“Supermassive Stars Explain Chemical Anomalies in Star Clusters, Study Finds | University of Geneva”

A team of researchers from Geneva, Paris and Barcelona has found evidence that supermassive stars can explain the chemical anomalies observed in large star clusters.

Globular clusters are the most massive and oldest star clusters in the Universe. They can contain up to a million stars. The chemical composition of these stars born at the same time shows oddities not seen in other stellar populations. Explaining this is one of the great challenges of astronomy. Supermassive stars could explain the anomalies, according to colleagues from the University of Geneva and Barcelona and the Institute of Astrophysics in Paris. Researchers believe they have found the first chemical signature confirming their presence in proto-globular clusters that formed roughly 440 million years after the Big Bang. It is about the results of research based on observations made with the Webb Space Telescope Astronomy and Astrophysics were reported in the columns of the newspaper.

Globular clusters are very close associations of stars arranged in a spherical shape in space. The radius of the sphere can be ten or even a hundred light years. They can host up to a million stars and are found in many different types of galaxies. There are roughly 180 of them in our own galaxy. One of the biggest mysteries about globular clusters is the extremely varied composition of the stars that make them up: what could be causing it? For example, the ratio of oxygen, nitrogen, sodium, and aluminum varies from star to star, even though they were all born at the same time in the same gas cloud. The English technical term “abundance anomaly” created by astrophysicists refers to the fact that the relative element frequencies (e.g. the ratio of sodium and oxygen compared to the ratio of iron and hydrogen) can show a much larger deviation than expected.

In 2018, researchers from the universities of Geneva and Barcelona and the Institute of Astrophysics in Paris developed a theoretical model according to which the original gas clouds were “infected” by supermassive stars during the formation of the clusters, heterogeneously enriching their stars with chemical elements. “Today, thanks to the James Webb Space Telescope, we believe we have found the first evidence of the existence of these special stars.” – explains Corinne Charbonnel (UNIGE), lead author of the study.

Star monsters are at least 5,000 times, but at most, 10,000 times more massive than the Sun, and five times hotter in their centers (75 million degrees Celsius). However, proving their existence is not an easy task. “Globular clusters are between 10 and 13 billion years old, while superstars live up to 2 million years. So they disappeared very early from the clusters we now observe. Only indirect traces remained behind them.” – explains Mark Gieles (ICREA), co-author of the study.

Thanks to the excellent infrared sensor of the Webb Space Telescope, the researchers were able to verify their theory. The space telescope also captured the light of one of the most distant and youngest galaxies known in the Universe. GN-z11, 13.3 billion light-years away, is barely a few tens of millions of years old. In astronomy, many characteristics of celestial bodies are determined by analyzing their light spectrum. The light from this galaxy told researchers two important pieces of information.

“We found that it has a very high proportion of nitrogen and a very high density of stars.” says Daniel Schaerer (UNIGE), co-author of the study. This suggests that multiple globular clusters are forming in the galaxy, and that an active supermassive star is still present in them. “The significant amount of nitrogen can only be explained by the ‘burning’ of hydrogen at extremely high temperatures, which only the cores of supermassive stars can do, as shown by the models of Laura Ramirez-Galeano, a master’s student in our research group.” explains Corinne Charbonnel.

These new results confirm the theory of the international research team. This is currently the only explanation for the abundance anomaly in globular clusters. The researchers’ next task is to test the model based on data from the Webb Space Telescope with other globular clusters forming in distant galaxies.

Source: University of Geneva

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