Astronomers are discovering new ways to study the universe's first stars, objects too distant and faint to observe directly, by examining the 21-centimeter signal, an ancient radio signal left behind by hydrogen atoms shortly after the Big Bang.
Understanding how the world transitioned from total darkness to the first glow of starlight is an important milestone in the history of the universe, a period known as the cosmic dawn. But even today's most advanced telescopes cannot directly capture these earliest stars, so discovering their fundamental properties is one of the most difficult tasks in astronomy.
A team of researchers has now shown that clues to the masses of these first stars can be found in a particular radio signal. This signal is produced by hydrogen atoms that filled the space between early star-forming regions and was created just 100 million years after the Big Bang.
Scientists have examined how the earliest stars and their remnants changed this signal, known as the 21-cm signal, and show that the soon-to-be-launched radio observatories have the potential to reveal how the young universe evolved from a nearly uniform cloud of hydrogen into the universe with the rich structure we see today.
"This is a unique opportunity to learn how the universe's first light emerged from darkness," said co-author astronomer Anastasia Pialkov. "The transition from a cold, dark place to a universe full of stars is a story we are only beginning to understand."
The study of the universe's first generation of stars depends on the very faint 21-cm glow, an ancient form of energy that has been around for more than 13 billion years. This signal is shaped by radiation from early stars and black holes, making it one of the few ways scientists can study the universe in its earliest stages.
REACH, SKA and the search for ancient starlight
Pialkov leads the REACH theory group, a radio antenna that is one of two important projects that could help us learn about the cosmic dawn and the era of deionization, when the first stars reionized neutral hydrogen atoms in the universe.
Although REACH, which captures radio signals, is still in its calibration phase, it promises to reveal data about the early universe. The SKA (Square Kilometre Array) – a massive array of antennas under construction – will map fluctuations in cosmic signals across vast areas of the sky.
Both projects are essential in investigating the mass, luminosity, and dispersion of the earliest stars in the universe. In the current study, Pialkov and her colleagues developed a model that makes predictions about the 21-cm signal for REACH and SKA, and found that the signal is sensitive to the masses of the first stars.
"We are the first group to consistently model the dependence of the 21 cm signal on the masses of the first stars, including the influence of UV starlight and X-ray emission from binary X-ray systems that form when the first stars die," said Pialkov. "These insights come from simulations that incorporate the hyalological conditions of the universe, such as the hydrogen-helium composition created by the Big Bang."
When the researchers developed their theoretical model, they studied how the 21 cm signal responds to the mass dispersion of the first stars, called Population III stars. They found that previous studies had underestimated this relationship because they had not taken into account the number and brightness of X-ray binaries – binaries that include a normal star and a collapsed star – among Population III stars, and how they affect the 21 cm signal.
A statistical look at the first stars
Unlike optical telescopes like the Webb Space Telescope, which take live images, radio astronomy relies on statistical analysis of weak signals. REACH and SKA will not be able to image individual stars, but will provide information about different populations of stars, binary X-ray systems, and galaxies.
"It takes imagination to connect radio data to the story of the first stars, but the implications are profound," said Pialkov.
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