Astronomers may have glimpsed evidence of the biggest stars ever seen

The James Webb Space Telescope (JWST) is allowing astronomers to explore distant galaxies in the far reaches of the early Universe for the first time. Some of them have chemical signatures that seem to indicate exotic supermassive stars with masses up to 10,000 times the mass of the Sun.

These monsters are bizarre because stars in the neighboring universe seem to have an innate size limit. “All of our models of galaxy evolution… are based on the fact that stars cannot be more massive than 120 solar masses or so,” says Devesh Nandal at the Harvard-Smithsonian Center for Astrophysics in Massachusetts. “There have certainly been theoretical ideas to study stars…more massive, but there has never been an actual observation to point to.”

That is, until now. Nandal and his colleagues studied JWST observations distant galaxy called GS 3073 and discovered an unusually large amount of nitrogen in its chemical signature. This is not entirely anomalous: high levels of nitrogen have been observed in several other galaxies at similarly large distances.

However, in most other galaxies, nitrogen levels are not high enough to cause any confusion – they can be explained by certain types of relatively normal stars and other cosmic events. “This is not the case with the GS 3073,” says Nandal. It simply has too much nitrogen.

There is a special type of putative primordial star called Population of III staras models show, it can grow to extremely large sizes. When these stars do this, modeling also shows that they should produce much more nitrogen than normal stars. Nandal and his colleagues estimate that just a few Population III stars with masses between 1,000 and 10,000 times the Sun could be responsible for the excess nitrogen in GS 3073. “Our work provides the strongest evidence to date for the existence of supermassive Population III stars in the early Universe,” he says.

However, some other researchers have questioned whether supermassive Population III stars are the only ones that fit these data, or if they fit at all. “Population III must be associated with a nearly pristine environment” without many elements heavier than helium, he says. Roberto Maiolino at Cambridge University. “In contrast, GS 3073 is a chemically quite mature galaxy. So it doesn't seem to fit the kind of environment in which you would expect to find Population III.”

Although it could just be a strange galaxy, says John Regan at Maynooth University in Ireland. “When we look at the early Universe, all we see are really weird, really exotic galaxies. So it's hard to turn around and say yes, but I don't expect supermassive stars to form because that would be too weird. Well, you just said it's really weird,” he says.

If these colossal stars really exist, it could help researchers understand the origins supermassive black holeswhich exist in the Universe much earlier than they should. If they formed from supermassive stars rather than ordinary ones, this would give them an advantage and could explain how they grew to the enormous sizes we see in a relatively short period of time.

Confirming that GS 3073 and other nitrogen-rich galaxies in the early Universe actually contain supermassive stars will be difficult, and will likely require the discovery of additional chemical signatures of these strange giants. “Strengthening the case for their existence is extremely difficult—it's very difficult for us to have an explicit signature,” Regan says. “But this signature is very strong.”