ESO’s VLT detects chemical traces of first stellar explosions in distant gas clouds

In a groundbreaking discovery, scientists using ESO’s Very Large Telescope (VLT) have detected the chemical remnants of the first stars to have exploded in the universe. This marks the first time that researchers have been able to identify the fingerprints of these ancient stars in the composition of distant gas clouds, bringing us closer to understanding the nature of the very first stars that formed after the Big Bang.

According to Andrea Saccardi, a Ph.D. student at the Observatoire de Paris–PSL who led this study during his master’s thesis at the University of Florence, “We were able to identify, for the first time, the chemical traces of the explosions of the first stars in very distant gas clouds.”

The first stars that emerged in the universe over 13.5 billion years ago are believed to have been vastly different from those we observe today. These early stars were composed solely of hydrogen and helium, the simplest elements in the periodic table, and were likely tens or hundreds of times more massive than our sun. After their brief but powerful lives, they exploded in a supernova, scattering heavier elements into the surrounding gas for the first time. Later generations of stars were formed from this enriched gas and, in turn, expelled heavier elements as they too perished.

Although the first stars in the universe have long since vanished, researchers can still glean information about them indirectly. As Stefania Salvadori, co-author of the study and Associate Professor at the University of Florence, explains, “Primordial stars can be studied indirectly by detecting the chemical elements they dispersed in their environment after their death.”

Using data collected by ESO’s VLT in Chile, the team of researchers located three gas clouds situated in the distant cosmos, dating back to when the universe was only 10% to 15% of its current age. Through their chemical composition, these clouds left behind a chemical signature that corresponds to what is expected from the explosions of the first stars. When the earliest stars died, they dispersed heavier elements such as oxygen, carbon, and magnesium, which are present in their outer layers and can be detected in the surrounding gas.

However, some of these explosions did not release enough energy to disperse heavier elements like iron, which is solely located in the cores of stars. Therefore, to identify low-energy supernovae that mark the explosions of the first stars, the team searched for gas clouds in the early universe that were deficient in iron but abundant in other elements such as carbon. Ultimately, they identified three such clouds, providing crucial evidence of the explosions of the very first stars in the universe.

The unusual chemical composition detected in the gas clouds studied by the researchers has also been observed in old stars in our galaxy, believed to be second-generation stars that formed directly from the remains of the first ones. The discovery of these “ashes” in the early universe by the team fills in an essential piece of the puzzle, as Associate Professor Stefania Salvadori explains, “Our discovery opens new avenues to indirectly study the nature of the first stars, fully complementing studies of stars in our galaxy.”

To identify and investigate these distant gas clouds, the team used quasars as light beacons. Quasars are intensely bright sources powered by supermassive black holes at the centers of distant galaxies. As the light from quasars travels through the universe, it passes through gas clouds, which leave a chemical imprint on the light.

The team analyzed data from multiple quasars using the X-shooter instrument on ESO’s VLT. X-shooter divides light into an extensive range of wavelengths, allowing for the detection of numerous chemical elements present in these distant clouds.

This study opens up new avenues for next-generation telescopes and instruments, such as ESO’s upcoming Extremely Large Telescope (ELT) and its high-resolution ArmazoNes high Dispersion Echelle Spectrograph (ANDES). According to Valentina D’Odorico, a co-author of the study and researcher at the National Institute of Astrophysics in Italy, “With ANDES at the ELT, we will be able to study many of these rare gas clouds in greater detail, finally uncovering the mysterious nature of the first stars.”

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