Glimpse of the earliest supernovae could reveal our cosmic origins

The James Webb Space Telescope may have glimpsed the remnants of the very first supernovae in the universe, a chemical fingerprint of the first stars and the start of a process that eventually created almost every element in the periodic table and the “stardust” that makes up our bodies.

When stars formed after the big bang, they contained mostly just hydrogen and helium. As these first-generation stars, known as population III stars, reached the end of their life, they exploded in supernovae, producing heavier elements that were incorporated into a new generation of stars.

Astronomers haven’t been able to observe these ancient stars or their explosions directly, as they would have been born and died just a couple of hundred million years after the big bang. Instead, Roberto Maiolino at the University of Cambridge and his colleagues used the James Webb Space Telescope (JWST) to study one of the earliest galaxies ever seen, GLASS-z12, for more than 50 hours. The galaxy, which was born 350 million years after the big bang, was itself discovered using JWST shortly after the telescope began operations last year.

By analysing the light from the galaxy, the team found it contained carbon, as well as weaker detections of elements like oxygen and neon. “This is the most distant signature of any heavy element in the universe, which is remarkable,” says Maiolino.

The team focused on the ratio of carbon and oxygen within the galaxy because it can tell us what stellar processes are happening. The ratio tends to decrease in galaxies as we look back further in the universe, as there has been less time for stars to repeatedly explode and “pollute” their galaxies with carbon. But strangely, GLASS-z12’s carbon-to-oxygen ratio was higher than in many newer galaxies.

It isn’t clear what is producing this carbon, but the brief time the galaxy has existed for and its high mass rule out many scenarios, such as regular supernovae from stars like our sun, says Maiolino. One of the few mechanisms that could explain this pattern, he says, is exploding population III stars, which are low-energy and extremely pure in hydrogen and helium, allowing them to produce more carbon than other stars.

If this is the correct explanation, the observation could give us our earliest glimpse of the stellar process that created all of the elements, even without seeing the population III stars directly. “The universe gets polluted astonishingly quickly,” says Emma Chapman at Imperial College London, as just one population III supernova is enough for less-pristine stars to start forming. “It really is just a blink of an eye that they’re trying to capture.”

Finding out more about GLASS-z12 might be tricky because there is only so much of JWST’s time that can be devoted to a single object and this galaxy is now comparatively well studied, says Maiolino. But finding other similarly old galaxies and measuring their carbon ratios could also help shed light on population III stars, he says.

The detection of high levels of carbon in such an old galaxy is surprising, says Richard Ellis at University College London, but uncertainty in both the carbon and oxygen ratio measurement and models of population III stellar explosions means there might be other explanations from different kinds of stars. “We have no idea what a population III supernova would look like, it might not even be like a supernova that we see in the nearby universe,” says Ellis.