Stars made from only primordial gas finally spotted, astronomers claim

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28 Feb 2025

Staring deep into space and far back in time, a team of astronomers may have spotted a galaxy full of stars made from only the primordial gas created in the Big Bang. Such “population III stars” would have formed from hydrogen and helium and nothing else, and researchers have been searching for them for decades, racking up many disputed sightings.

If confirmed, the discovery, made with NASA’s JWST space observatory, opens a window on the starting point of the chemical enrichment of the universe, in which the heavier elements needed to make planets and life began to be forged in stellar explosions.

“It’s very exciting,” says astronomer Elka Rusta of the University of Florence. “We hypothesize that [population III stars] exist from theory, but they have never been directly observed.”

The nature of population III stars remains uncertain. Most theorists think they were huge, with masses up to 1000 times that of the Sun, 10 times larger than any star around today.

That’s because a cloud of gas collapsing to form a star needs to cool, which requires ionizing the atoms in the gas when they collide. But tightly bound hydrogen and helium atoms are hard to ionize, unlike the heavier elements found in later generations of stars.

So a cloud of primordial gas would just keep growing as it pulled in more gas under its own gravity, reaching an enormous size before finally becoming dense enough to ignite nuclear fusion in its core.

The gigantic stars that resulted would also burn hot and fast, ending in a supernova explosion after just a few million years. That brief first flash of population III stars is hard for astronomers to spot in galaxies that went on to shine steadily for billions of years with smaller, longer lived stars. But the spectrum of the light from the giant stars might give them away. Different elements absorb and emit characteristic wavelengths of light. Population III stars would produce very strong emission lines for hydrogen and helium and would lack completely spectral lines produced by heavier elements.

Astronomers have struggled to find stars that bear all the hallmarks of population III, however. Even with JWST’s sharp vision, gathering enough light to measure a galaxy’s spectrum takes time. And, to perform such spectroscopy, the telescope focuses on only small patches of sky, making the odds of finding a rare population III galaxy very slim. However, Rohan Naidu, an astronomer at the Massachusetts Institute of Technology, and his colleagues found a way to speed up the search, as they described in a preprint posted to the arXiv server last month.

Since it launched in 2021, JWST has surveyed millions of objects with its near-infrared camera. These surveys don’t record a detailed spectrum for each object, but use a series of filters to measure a source’s brightness in as many as a dozen broad wavelength bands.

Naidu and colleagues realized they could do a quick and dirty search for population III galaxies by looking for a few key signatures, such as objects that were bright in a band containing a prominent hydrogen line and dim in another containing a line from a heavier element, such as oxygen. “This is an approach that really yields results if you’re looking for very rare objects,” says astronomer David Sobral of Lancaster University.


Scanning data on hundreds of thousands of galaxies, the team came up with two candidates, and one known as GLIMPSE-16043 was a particularly good match. “If you had to manufacture a population III galaxy in a factory, you couldn’t come up with a better specimen,” Naidu says. Viewed as it was when the universe was about 850 million years old, GLIMPSE-16043 has a mass of roughly 100,000 solar masses, the team estimates. That low mass makes it a dwarf galaxy, and its stars had been burning for less than 5 million years, the researchers think. “It has these signatures of extremely hot stars and barely any oxygen,” Naidu says, “and it’s very, very faint, which is what you expect for these types of sources.”

The team still refers to it as a candidate because without a detailed spectrum it’s impossible to rule out other, less exciting possibilities. For example, GLIMPSE-16043 could be a cloud of lingering primordial gas that is being energized by light from a black hole gorging on matter. Or it could simply be a smaller cluster of stars much closer to Earth that is mimicking a population III spectrum. To settle the issue, “ultimately, you will need spectroscopy,” Sobral says. Naidu says JWST officials have awarded the project some high-priority observing time in June to get a spectrum.

If population III stars prove to be big and bright, the ultraviolet light they emit could have played a key role in the youthful universe: ionizing the neutral hydrogen gas between galaxies. And small primordial galaxies like GLIMPSE-16043 could be the predecessors of ultrafaint dwarf galaxies close to our own Milky Way that appear to contain very ancient stars only slightly enriched by heavier elements, notes Tim Beers of the University of Notre Dame.

Some astrophysicists think those current stars are the children of population III stars, Beers says, and by studying them astrophysicists could learn about their ancient forebears. “I find it exciting that you can draw a straight line from what we see around the Milky Way to this proposed birthplace.”