Evidence of a first-generation star that died in a “super-supernova” explosion is discovered by Gemini’s observation of a far-away quasar.
The ancient chemical remains of the first stars to light the universe may have been found by astronomers. The researchers discovered an unusual ratio of elements that, in their opinion, could only come from the debris produced by the all-consuming explosion of a 300 solar-mass first-generation star using an innovative analysis of a distant quasar observed by the 8.1-meter Gemini North telescope on Hawai’i, operated by the National Science Foundation’s NOIRLab.
The earliest stars most likely formed when the Universe was barely 100 million years old, or less than one percent of its present age. These early stars, known as Population III, were so colossally huge that when they died as supernovae, they tore themselves apart, dispersing a unique mixture of heavy elements across interstellar space. However, despite astronomers’ careful investigation over many years, there hasn’t been any conclusive proof of these ancient stars until now.
Astronomers now believe they have discovered the remnants of the explosion of a first-generation star after studying one of the most distant known quasars using the Gemini North telescope, one of the two identical telescopes that make up the International Gemini Observatory. They discovered a very unusual composition by using an innovative method to determine the chemical elements included in the clouds around the quasar – the material contained nearly 10 times more iron than magnesium compared to the ratio of these elements seen in our Sun.
The scientists believe that 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 witnessed, but are theorized to be the end of life for gigantic stars with masses between 150 and 250 times that of the Sun.
Pair-instability supernova explosions happen when photons in the center of a star spontaneously turn into electrons and positrons — the positively charged antimatter counterpart to the electron. This conversion reduces the radiation pressure inside the star, allowing gravity to overcome it and leading to the collapse and subsequent explosion.
Unlike other supernovae, these dramatic events leave no stellar remnants, such as a neutron star or a black hole, and instead eject all their material into their surroundings. There are only two ways to find evidence of them. The first is to catch a pair-instability supernova as it happens, which is a highly unlikely happenstance. The other way is to identify their chemical signature from the material they eject into interstellar space.
Astronomers may have discovered the ancient chemical remains of the first stars to light up the Universe. Using an innovative analysis of a distant quasar observed by the 8.1-meter Gemini North telescope on Hawai‘i, operated by NSF’s NOIRLab, the scientists found an unusual ratio of elements that, they argue, could only come from the debris produced by the all-consuming explosion of a 300-solar-mass first-generation star. Credit: Images and Videos: PROGRAM/NOIRLab/NSF/AURA, S. Brunier/Digitized Sky Survey 2, E. Slawik, J. Pollard Image Processing: T.A. Rector (University of Alaska Anchorage/NSF’s NOIRLab), M. Zamani (NSF’s NOIRLab) & D. de Martin (NSF’s NOIRLab) Music: Stellardrone – Airglow
For their research, the astronomers studied results from a prior observation taken by the 8.1-meter Gemini North telescope using the Gemini Near-Infrared Spectrograph (GNIRS). A spectrograph splits the light emitted by celestial objects into its constituent wavelengths, which carry information about which elements the objects contain. Gemini is one of the few telescopes of its size with suitable equipment to perform such observations.
Deducing the quantities of each element present, however, is a tricky endeavor because 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 of the University of Tokyo, have tackled this problem by developing a method of using the intensity of wavelengths in a quasar spectrum to estimate the abundance of the 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 supernova candidate for this would be a pair-instability supernova of a Population III star, in which the entire star explodes without leaving any remnant behind,” said 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.”
Searches for chemical evidence for a previous generation of high-mass Population III stars have been carried out before among the stars in the halo of the Milky Way and at least one tentative identification was presented in 2014. Yoshii and his colleagues, however, think the new result provides the clearest signature of a pair-instability supernova based on the extremely low magnesium-to-iron abundance ratio presented in this quasar.
If this is indeed evidence of one of the first stars and of the remains of a pair-instability supernova, this discovery will help to fill in our picture of how the matter in the Universe came to evolve into what it is today, including us. To test this interpretation more thoroughly, many more observations are required to see if other objects have similar characteristics.
But we may be able to find the chemical signatures closer to home, too. Although high-mass Population III stars would all have died out long ago, the chemical fingerprints they leave behind in their ejected material can last much longer and may still linger on today. This means that astronomers might be able to find the signatures of pair-instability supernova explosions of long-gone stars still imprinted on objects in our local Universe.
“We now know what to look for; we have a pathway,” said co-author Timothy Beers, an astronomer at the University of Notre Dame. “If this happened locally in the very early Universe, which it should have done, then we would expect to find evidence for it.”
Reference: “Potential Signature of Population III Pair-instability Supernova Ejecta in the BLR Gas of the Most Distant Quasar at z = 7.54*” by Yuzuru Yoshii, Hiroaki Sameshima, Takuji Tsujimoto, Toshikazu Shigeyama, Timothy C. Beers and Bruce A. Peterson, 28 September 2022, The Astrophysical Journal.
The study was funded by the National Science Foundation.