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China-led team finds first evidence of oldest stars in the universe

Tan KW
Publish date: Sat, 10 Jun 2023, 06:33 PM
Tan KW
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A China-led study has yielded the first observational evidence of the lives - and deaths - of the oldest stars in our universe.
Scientists from China, Japan and Australia found the stars’ unique chemical footprints in the halo of the Milky Way galaxy, using the combined power of two of the world’s largest land-based telescopes.
Their findings - from the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) near Beijing and the Subaru Telescope in Hawaii - showed that so-called first-generation stars - which lit up the universe some 100-250 million years after the Big Bang - could be as colossal as 260 times the sun’s mass. 
Their study, published in the journal Nature on Wednesday, also provided the first observation-based proof that the stars ended their lives in an unusual explosion, quite different from the supernovas we know today, and only predicted in theory until now.
Harvard University theoretical physicist Avi Loeb, who was not involved in the research, hailed the discovery as “extremely important in confirming our theoretical ideas about the first generation of stars”.
Loeb said first-generation stars are among the universe’s biggest unsolved mysteries. Scientists predicted that they formed from pristine gas after the Big Bang and were made of just hydrogen and helium, without the heavier elements that were synthesised in the cores of later giant stars.
Astronomy theory also suggested that these ancient bodies may have had masses equivalent to hundreds of suns and went through a unique partial explosion when they died.
The first-generation stars were short-lived and very hard to detect, leaving only their chemical signatures in the next generation of stars.
For their groundbreaking study, Zhao Gang from the National Astronomical Observatories of China and his colleagues sifted through the spectra of more than five million stars gathered by LAMOST.
The information included chemical composition, temperature, brightness and other key properties which the scientists compared until they found a candidate, dubbed LAMOST J1010+2358.
At about half the sun’s mass, and some 3,300 light years away in the galactic halo, the star was extremely low in metals such as sodium and cobalt.
The team compared the star’s spectrum with theoretical models and concluded that it was most likely to have formed in a gas cloud dominated by the remnants of a first-generation star with a mass equivalent to 260 suns.
Loeb explained that, unlike stellar explosions in the later universe - which occur as the star uses up its fuel and collapses into a neutron star or black hole - the explosion of LAMOST J1010+2358’s parent star involved the generation of electrons and their antimatter counterpart positrons.
The electron-positron pairs reduced the internal radiation pressure needed to support such a massive star against its own gravity. This led to a partial collapse, triggering a thermonuclear explosion that left nothing but a cloud of gas.
Astronomers had also predicted that this process would result in a big difference in the abundance of ions with odd and even charge numbers - like sodium/magnesium and cobalt/nickel, Loeb said.
And this was exactly what the researchers observed with LAMOST and Subaru - “a pronounced odd-even effect” as the paper’s co-author Xing Qianfan put it.
In a press release on Wednesday, Zhao said there was no direct evidence of supernova explosions of such massive, metal-poor stars before this study.
“Hopefully, our work will have a positive impact on the study of the origin of elements, star formation in the early universe, and the chemical evolution of galaxies,” he said.
Zhao said the team intends to continue the search for stars with unusual chemical compositions, to help better understand how first-generation stars were formed, both within and outside the Milky Way.
They plan to combine LAMOST’s power with the Xuntian space telescope, a Hubble-class device that could launch later next year.
The Chinese co-orbiting optical space telescope module will have a field of view more than 300 times greater than the Hubble. Its wide field of view will allow Xuntian to map up to 40 per cent of the sky over 10 years, using a huge 2.5 billion pixel camera.
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