Astronomers see tantalizing evidence for one of the first stars to form in the universe

According to the prevailing cosmological model, the first stars in the universe formed about 100,000 years after the Big Bang. These early stellar masses, known as Population III stars, were very large, short-lived, and did not contain virtually any metals or heavier elements. Over time, elements such as carbon, nitrogen, oxygen and iron were formed in the interior by nucleosynthesis. When these stars reached the end of their lives, they exploded in a supernova many times more than anything we see today (“supernovae”), causing these elements to scatter throughout the universe.

For decades, astronomers have been trying to find evidence of these first stars, but all attempts so far have been unsuccessful. But thanks to a recent study, a team led by the University of Tokyo believes they may have finally discovered the first traces of one of the oldest stars in the universe. While analyzing data previously obtained by the Gemini North Telescope for the farthest quasar ever observed, the team noticed an enormous cloud of material around it. Based on their analysis, they believe the material came from a star of the first generation after it became a “supernova”.

The study that appeared recently in Astrophysical Journal, led by Yozuru Yoshi, Professor of Astronomy at the University of Tokyo and Steward Observatory at the University of Arizona. He was joined by researchers from the National Astronomical Observatory of Japan (NAOJ), Tokyo University’s Research Center for the Early Universe (RESCEU), the JINA Center for the Evolution of the Elements (JINA-CEE) at the University of Notre Dame, and the Australian National University’s Mount Stromlo Observatory.

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Society III stars are believed to have formed about 100,000 years after the Big Bang. Credit: ESA/Hubble, M. Kornmesser.

As they point out in their study, the team believes the most likely explanation for what they observed is that the material is the remains of a first-generation star that exploded as an unstable binary supernova (aka a “supernova”). This happens when photons in the star’s center spontaneously convert into electrons and positrons – the antimatter counterpart of the electron – reducing radiation pressure inside the star, causing it to collapse. Much like how the stars of Group I and II collapse after depleting hydrogen and helium, this process caused the stars of Group III to become a supernova.

Although such an event has never been seen, it is assumed to occur when giant stars (between 150 and 250 solar masses) reach the end of their lives. Unlike other supernovae, a bistable supernova leaves no stellar remnants and expels all the star’s material in its surroundings. In addition, astronomers assumed that this substance contains ten times more iron than magnesium compared to the ratio of these elements in our Sun. Because of their distinct properties, there are only two ways to find evidence of Population III stars.

First, astronomers can try to notice supernova pair instability as it happens, and the odds of it happening are very slim. Second, they can try to detect the material these stars are ejecting into interstellar space by determining its chemical signature. In this case, Yozuru and colleagues relied on the latter method, which consists of consulting previous observations made with the Gemini Near Infrared Spectrometer (GNIRS) on the 8.1-meter Gemini North Telescope.

This telescope is one of two (located in the northern and southern hemispheres) that make up the Gemini International Observatory, which is operated by the National Optical and Infrared Astronomy Research Laboratory (NOIRLab). To determine the quantities of each element present, the team used an analytical method developed by Yozuru and co-author Hiroaki Samshima, a research associate on the project at the University of Tokyo Graduate School. This method involves measuring the intensity of wavelengths in the quasar spectrum, from which the chemical spectra of the substance are extracted.

Gemini North Observatory, located above Maunakea, Hawaii. Credit: Gemini Observatory / Ora

From their analysis, Yozuru and colleagues note that the ejected material contains more than ten times more iron than magnesium compared to the proportion of these elements found in our sun. As Yuzuru explained in a NOIRLab press release:

“It was clear to me that the supernova candidate for this would be an unstable binary supernova of Population III star, in which the entire star explodes without leaving any remnants behind.. I was somewhat surprised to discover that a supernova of a star about 300 times the mass of the Sun provided a ratio of magnesium to iron consistent with the low value we derived for the quasar.

Similar searches have been conducted in the past, as astronomers have searched for chemical clues to the stars of the third clusters in the Milky Way. And while one initial identification was made in 2014, Yozuru and his colleague believe these new findings are the clearest indication of an unstable binary supernova to date. If their findings are confirmed, they will provide new insight into how our universe has evolved since the first stars and galaxies formed. In the meantime, further observations are necessary to see if there are other things with similar properties.

Evidence for these stars can also be found within the Milky Way, where projectiles of primordial stars can be found among the objects in our local universe. With this latest study, astronomers now have a potential path to identifying the chemical signatures of stars that played a vital role in the evolution of the universe, and led to the first planets, and even life itself.

Further reading: NOIR LabAnd the Astrophysical Journal

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