The first potential traces of one of the first stars in the universe

A pocket of gas surrounding a distant quasar may contain the remains of a Population III star, one of the first stars in the universe. Until now, these objects had remained elusive. Details of the study are published in The Astrophysical Journal.

The first stars in the universe

There are three populations of stars. The latter were named in the order in which they were observed, so stars in Population III are counterintuitively the oldest. In other words, they are the first to form after the Big Bang.

Also note that according to Big Bang cosmology, nucleosynthesis does not produce heavy elements due to the rapid decrease in density and temperature as the universe expands. Thus, we know that the heavy elements observed in various objects were synthesized later, inside massive stars, and then dispersed by supernovae.

Thus we know that the very first generation of stars, that of Population III, did not consist of heavy elements, but only of hydrogen and helium, the original elements of the universe. These stars then forged heavy elements that later formed Population II and then Population I stars.

Astronomers estimate that these very first stars (which must have been gigantic) probably formed when the universe was only a hundred million years old. But despite decades of research, we still had no direct evidence of their presence in the early universe. It’s done now.

Astronomers Yuzuru Yoshii and Hiroaki Sameshima from the University of Tokyo made the discovery by analyzing 13.1 billion-year-old light from gas around one of the most distant known quasars thanks to the Gemini North Telescope in Hawaii.

Artist’s impression of the distant quasar. Credit: NOIRLab/NSF/AURA/J. da Silva/space engine

Pair-unstable supernova

Using an innovative method that allows the deduction of the chemical elements contained in the clouds surrounding this quasar, the researchers actually noted that this material contained more than ten times more iron than magnesium compared to the ratio of these elements found in the Sun. It was a very unusual composition. How to explain it?

For the two astronomers, this material would actually be the remains of a first-generation star that exploded in pair-unstable supernova.

These explosions occur when photons at the center of a star are spontaneously converted into electrons and positrons, the electron’s positively charged antimatter counterpart. This process reduces the radiation pressure inside the star, ultimately leading to its collapse and the resulting explosion.

Unlike other supernovae, these events would leave no stellar remnants, such as a neutron star or black hole. Otherwise, all their matter would be thrown into their environment.

Note that these supernova versions have never been observed. However, they are theorized as the end of life for giant stars between 150 and 250 times more massive than the Sun. To identify their tracks, two solutions are possible: either astronomers detect one when it occurs, which is highly unlikely, or they identify their chemical signature from the material ejected into interstellar space. So that’s what happened here.

Astronomers hope to one day be able to identify the chemical signatures of one of these stars closer to home, in our galaxy. Although these Population III objects are all long extinct, their chemical fingerprints may still linger in their ejected material.

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