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Globular clusters are the largest and oldest star clusters in the universe. It can contain up to a million of them. Born at the same time, the chemical composition of these stars shows anomalies not found in any other group of stars. Explaining this peculiarity is one of astronomy’s greatest challenges. Having imagined that massive stars could have been in origin, a team from the Universities of Geneva and Barcelona and the Astrophysical Institute in Paris (CNRS and Sorbonne University) believe they have discovered the first chemical trace attesting to their presence on the globe. Proto-clusters, born about 440 million years after the Big Bang. These results, obtained thanks to observations by the James Webb Space Telescope, can be found Astronomy and astrophysics.
Globular clusters are very dense collections of stars distributed in a sphere, the radius of which ranges from ten to one hundred light years. They can contain up to a million stars and are found in all kinds of galaxies. Our country is home to about 180 of them. One of their greatest mysteries is their star formation: why is it so diverse? For example, the ratio of oxygen, nitrogen, sodium, and aluminum varies from star to star. However, they were all born at the same time, within the same gas cloud. Astrophysicists talk about “abundance anomalies”.
Short-lived monsters
A team from the Universities of Geneva (UNIGE) and Barcelona and the Astrophysical Institute of Paris (CNRS and Sorbonne) have made new progress in explaining this phenomenon. In 2018, he developed a theoretical model according to which the formation of massive stars would have “polluted” the original gas cloud during the formation of these clusters, enriching their stars with chemical elements in a heterogeneous manner. “Today, thanks to data collected by the James Webb Space Telescope, we believe we have found the first evidence of the existence of these unusual stars,” explains Corinne Charbonnell, a professor in the Department of Astronomy at UNIGE. Faculty of Science, and first author of the study.
These celestial monsters have 5,000 to 10,000 times more mass, and five times hotter at their center (75 million degrees Celsius) than the Sun. But proving its existence is complicated. Globular clusters are between 10 and 13 billion years old, while the maximum age of stars is 2 million years. So it disappeared very early from the currently observable groups. “Only indirect effects remain,” explains Marc Gillies, ICREA Professor at the University of Barcelona and co-author of the study.
revealed by the light
Thanks to the very strong infrared vision of the James Webb Telescope, the co-authors were able to support their hypothesis. The satellite captured the light emitted from one of the smallest, most distant galaxies known to date in our universe. GN-z11 is located about 13.3 billion light-years away, and is a few tens of millions of years old. In astronomy, analysis of the optical spectrum of cosmic bodies is a key element in determining their properties. Here, the light from this galaxy provided two valuable pieces of information.
“It has been shown to have very high proportions of nitrogen and very high densities of stars,” says Daniel Shearer, associate professor in the Department of Astronomy at UNIGE College of Science, and co-author of the study. This indicates that several globular clusters are forming in this galaxy and that it still harbors an active massive star. “The strong presence of nitrogen can only be explained by the combustion of hydrogen at extremely high temperatures, which only the cores of massive stars can reach, as shown by the models of Laura Ramirez-Galeano, a master’s student on our team,” explains Corinne Charbonnel.
These new findings reinforce the international team model. The only one currently able to explain the anomalies of abundance in globular clusters. The scientists’ next step will be to test the validity of this model on other globular clusters that form in distant galaxies, using James Webb’s data.
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