They illuminated the nascent cosmos with the light of millions of Suns and for the first time astronomers detected the chemical trace of supermassive stars, “celestial monsters” in a galaxy that appeared more than 10 billion years ago.
“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 extraordinary 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 an estimated mass 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 Institute of Astrophysics in 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 are revealing an increasing number of them, like 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 enigma lies in the fact that many of the stars in these clusters contain elements requiring colossal temperatures to be produced, up to 70 million degrees for aluminum. Temperatures well above those that stars reach in their hearts, 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 able to reach such an extreme temperature. Scientists imagine that such supermassive stars are born by successive collisions in the restricted and very dense space of the cluster.
A “seed star”
A “kind of star-seed will engulf more and more stars”, explains Ms. Charbonnel to AFP. And become “like a huge nuclear reactor, continuously fed with matter and which will eject a lot” into the cluster. This material will feed the young stars in formation, in proportion to “their proximity to the supermassive star”.
It remained to find proof of the phenomenon. The team unearthed it in a galaxy from the early ages of the Universe, GN-Z11.
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 its successor James-Webb delivered two key clues: a very high density of stars and above all 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 stuck with its theory “like a kind of footprint of our supermassive star, there it is a bit as if we had found a bone”, continues Ms. Charbonnel: “And we speculate on the head of the beast behind it all…”.
The hope of observing one one day is slim. Scientists estimate the life expectancy of a supermassive star at around two million years, a blink of an eye in cosmic timescales.
But they suspect they may have appeared in globular clusters as recently as two billion years ago. And therefore leave a trace to better identify them.
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