We may have just glimpsed the first stars of the universe

Astronomers have had their most impressive look yet at some of the earliest stars in the universe. These stars are unlike any other stars we’ve seen and could help us understand fundamental properties of the early universe, such as how massive the earliest stars were and how they shaped those that formed later.

The first stars to form in our universe are thought to have been made almost entirely of hydrogen and helium, with no heavier elements. They were also huge and blazing hot, hundreds of times more massive and tens of thousands of degrees hotter than the Sun.

But because most of these so-called population III stars lived only a relatively short time before exploding, astronomers have yet to conclusively find a galaxy filled with them because they existed so early in the universe’s history.

Now, Roberto Maiolino at the University of Cambridge and his colleagues found that the Hebe galaxy, a group of stars that existed only 400 million years after the Big Bang, has all the hallmarks of a population III-filled galaxy.

In addition to containing no observable elements heavier than hydrogen or helium, the light coming from the galaxy is centered around a carrier frequency associated with helium that has been stripped of its electrons—something only extremely hot stars like Population III can do. “Population III stars seem to be the most likely explanation as far as we can see,” says Maiolino. “All other explanations are very unsatisfactory.

Hebe was originally spotted by Maiolini and his team in 2024 using the James Webb Space Telescope (JWST), where its spectra appeared to show a line of ionized helium reminiscent of Population III stars, but it was unclear whether the line was real or from another galaxy, and whether the stars might contain heavier metals.

But after another observation with JWST, scientists have now found a second line associated with ionized hydrogen and originating from the same source, suggesting that the helium line was in fact real.

“I spent a lot of time reviewing the data to make sure it was a safe line detection,” says a team member Hannah Übler at the Ludwig Maximilian University in Munich, Germany. “Once it was clear where we see it. [ionised hydrogen] peak and otherwise no line detection, it was really amazing. It was a great moment to know and have proof that what we claimed a few years ago was indeed true, that we have helium and hydrogen here, suggesting a Population III scenario.

The results are convincing, and the presence of a line of ionized helium suggests that we are seeing extremely hot objects, which is what we expect for a Population III star, he says Daniel Whalen at the University of Portsmouth in the UK, but it’s not conclusive because we still don’t have the level of precision needed to rule out that there aren’t heavier elements that would make them more mature Population II stars.

A galaxy filled with as many Population III stars as Maiolino and his team predict is also difficult to explain using simulations astronomers have done of the early universe, which found that the first stars often formed in relatively isolated and sparse clusters.

“It’s not just a race to put a flag on Population III, saying we’ve found it and that’s it. We’re actually learning a lot,” Maiolino says. If the stars in Hebe are confirmed to be Population III stars, they could provide us with crucial information about the early universe, he says.

Maiolino and his team have already used the first observations of Hebe to simulate how massive the first stars might have been, finding that most of them are about 10 to 100 times more massive than the Sun, and much fewer are smaller.

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