First potential traces of the first stars in the universe

Astronomers may have discovered the ancient chemical remnants of the first stars to light up the Universe.

Using innovative analysis of a distant quasar observed by the 8.1-meter Gemini North Telescope in Hawai’i, operated by NSF’s NOIRLab, scientists have found an unusual ratio of elements that, according to them, could only have come from the debris produced by the all-consuming explosion of a 300 solar mass first-generation star.

The very first stars probably formed when the Universe was only 100 million years old, less than one percent of its current age. These early stars – known as Population III – were so massive that when they ended their lives as supernovae, they tore apart, seeding interstellar space with a distinctive mix of heavy elements. Despite decades of diligent research by astronomers, however, there has been no direct evidence for these primordial stars, until now.

By analyzing one of the most distant known quasars using the Gemini North Telescope, one of two identical telescopes that make up the Gemini International Observatory, operated by NSF’s NOIRLab, astronomers now believe they have identified the residual material from the explosion of a first- generation star. Using an innovative method to infer the chemical elements contained in the clouds surrounding the quasar, they noticed a very unusual composition – the material contained more than 10 times more iron than magnesium compared to the ratio of these elements found 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 theorized as the end of life of gigantic stars with masses between 150 and 250 times that of the Sun.

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

Unlike other supernovae, these dramatic events leave no stellar remnants, such as a neutron star or black hole, and instead eject all of their material into their surroundings. There are only two ways to find proof. The first is to catch a pair instability supernova as it happens, which is a highly unlikely coincidence. The other way is to identify their chemical signature from the material they eject into interstellar space.

For their research, the astronomers studied the results of an earlier observation taken by the Gemini North 8.1-meter telescope using the Gemini Near-Infrared Spectrograph (GNIRS). A spectrograph splits the light emitted by celestial objects into its constituent wavelengths, which contain information about the elements the objects contain. Gemini is one of the few telescopes of its size with the right equipment to perform such observations.

Deducing the amounts of each element present, however, is a tricky business because the brightness of a line in a spectrum depends on many other factors besides the abundance of the element.

Two co-authors of the analysis, Yuzuru Yoshii and Hiroaki Sameshima of the University of Tokyo, tackled this problem by developing a method using the intensity of wavelengths in a quasar spectrum to estimate the abundance elements present there. It was by using this method to analyze the quasar’s spectrum that they and their colleagues discovered the conspicuously low magnesium to iron ratio.

“It was obvious to me that the candidate supernova for this would be a pair-instability supernova of a Population III star, in which the entire star explodes without leaving a remnant,” Yoshii said. “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 magnesium to iron ratio that matches the low value we have derived for the quasar.”

Searches for chemical evidence for a previous generation of high-mass Population III stars have already been conducted among Milky Way halo stars and at least one attempt at identification was presented in 2014. Yoshii and colleagues , however, believe that the new result provides the clearest signature of a pair-instability supernova based on the extremely low magnesium to iron abundance ratio exhibited in this quasar.

If this is indeed evidence of one of the first stars and 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 further, many more observations are needed to see if other objects have similar characteristics.

But we may also be able to find the chemical signatures closer to home. Although the high-mass Population III stars all died out long ago, the chemical fingerprints they leave in their ejected material may last much longer and may still persist today. This means that astronomers might be able to find the signatures of instability supernova explosions of long-extinct star pairs 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 it happened locally at the very beginning of the Universe, which it should have, we would expect to find evidence of it.”

Association of Universities for Research in Astronomy (AURA)

Image credit: NOIRLab/NSF/AURA/J. da Silva/space engine

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