These stars, referred to as Population III Stars, are thought to have been massive stars, between 150 and 250 times the sun, that formed within the first 100 million years of the universe.
There is a good chance that the very first stars formed within the first 100 million years of the universe, which is less than a percent of the current age. These first stars, known as Population III, were so massive that when these stars blew up as supernovae, they left a distinctive blend of heavy elements in interstellar space. Although astronomers have diligently searched for these primordial stars for decades, there has not yet been any direct evidence of their existence.
As part of the International Gemini Observatory, one of the two identical telescopes operated by the National Science Foundation’s NOIRLab, astronomers have identified the remnants of a first-generation star based on their analysis of one of the most distant known quasars. Detecting the chemical composition of the clouds surrounding the quasar with an innovative method revealed an unusual composition — the material contained over ten times as much iron as magnesium as our sun.
This striking feature is believed to be the result of a pair-instability supernova that exploded as a first-generation star. There have never been any visible instances of these remarkably powerful supernova explosions. Still, they are theorized to be the death throes of massive stars that weigh between 150 and 250 times the sun.
Supernova explosions caused by pair-instability occur when photons transform spontaneously into electrons and positrons — the electron’s positively charged antimatter counterparts. Radiation pressure inside the star decreases due to this conversion, allowing gravity to prevail and the star to collapse and explode.
This dramatic event does not leave any stellar remnants, like a neutron star or black hole, but instead ejects all its material into the surrounding space. Therefore, it is only possible to find evidence of them in two ways. Firstly, you might be able to catch a pair-instability supernova as it happens, but this is highly unlikely. The material they eject into interstellar space can also be examined for its chemical signature.
A previous observation taken by Gemini North’s 8.1-meter telescope, Gemini Near-Infrared Spectrograph (GNIRS), was analyzed for the study, now published in The Astrophysical Journal. By splitting light emitted by celestial objects into its constituent wavelengths, spectrographs reveal what elements are present in the objects. Among telescopes with suitable equipment for such observations, Gemini is among the most powerful.
A spectrum’s brightness is dependent on other factors besides the abundance of each element, so it can be difficult to determine the amount of each element present.
Yuzuru Yoshii and Hiroaki Sameshima, two co-authors of the analysis, developed a method to estimate the abundance of elements in quasar spectrums based on wavelength intensities. The magnesium-to-iron ratio of the quasar was discovered by using this method to analyze its spectrum.
Rare ancient stars
I instantly recognized that a pair-instability supernova from a Population III star is a candidate since the entire star explodes without leaving any remnants, explained Yoshii. “I was delighted and somewhat surprised to find that a pair-instability supernova of a star with a mass about 300 times that of the sun provides a ratio of magnesium to iron that agrees with the low value we derived for the quasar.”
Tentative identification of a high-mass Population III star was made among the stars in the Milky Way’s halo in 2014. As a result of the extremely low magnesium-to-iron abundance ratio in this quasar, Yoshii and his colleagues believe the new result is the strongest indication that this supernova was a pair-instability supernova.
This discovery may aid in filling in the picture of how matter in the universe evolved into what it is today, including us, if it is indeed evidence of one of the first stars and the remains of a pair-instability supernova. However, for a thorough test of this interpretation, more observations are needed to verify that other objects share the same characteristics.
The chemical signatures might also be found closer to home. For example, the chemical fingerprints left by high-mass Population III stars in their ejected material may still linger even today, even though all of them would have been destroyed long ago. As a result, astronomers may discover signatures of long-gone supernova explosions in our local universe that were caused by pair-instability supernova explosions.
As co-author Timothy Beers of the University of Notre Dame explained, “Now we know what to look for. “If this happened locally in the very early universe, which it should have done, then we would expect to find evidence for it.”
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