III stars were the most plausible source, despite the fact that such ancient stars had never been observed directly.

A closer look

Using the higher-resolution capabilities of NIRSpec-IFU, Maiolino’s team have now confirmed that this helium signal is real, and resolved it into two distinct components.

In their separate study, Rusta’s team independently detected a hydrogen emission line from the same location, providing a second anchor for the identification. Neither study found any evidence for heavier elements in the emissions.

Using theoretical modeling, Rusta’s team were then able to use Hebe’s observed helium-to-hydrogen ratio to constrain how massive these first stars likely were. Their analysis favors a top-heavy mass distribution, with most stars falling between roughly 10 and 100 times the mass of the sun—consistent with predictions that the first stars were hot and massive, forming in a universe not yet enriched by heavier elements.

While many more observations will be needed before astronomers can glean deeper insights into the lives of these ancient stars, these independent results offer some of the clearest evidence yet that they existed in the first place. By building on them, astronomers may soon learn more about the origins of the structures that shape our universe today.

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