The James Webb Space Telescope (JWST) may have just found the first stars in the universe.
It’s something that you’d think we’d have seen by now, considering how far Hubble has been able to look back in time for decades. But the search for these stars has been long and fruitless for many, many years.
Researchers call them Population III (or Pop III) stars, because sometimes astronomers name things in reverse for some reason. Pop III are the oldest stars, Pop II are in the middle, and Pop I are the newest. Our Sun is a Pop I star.
Crucially, this has nothing to do with how far along a star is in its individual life cycle. Think of it more like generations—if Pop I are the equivalent of Gen Z stars, Pop III are the boomers. And we’ve never seen a boomer star. Technically, we don’t know for sure that they ever existed.
Until, potentially, now. An international team just announced in a new paper that they have found the first evidence of Pop III stars with the help of JWST. The paper was uploaded to the preprint database arXiv, and has yet to be peer-reviewed.
There were two keys to this discovery. One was the simple power of JWST. Because light has a finite speed, the further away you look, the further back in time you can see. And JWST can look really, really far. The team used the telescope to spy on GN-z11, a bright, really-far-away galaxy, in the hopes of getting a strong and clear spectrum from when the universe was only about 400 million years old (today, it’s around 13.7 billion years old).
The other is a property of starts called metallicity—the amount of metals a star has. (Important caveat: astronomers call anything heavier than hydrogen or helium a metal. So, in space terms, metals are just any sufficiently heavy elements.)
The metals contained within and burned by a star are how we categorize these celestial furnaces into their respective populations. Pop I have the highest metallicities, and Pop III have the lowest. This is because the populations-as-generations idea is a little bit more literal than just a metaphor.
Heavy elements, by and large, are created by stars—whether it be through fusion in their inner layers or in the moments of intense heat when they go supernova. The Pop I stars currently littering the universe were made from the debris left behind when Pop II stars exploded. But Pop II stars had to get their start from older stars, as they have more metals than would have been available just after the Big Bang, but less than Pop I.
So, if Pop II needed more metals than the Big Bang could provide, they must have been forged in Pop III stars. Pop III stars are believed to be made almost entirely of hydrogen and helium—very low metallicity. They would have been truly massive, and likely would not have lived very long (on star timescales at least) before exploding in the supernovas that researchers believe seeded the entire rest of the universe with metals.
So, those are the signs scientists have been looking for—very, very old and very low metallicity. And in GN-z11, they believe they’ve found their long-sought-after signature.
The researchers looked at the halo of gas around the outskirts of the galaxy, where they believe Pop III stars may have formed. In that region, they found a very strong HeIIλ1640 spectral line, which shows up in a spectrum when helium is extremely hot. And the thing is—there’s no metal around it. Not like there would usually be if there was a higher-metallicity star burning nearby. Something made helium incredibly hot without any metals present, and the team thinks it was, at long last, a Pop III star.
This detection is incredibly exciting, but it’s important to temper that excitement appropriately right now. For one, there are alternative propositions to what could have caused this HeIIλ1640 line. One of these is the potential presence of an active galactic nucleus at the center of the GN-z11 galaxy (though, the team working on the study claims that the evidence doesn’t really fit that model).
In addition, the detection needs to be followed up in order to be confirmed, something which will require future JWST time. That’s a hot commodity, and not easy to get, but the team is planning to follow up on their observations as soon as possible. And all of this is still awaiting peer review.
But there’s potential here—potential that researchers have succeeded in one of the biggest goals in the entire field of astrophysics. If this is eventually confirmed as the first true detection of a Population III star, it unlocks a whole new universe of scientific exploration and discovery.