Evrim Yazgin
Cosmos science journalist
The chemical remains of the earliest stars in the universe may have been discovered by astronomers.
Using an innovative technique at the Gemini North telescope on Hawaii, researchers found an unusual ratio of elements, which they say , could only have come from debris produced by the explosive death of a first-generation star 300 times more massive than our sun.
The universe is now estimated to be 13.7 billion years old, and contains an estimated 1 septillion (1 with 24 zeroes after it, or 1 million billion billion) stars. But scientists believe the very first stars likely formed when the universe was only 100 million years old.
These first-generation stars are known as “Population III” stars. They are believed by astronomers to have kickstarted the production of chemical elements heavier than hydrogen, which are needed for planet formation and, of course, life.
Population III stars would have been so massive that the supernovae that saw the tumultuous ends of their lives would have seeded vast expanses of space with heavy elements. But direct evidence of these primordial stellar behemoths has evaded astronomers, until now.
Astronomers who believe they have come across the remnants of a first-generation star published their results in the Astrophysical Journal.
The researchers used the 8.1-metre Gemini North telescope to look at clouds surrounding one of the most distant known quasars and noticed over 10 times more iron than magnesium compared to the ratio of these elements in our sun. The quasar itself is 13.1 billion lightyears away, so what the scientists see through the telescope is the quasar as it was only 700 million years after the Big Bang.
The scientists believe the most likely explanation for this chemical composition is that the material was left behind by a first-generation star that exploded as a ‘pair-instability supernova.’ Though never witnessed, these versions of a supernova are theorised to mark the death of gigantic stars of between 150 and 250 solar masses (one solar mass is the mass of our sun)
By spectrographically analysing the different wavelengths of infrared light coming from the quasar, the team was able to deduce the elements inside the cloud. But, since the brightness of lines on a spectrograph which correspond to different elements depends on several factors, determining the amount of each element is a bit tricky.
The researchers tackled this problem by analysing the intensity of wavelengths in the spectrum to estimate the abundance of elements in the cloud around the quasar.
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“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,” says first author Yuzuru Yoshii from the University of Tokyo. “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.”
An earlier tentative identification of a first-generation star was published in 2014 out of research analysing the halo around our own Milky Way galaxy. But Yoshii believes their new result showing extremely low magnesium-to-iron abundance is the clearest indication of a pair-instability supernova.
If they are correct and they have found the remnants of one of the universe’s first stars, the team’s results help develop our understanding of how the matter in the universe came to evolve into what it is today.
The chemical fingerprints of these first stars can even be found closer to home.
“We now know what to look for; we have a pathway,” says co-author Timothy Beers, an astronomer at the University of Notre Dame in the US. “If this happened locally in the very early Universe, which it should have done, then we would expect to find evidence for it.”