Astronomers may have discovered the ancient chemical remains of the first stars that lit up the Universe. Using groundbreaking analysis of a distant quasar observed by the 8.1-meter Gemini North telescope in Hawaii, operated by NSF’s NOIRLab, scientists found an unusual proportion of elements that they argue could only come from debris produced by all the -devouring explosion of a first-generation star of 300 solar masses.
The 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 titanically 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, there has been no direct evidence of these primordial stars, until now.
By analyzing one of the most distant known quasars [1] 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 remnant material from the explosion of a first-generation star. Using an innovative method to deduce the chemical elements contained in the clouds surrounding the quasar, they observed a very unusual composition: the material contained more than 10 times more iron than magnesium compared to the proportion of these elements found in our Sun .
Scientists believe the most likely explanation for this surprising 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 giant 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 convert into electrons and positrons, the positively charged antimatter counterpart of the electron. This conversion reduces the radiation pressure inside the star, allowing gravity to overcome it, leading to collapse and subsequent explosion.
Unlike other supernovae, these dramatic events leave no stellar remnants, such as a neutron star or 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 highly unlikely. 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 a previous observation made by the 8.1-meter Gemini North telescope using the Gemini Near-Infrared Spectrograph (GNIRS). A spectrograph divides the light emitted by celestial objects into their constituent wavelengths, which carry information about what elements the objects contain. Gemini is one of the few telescopes of its size equipped to make these observations.
Deducing the amounts of each element present, however, is a complicated endeavor 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, have tackled this problem by developing a method to use the intensity of wavelengths in a quasar spectrum to estimate the abundance of the elements present in it . It was by using this method to analyze the spectrum of the quasar that they and their colleagues discovered the remarkably low magnesium/iron ratio.
“It was obvious to me that the candidate supernova for this would be a pair-instability supernova from a population III star, in which the entire star explodes without leaving any remnants,” Yoshii said. “I was delighted and somewhat surprised to find that a pair-instability supernova from a star roughly 300 times the mass of the Sun provides a magnesium-to-iron ratio that matches the low value we derived for the quasar “.
Searches for chemical evidence of an earlier generation of high-mass population III stars have been made before among stars in the Milky Way’s halo, and at least one tentative identification was presented in 2014. Yoshii and colleagues, however, think that the 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 evidence of one of the first stars and the remnants of a pair instability supernova, this discovery will help fill in our picture of how the 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 high-mass Population III stars would have disappeared long ago, the chemical fingerprints they leave in their ejected material can last much longer and may still persist today. This means that astronomers could find the signatures of instability supernova explosions from pairs of vanished stars still imprinted on objects in our local Universe.
“Now we 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 it should have, then we would expect to find evidence for it.”
notes
[1] Light from this quasar has traveled for 13.1 billion years, meaning astronomers are observing this object as it appeared when the Universe was only 700 million years old. This corresponds to a redshift of 7.54.