Possible first traces of the earliest stars in the universe

Possible first traces of the earliest stars in the universe
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Astronomers may have discovered the ancient chemical remains of the first stars that lit the universe.

Using an innovative analysis of a distant quasar observed by the 8.1-metre Gemini North Telescope in Hawaii, operated by NSF’s NOIRLab, the scientists found an unusual ratio of elements they believe is unique to the Debris produced by the all-consuming explosion of a 300 solar-mass first-generation star.

The very first stars likely formed when the universe was only 100 million years old, less than 1 percent of its current age. These first stars — known as Population III — were so massive that they tore themselves apart as they ended their lives as supernovae, seeding interstellar space with a distinctive mix of heavy elements. However, despite decades of diligent searches by astronomers, until now there has been no direct evidence for these primordial stars.

By analyzing one of the most distant known quasars with the Gemini North Telescope, one of the two identical telescopes that make up the International Gemini Observatory operated by NSF’s NOIRLab, astronomers now believe it may be the remnant of the explosion of a first generation star. Using an innovative method to derive the chemical elements contained in the clouds surrounding the quasar, they noticed a highly unusual composition – the material contained over 10 times more iron than magnesium compared to the ratio of these elements in our Sun.

Scientists believe the most likely explanation for this striking feature is that the material was left behind by a first-generation star that exploded as a pair-instability supernova. These remarkably powerful versions of supernova explosions have never been observed, but are believed to spell the end of life for gigantic stars with masses between 150 and 250 times that of the Sun.

Pair-instability supernova explosions occur when photons spontaneously transform into electrons and positrons—the positively charged antimatter counterpart of the electron—at the center of a star. This conversion reduces the radiation pressure inside the star, allowing gravity to overcome it and leading to its collapse and subsequent explosion.

Unlike other supernovae, these dramatic events do not leave stellar remnants such as neutron stars or black holes in their wake, but eject all of their material into their surroundings. There are only two ways to find evidence of this. The first is to catch a pair instability supernova as it happens, which is a highly unlikely coincidence. The other possibility is to identify their chemical signature from the material they eject into interstellar space.

For their research, the astronomers examined the results of an earlier observation made with the 8.1-metre Gemini North Telescope using the Gemini Near-Infrared Spectrograph (GNIRS). A spectrograph breaks down the light emitted by objects in the sky into its wavelengths, which contain information about what elements the objects contain. Gemini is one of the few telescopes of its size with the proper equipment to make such observations.

However, deriving the amounts of each element present is tricky, since the brightness of a line in a spectrum depends on many other factors besides the element’s abundance.

Two co-authors of the analysis, Yuzuru Yoshii and Hiroaki Sameshima from the University of Tokyo, tackled this problem by developing a method that uses the intensity of wavelengths in a quasar spectrum to estimate the abundances of the elements present there. Using this method to analyze the quasar’s spectrum, she and her colleagues discovered the strikingly low ratio of magnesium to iron.

“It was clear to me that the supernova candidate for this would be a population III star pair-instability supernova, where the entire star explodes without leaving a residue,” Yoshii said. “I was pleased and somewhat surprised to find that a pair-instability supernova from a star about 300 times the mass of our Sun yields a magnesium-to-iron ratio consistent with the low value we derived for the quasar.”

Chemical evidence for an earlier generation of high-mass Population III stars has already been sought among the stars in the Milky Way’s halo, and at least one tentative identification was presented in 2014. However, Yoshii and his colleagues believe that the new result provides the clearest signature of a pair-instability supernova, based on the extremely low magnesium-to-iron ratio present in this quasar.

If this is indeed evidence of one of the first stars and the remnants of a pair-instability supernova, this discovery will help complete our picture of how matter in the Universe evolved into what it is today, including us. To test this interpretation more thoroughly, many more observations are needed to see if other objects have similar properties.

But maybe we can find the chemical signatures closer to home. Although massive Population III stars would all have died out long ago, the chemical fingerprints they leave in their ejected material may be much longer lasting and still present today. This means astronomers may be able to find the signatures of pair-instability supernova explosions from long-ago stars that are still imprinted on objects in our local Universe.

“We now know what to look for; We have a way,” said co-author Timothy Beers, an astronomer at the University of Notre Dame. “If this happened locally in the very early Universe, which should have happened, then we would expect to find evidence of it.”

Association of Universities for Research in Astronomy (AURA)

Credit: NOIRLab/NSF/AURA/J. da Silva / space machine

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