Cosmic Enigmas Unveiled: Celestial Giants and the Birth of Globular Clusters

by Henrik Andersen
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cosmic mysteries

A groundbreaking discovery by scientists indicates that globular clusters, ancient and massive gatherings of stars, were home to supermassive stars when they first formed approximately 13 billion years ago. An awe-inspiring image of M13, a globular cluster situated 22,000 light years away from Earth, showcases a million stars packed into a compact space spanning 150 light-years. Image Credit: HST STScI NASA ESA.

Researchers hailing from the universities of Geneva, Paris, and Barcelona have recently unearthed compelling evidence linking the peculiarities observed in large star clusters to the presence of supermassive stars.

Globular clusters, renowned as the most massive and ancient assemblies of stars in the universe, encompass up to a million individual stars. These stars, born simultaneously, possess distinct chemical compositions and exhibit anomalies that are absent in any other stellar population.

Comprehending this unique attribute presents a significant quandary in the realm of astronomy. Previously, scientists postulated that supermassive stars might be responsible for these anomalies. Now, researchers from the Universities of Geneva and Barcelona, in collaboration with the Paris Institute of Astrophysics (affiliated with CNRS and Sorbonne University), believe they have detected initial chemical evidence of these stars in proto-globular clusters, which came into existence around 440 million years after the Big Bang.

These significant findings, made possible through observations made by the James Webb Space Telescope, have been published in the journal Astronomy and Astrophysics.

Globular clusters are densely packed groups of stars arranged in a spherical shape, with a radius ranging from a dozen to a hundred light-years. They exist in various types of galaxies and can harbor up to one million stars. Our own galaxy is home to around 180 of these clusters. One of the enduring mysteries surrounding globular clusters is the composition of their stars: why does it exhibit such diversity? For instance, the proportions of oxygen, nitrogen, sodium, and aluminum vary from one star to another, despite all of them originating from the same gas cloud. Astrophysicists refer to this as “abundance anomalies.”

Monsters with ephemeral lives

A team of scientists from the universities of Geneva (UNIGE) and Barcelona, alongside the Institut d’Astrophysique de Paris (CNRS and Sorbonne University), has made significant progress in explaining this phenomenon. In 2018, they developed a theoretical model proposing that supermassive stars would have “contaminated” the original gas cloud during the formation of these clusters, leading to a heterogeneous enrichment of chemical elements among their stars.

“Today, thanks to the data collected by the James Webb Space Telescope, we believe we have discovered the first clue indicating the presence of these extraordinary stars,” explains Corinne Charbonnel, a professor in the Department of Astronomy at the Faculty of Science, UNIGE, and the lead author of the study.

These celestial giants are 5,000 to 10,000 times more massive than our Sun and possess a core that is five times hotter, reaching temperatures as high as 75 million degrees Celsius. However, proving their existence presents a complex challenge.

“Globular clusters are between 10 and 13 billion years old, while the maximum lifespan of superstars is two million years. As a result, they vanished from the observable clusters very early on. Only indirect traces remain,” explains Mark Gieles, an ICREA professor at the University of Barcelona and a co-author of the study.

Revealed through light

With the aid of the James Webb Telescope’s exceptionally powerful infrared capabilities, the researchers were able to support their hypothesis. The satellite captured the light emitted by GN-z11, one of the most distant and youngest galaxies known in our universe, located approximately 13.3 billion light-years away. This galaxy is merely tens of millions of years old. In astronomy, analyzing the light spectrum of celestial objects is a crucial method for determining their properties. In this case, the light emitted by GN-z11 offered two valuable pieces of information.

“We have determined that this galaxy exhibits exceptionally high levels of nitrogen and a remarkably dense population of stars,” says Daniel Schaerer, an associate professor in the Department of Astronomy at the Faculty of Science, UNIGE, and a co-author of the study.

These observations suggest that multiple globular clusters are forming within this galaxy and that they still host an active supermassive star. “The significant presence of nitrogen can only be explained by the hydrogen combustion occurring at extremely high temperatures, a process achievable only within the core of supermassive stars, as demonstrated by the models developed by Laura Ramirez-Galeano, a Master’s student on our team,” explains Corinne Charbonnel.

These fresh results provide further support for the international team’s model, which is currently the sole explanation capable of accounting for the abundance anomalies observed in globular clusters. The next step for the scientists will involve testing the validity of this model on other globular clusters forming in distant galaxies, utilizing data from the James Webb Telescope.

Reference: “N-enhancement in GN-z11: First evidence for supermassive stars nucleosynthesis in proto-globular clusters-like conditions at high redshift?” by C. Charbonnel, D. Schaerer, N. Prantzos, L. Ramírez-Galeano, T. Fragos, A. Kuruvanthodi, R. Marques-Chaves, and M. Gieles, 5 May 2023, Astronomy & Astrophysics.
DOI: 10.1051/0004-6361/202346410

Frequently Asked Questions (FAQs) about cosmic mysteries

What are globular clusters?

Globular clusters are massive and ancient groupings of stars in the universe, housing up to a million individual stars. They are densely packed spherical clusters found in various types of galaxies, including our own Milky Way.

What are the peculiarities observed in globular clusters?

Globular clusters exhibit anomalies in the chemical makeup of their stars. Despite being born from the same gas cloud, the proportions of elements like oxygen, nitrogen, sodium, and aluminum vary from one star to another, presenting what astrophysicists call “abundance anomalies.”

What is the significance of supermassive stars in globular clusters?

Scientists have found evidence suggesting that supermassive stars played a role in the formation of globular clusters. These massive stars are believed to have enriched the gas cloud from which the clusters formed, contributing to the unique chemical compositions observed in their stars.

How were supermassive stars detected in globular clusters?

Directly observing supermassive stars in present-day globular clusters is challenging since their lifespan is relatively short compared to the age of these clusters. Instead, scientists rely on indirect traces and evidence, such as analyzing the chemical composition of stars in proto-globular clusters and studying light emitted by distant galaxies.

What does the presence of nitrogen in a distant galaxy suggest?

Observations of a distant and young galaxy, GN-z11, with high levels of nitrogen and dense star populations indicate the potential formation of globular clusters within the galaxy. The presence of nitrogen is attributed to the combustion of hydrogen at extremely high temperatures, which is characteristic of supermassive stars.

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