The James Webb Space Telescope (JWST) was built primarily to transform our understanding of the early universe. Less than a year after it was turned on, it is performing and finding galaxies earlier in the universe than ever before. Still, the telescope has another, lesser-known purpose: to investigate those earliest moments after the Big Bang 13.8 billion years ago. It is on the hunt for signs of the first stars to light up in the Universe, so-called Population III stars, giant balls made purely of hydrogen and helium that shone brilliantly and brightly to be the first to bring light into the cosmos. “They’ve sort of been in the background,” says Garth Illingworth of the University of California, Santa Cruz, mainly because it’s so hard to find them. No definitive detection of such stars has ever been made, but we know they must exist. Now two new results bring us closer than ever to their discovery.
In a pair of papers posted to the preprint server arXiv.org, two teams of astronomers report promising signs of Population III stars. In the first study, led by Roberto Maiolino of the University of Cambridge, researchers think so may have found a pocket of Population III stars nestled in the outskirts of a remote galaxy. The second study, led by Eros Vanzella of the National Institute of Astrophysics in Italy, hints at a small galaxy which may be composed of, if not Population III stars per se, extremely primordial stars born early in the cosmos. “These papers highlight very nicely the different aspects of the search,” says Jorryt Matthee of the Swiss Federal Institute of Technology in Zurich, who was not involved in either paper. “We are almost there.”
When the universe had cooled and calmed down sufficiently some 400,000 years after the big bang, the first atoms could form: hydrogen and helium. These atoms would have gravitationally clumped into immense clouds and eventually formed Population III stars. Unfettered by competition from other stars, these stars within these clumps could have grown to enormous sizes – at least hundreds or even thousands of times more massive than our sun. This mass meant that the stars were short-lived, exhausted their fuel, and exploded as supernovae in just a few million years. Still, those explosions were vital to the universe. They released heavier elements that had formed in the stars, such as oxygen and carbon, which gave rise to Population II stars and later Population I stars like our sun and even planets like Earth and life itself.
Those short lives have made tracking down Population III stars difficult but not impossible. Some clouds of primordial gas should have existed for some time after the Big Bang, perhaps hundreds of millions of years. Meanwhile, the stars’ immense heat, about 90,000 degrees Fahrenheit on their surfaces — 10 times the temperature of our sun — should give off a telltale whiff of helium that could only have been produced at such temperatures. However, because they are very small from our point of view and likely confused with later Population II stars, they were not observable by most telescopes – until JWST. From the beginning of the telescope’s planning in the 1980s, the focus has been on galaxies. “When it came to the scientific discussions about JWST’s potential, it was always about the earliest galaxies,” said Illingworth, former deputy director of the Space Telescope Science Institute, which directs JWST. “For political reasons, we tried to keep it very simple.” Still, the possibility of finding Population III stars has always been a tempting, if extremely difficult, possibility. “Searching for Population III stars in the early universe is an important part of the science that JWST was built to accomplish,” said Jane Rigby of NASA’s Goddard Space Flight Center, who is an operations project scientist for JWST.
That would be “a huge discovery,” says Rebecca Bowler of the University of Manchester in England. “We still haven’t seen the smoking gun of Population III stars. They are a missing piece in our understanding of the history of the universe.” A galaxy touted as a potential host of Population III stars prior to JWST, dubbed CR7, was apparently excluded in 2017. A more recent result from JWST earlier this year tentative hints found in a distant galaxy, but the results remain unclear. “We’ve never had an instrument she could find,” says Bowler. “JWST is our best shot — it’s huge and it has the right wavelength coverage.”
Maiolino’s team used JWST to observe a galaxy called GN-z11 that had been previously discovered by the Hubble Space Telescope in 2015. Dating back just 400 million years after the Big Bang, GN-z11 was the most distant known galaxy until JWST discovered those are further away. By picking apart the light at the edge of the galaxy in a process called spectroscopy, they found hints of helium that could be linked to small pockets of Population III stars in the galaxy’s outer regions. If correct, the stars would have masses at least 500 times that of our sun, with a total mass of 600,000 solar masses, which would explain the signal observed by the team. “We push the telescope to its limits,” says Maiolino. “These could be clumps of gas that didn’t fully mix with the rest of the galaxy.” Another possibility is that the signal came from a directly collapsing black hole, an example of theorized, never-before-seen objects tens of thousands of times the mass of our sun that were the seeds of supermassive black holes. That would “still be an extremely exciting discovery as well,” he says.
The Vanzella team takes a different approach. Using the gravitational mass of a galaxy cluster called MACS J0416, the team detected what appears to be a magnified emission of hydrogen and a small amount of oxygen from a very small and very distant galaxy. “It’s been magnified by a factor of maybe 500,” said Mark Dickinson of the National Science Foundation’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab) in Arizona, who is a co-author on the paper. While the researchers couldn’t see the light from the galaxy directly, their findings suggest the presence of two extremely small clumps of stars in the early universe, perhaps less than 10,000 solar masses in total, and observed about 800 million years after the Big Bang. . Not only do the clumps appear to consist of Population III stars, but the amount of heavy elements present is incredibly small. “The abundance of heavy elements is lower than anything else we’ve seen in the universe,” says Dickinson. “It’s as close to a primordial galaxy as we’ve seen.”
While neither paper is a definitive detection of Population III stars, both are some of our best evidence to date for their existence. “They’re closer, but they’re not convincing,” said Daniel Whalen of the University of Portsmouth in England, who was not involved in the study. Bowler says that while neither paper “ticks all the boxes,” both are “very interesting and point in the direction of Population III.” Further studies of both targets would be needed to really determine whether they contain primordial stars or at least nearby primordial stars.
Another result published last week in Nature finds a touch of Population III stars closer to home. Astronomers studied a star in our galaxy’s halo and found that it contained an unusual composition of heavy elements and was sodium deficient. This suggests it may have formed from the ashes of a Population III star in a theoretical pair-instability supernova, which occurs when a star between 140 and 260 times the mass of our Sun runaway thermonuclear explosion. “We know that first-generation stars can produce such patterns of chemical elements,” said Gang Zhao of the Chinese Academy of Sciences, one of the paper’s co-authors. The team estimates that the second-generation star they observed is more than 13 billion years old and formed just 500 million years after the Big Bang following the death of a Population III star.
Future work, especially by researchers using JWST, will bring us closer to seeing light directly from Population III stars. Hannah Übler of the University of Cambridge and Maiolino have successfully proposed using JWST in its second year of science, starting in July, to observe seven galaxies in the early universe that appear to be low in heavy elements. “We want to look around these galaxies to see if we can find Population III stars in their suburbs,” says Übler. “We would then be able, with some assumptions, to constrain the mass of the stars.” Another JWST program, led by Matthee, will look for clouds of gas “in or around galaxies” in the early Universe that don’t contain heavy elements, Matthee says.
Seeing such clusters of Population III stars could be the limit of what’s possible with JWST, allowing us to confirm the existence of these stars in certain eras of the universe and tell us something about the sizes they grew to. However, there is a small chance that the telescope will be able to discern individual Population III stars in future observations if they are magnified enough, perhaps around a cluster of galaxies. “It’s possible in principle, but you have to be extremely lucky,” says Übler, adding that magnifications of 1,000 times or more are needed. For now, our best bet is to look for the emission of small nuggets of stars, a task for which JWST is perfectly suited. If we can find them, it will open up a whole new understanding of how our universe began. “We want to know how everything started,” says Maiolino. “Without the chemical enrichment of first-generation stars, there would have been nothing else. It is an important era in the formation of our universe.”