Telescopes are time machines, and astronomers are using them to find the first stars ever formed in the universe. These early generations of stars, known as Population III stars, were crucial to shaping the universe we know today.
Back in the early days of the cosmos, there were no stars to light up the universe yet. At that time known as the “dark ages,” a few hundred thousand years after the big bang, only the very lightest elements—mainly hydrogen and helium—had formed, spread across the infant universe in a dark, cold haze.
At some point in the subsequent hundred million years, the first stars coalesced from that hydrogen and helium haze, lighting up the universe and gathering into the first galaxies. This period (from the time of the first stars’ birth to about a billion years after the big bang) is known as the epoch of reionization.
“The transition from a dark Universe filled with just hydrogen and helium to today’s galaxies, stars, planets, and metals is a super fundamental one,” says UCLA astronomer William Lake. Because Population III stars were the first, driving these foundational changes in the universe, “they created the conditions and elemental abundances that the stars in our galaxy formed from,” he adds.
Those very early generations of Population III stars were nothing like stars like our sun, as the conditions of the universe were totally different at the time of their births billions of years ago. Stars today typically contain a variety of heavier elements, including carbon, nitrogen, oxygen, and iron—but none of those things existed when Population III stars were born. Heavier elements had yet to be made in the intense cosmic furnaces found inside the cores of stars.
Without those extra elements in their atmospheres, Population III stars lacked strong stellar winds and they grew to be “more massive on average than today’s stars,” says Lake. “This also means that they were hot and bright, with short lifetimes. So, we don’t see any around us today—they died long ago.” Once Population III stars died, having made some metals in the process, they would then seed the universe with those elements for future generations.
But how could we possibly study stellar ancestors that died billions of years ago? Thanks to the finite speed of light, it takes time for light to reach us from the distant cosmos. This effectively makes telescopes into a kind of “look back machine,” where objects far away appear as they did in the past when the light first began its lengthy journey. For a star four light years away (like our nearest neighbor Proxima Centauri), we’re seeing it as that star was four years ago. For a star 12 billion light years away, we would see it as it was 12 billion years ago, around the epoch of reionization—although this is a much more difficult observation to make. So difficult, in fact, that we haven’t actually seen any Population III stars yet.
Part of why it’s so hard is that everything in the distant universe is very red. As the literal fabric of spacetime itself stretches, it stretches out the light waves on their way to us, shifting them towards the infrared or even microwave part of the electromagnetic spectrum. This means we’ll need a specialized telescope to look in the infrared, in order to be able to spot these very early stars and galaxies. And astronomers just got the biggest, best kind of telescope ever made for that purpose: NASA’s James Webb Space Telescope (JWST).
JWST can’t reach back to the very beginning of the universe, but it can get pretty far—back to just a billion years after the universe began, in the epoch of reionization. Although Population III stars were totally alive at that time, they started to be hidden amongst the more “normal” stars, the next generation known as Population II. Plus, there’s still one more challenge for seeing such far away stars: any objects that are that distant from us are absolutely tiny from our point of view, and incredibly faint.
As a result, it’s fairly unlikely we’ll see individual Population III stars, but astronomers still have some ideas of how they’ll find evidence of these elusive early stars. With JWST, they hope to rely on some extra magnification from the universe itself—gravitational lensing, where light bends around massive objects in space—but that approach requires a bit of luck. Unlike a telescope that we can point in any direction, multiple gigantic objects that we have no control over must be fortuitously aligned across astronomical distances for a distant star to be gravitationally lensed; that is, we can’t just make a gravitational lens on demand, it has to be provided by the universe.
There are also some opportunities to spot these stars closer to home, a billion or so years later in the universe’s history, but with the many other modern stars around it’s a bit like finding a needle in a haystack. Astronomers do have some leads on where to look, though. “Some simulations find that pristine gas flowing into the outskirts of big galaxies could stimulate Pop III star formation,” explains UCLA astronomer Sahil Hegde. Another place to look is dwarf galaxies, which stopped forming stars a while ago and therefore might have fewer bits of hay to sort through on our way to finding Population III needles. “You could also in principle look for a gravitational wave background from Population III-seeded binary black holes, but this would require next-generation gravitational wave observatories like the Einstein telescope,” Hegde adds.
So far, astronomers do have one candidate for a possible Population III star they’ve spotted with JWST, known as Earendel. This star is from less than a billion years after the big bang, and was only visible thanks to one of those handy gravitational lenses. There’s still a lot more work to be done, however, before astronomers are confident they’ve really found the first generation of stars; “no conclusive evidence has yet been found for Earendel being Population III,” says Hegde. JWST is sure to continue searching, though, and perhaps we’ll finally see a stellar ancestor clearly in the coming years, illuminating our understanding of reionization just as they illuminated the early cosmos.