The first generation of stars (Population III) must have formed from the unenriched gas that permeated the infant Universe. These stars produced the first heavier elements and reilluminated the Universe, thus ending the cosmic dark ages and ushering the Universe into the Epoch of Reionization. Due to a lack of direct observations, the properties of the earliest stars remain highly uncertain. According to University of Cambridge’s Professor Anastasia Fialkov and her colleagues, astronomers will be able to learn about the masses of these stars by studying the cosmological 21-cm signal — created by hydrogen atoms filling the gaps between star-forming regions — originating just a hundred million years after the Big Bang.
This artist’s impression shows a field of Population III stars as they would have appeared a mere 100 million years after the Big Bang. Image credit: NOIRLab / NSF / AURA / J. da Silva / Spaceengine.
“This is a unique opportunity to learn how the Universe’s first light emerged from the darkness,” Professor Fialkov said.
“The transition from a cold, dark universe to one filled with stars is a story we’re only beginning to understand.”
The study of the Universe’s most ancient stars hinges on the faint glow of the 21-cm signal, a subtle energy signal from over 13 billion years ago.
This signal, influenced by the radiation from early stars and black holes, provides a rare window into the Universe’s infancy.
Professor Fialkov leads the theory group of REACH (the Radio Experiment for the Analysis of Cosmic Hydrogen).
“REACH is a radio antenna and is one of two major projects that could help us learn about the Cosmic Dawn and the Epoch of Reionization, when the first stars reionized neutral hydrogen atoms in the Universe,” the astronomers said.
“Although REACH, which captures radio signals, is still in its calibration stage, it promises to reveal data about the early Universe.
“Meanwhile, the Square Kilometre Array (SKA) will map fluctuations in cosmic signals across vast regions of the sky.”
“Both projects are vital in probing the masses, luminosities, and distribution of the Universe’s earliest stars.”
In the current study, Professor Fialkov and co-authors developed a model that makes predictions for the 21-cm signal for both REACH and SKA, and found that the signal is sensitive to the masses of first stars.
“We are the first group to consistently model the dependence of the 21-cm signal of the masses of the first stars, including the impact of ultraviolet starlight and X-ray emissions from X-ray binaries produced when the first stars die,” Professor Fialkov said.
“These insights are derived from simulations that integrate the primordial conditions of the Universe, such as the hydrogen-helium composition produced by the Big Bang.”
In developing their theoretical model, the researchers studied how the 21-cm signal reacts to the mass distribution of the Population III stars.
They found that previous studies have underestimated this connection as they did not account for the number and brightness of X-ray binaries among Population III stars, and how they affect the 21-cm signal.
REACH and SKA will not be able to image individual stars, but will instead provide information about entire populations of stars, X-ray binary systems and galaxies.
“It takes a bit of imagination to connect radio data to the story of the first stars, but the implications are profound,” Professor Fialkov said.
“The predictions we are reporting have huge implications for our understanding of the nature of the very first stars in the Universe,” said University of Cambridge’s Dr. Eloy de Lera Acedo.
“We show evidence that our radio telescopes can tell us details about the mass of those first stars and how these early lights may have been very different from today’s stars.”
“Radio telescopes like REACH are promising to unlock the mysteries of the infant Universe, and these predictions are essential to guide the radio observations we are doing from the Karoo, in South Africa.”
The paper was published today online in the journal Nature Astronomy.
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T. Gessey-Jones et al. Determination of the mass distribution of the first stars from the 21-cm signal. Nat Astron, published online June 20, 2025; doi: 10.1038/s41550-025-02575-x
