"We believe we have found a first clue to the presence of these extraordinary stars," Corinne Charbonnel, professor of astronomy at the University of Geneva, said in a statement. The superlative is not stolen to describe non-standard stars, hitherto only theorized.

The most massive star observed to date has a mass equivalent to that of just over 300 Suns. The one described in the study published in the May edition of Astronomy & Astrophysics leaves it far behind, with a mass estimated between 5,000 and 10,000 times that of the Sun.

The team led by the astrophysicist, with scientists from the Universities of Geneva and Barcelona and the Institut d'astrophysique de Paris, had theorized their existence in 2018 to explain an enigma of astronomy: the great diversity of composition of stars in globular clusters.

Generally very old, these clusters concentrate several million stars in a small volume. Advances in astronomy reveal a growing number of them, as a kind of "missing link" between the first stars and the first galaxies. Our Milky Way, which contains more than one hundred billion stars, has about 180 globular clusters, recalls the press release from the University of Geneva.

The conundrum lies in the fact that many of the stars in these clusters contain elements that require colossal temperatures to be produced, up to 70 million degrees for aluminum. Temperatures much higher than those that stars reach in their core, at most 15 to 20 million degrees – like our Sun.

The proposed solution is that of "pollution" by a young supermassive star, the only one capable of reaching such an extreme temperature. Scientists imagine that such supermassive stars were born by successive collisions in the cluster's tight and very dense space.

A "seed star"

A "kind of seed star will engulf more and more stars," Charbonnel told AFP. And become "like a huge nuclear reactor, continuously supplied with matter and which will eject many" into the cluster. This material will feed the young stars in formation, in proportion to "their proximity to the supermassive star".

Proof of the phenomenon remained to be found. The team found it in a galaxy from the early ages of the Universe, GN-Z11.

Photo of the Milky Way, taken from Badung, Indonesia, on March 23, 2023 © DICKY BISINGLASI / AFP/Archives

Discovered in 2015 by a colleague of Corinne Charbonnel, this galaxy among the most distant observed, more than 13 billion light years away, and therefore one of the oldest, already existed 440 million years after the Big Bang.

Discovered with the Hubble Space Telescope, the observation of this tiny red spot with his successor James-Webb has yielded two key clues: a very high density of stars and especially a lot of nitrogen. An element whose presence can only be explained in such proportions by the combustion of hydrogen at extreme temperatures. A phenomenon that can only occur in a supermassive star.

If the team held with its theory "like a kind of footprint of our supermassive star, then it's a bit like finding a bone," says Ms. Charbonnel: "And we speculate on the head of the beast behind all this ...".

The hope of ever observing one is slim. Scientists estimate the life expectancy of a supermassive star around two million years, a nod to cosmic time scales.

But they suspect they may have appeared in globular clusters until two billion years ago, relatively recently. And therefore leave a trace to better identify them.

© 2023 AFP