Astronomers May Have Spotted Traces of Material from Explosion of First-Generation Star

by johnsmith

The very first stars likely formed when the Universe was only 100 million years old. Known as Population III stars, these stellar objects were so massive that when they exploded as supernovae they tore themselves apart, seeding interstellar space with a distinctive blend of heavy elements. By analyzing ULAS J1342+0928, one of the most distant known quasars, astronomers have now identified the remnant material of the explosion of a first-generation star. They’ve noticed a highly unusual composition — the material contained over 10 times more iron than magnesium compared to the ratio of these elements found in our Sun. They 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.

This artist’s impression shows a field of Population III stars as they would have appeared a mere 100 million years after the Big Bang. Image credit: NOIRLab / NSF / AURA / J. da Silva / Spaceengine.

This artist’s impression shows a field of Population III stars as they would have appeared a mere 100 million years after the Big Bang. Image credit: NOIRLab / NSF / AURA / J. da Silva / Spaceengine.

“According to the Big Bang cosmology, nucleosynthesis does not produce heavy elements because of the rapid decrease in density and temperature as the Universe expands,” said University of Tokyo astronomer Yuzuru Yoshii and colleagues.

“This has led to an immediate interpretation that the heavy elements observed in various objects in the universe are synthesized in the interior of massive stars and ejected by supernovae.”

“Therefore, the first generation of stellar objects called Population III should be massive stars born from the gas of pristine composition consisting almost exclusively of hydrogen and helium.”

“If the initial mass function of the hypothetical Population III stars extended to masses as low as 1 solar mass, their lifetimes would be as long as the age of the Galaxy, and they would survive to be observed at the present day.”

“Contrary to expectation, despite the great observational efforts made during the past four decades, no single star without detectable metals has been found anywhere in the Galaxy.”

“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,” the astronomers added.

“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.”

For their research, the authors studied results from a prior observation taken by the 8.1-m 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.

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.

“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,” Dr. 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 ratio of magnesium to iron that agrees with the low value we derived for the quasar.”

The team’s results provide the clearest signature of a pair-instability supernova based on the extremely low magnesium-to-iron abundance ratio presented in ULAS J1342+0928.

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.

“We now know what to look for; we have a pathway,” said Dr. 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.”

The findings appear today in the Astrophysical Journal.


Y. Yoshii et al. 2022. Potential signature of Population III pair-instability supernova ejecta in the BLR gas of the most distant quasar at z = 7.54. ApJ, in press; doi: 10.3847/1538-4357/ac8163

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