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Not all stars are created equally. Astronomers believe that the first stars to form after the Big Bang were mostly made of only hydrogen and helium with trace amounts of lithium, as the heavier elements formed later on by nuclear fusion inside the stars. When these stars went supernova, heavier elements spread throughout space and formed more stars. Each successive generation contained more heavy elements, and these elements also became successively heavier.

While most stars still contain mostly hydrogen and helium, they now contain many heavy elements as well, especially as they get older. These elements show up in spectrographic data when astronomers gather light from these distant stars. Stars are considered "pristine" when the data shows a lack of heavy elements—meaning they are likely very rare, older stars from earlier generations. And now, a group of astronomers, led by Alexander Ji from the University of Chicago, believe they have found the most pristine star on record. The group has their findings on the arXiv preprint server.

The star, referred to as SDSS J0715-7334, is a red giant purported to have the lowest metallicity—or heavy element content—ever found. The team's detailed spectral and chemical analysis shows that SDSS J0715-7334 has a total metallicity "Z" of less than 7.8 x 10^-7. This is compared to the next lowest metallicity star currently known, a star located in the Milky Way with a total metallicity of around 1.4 × 10^-6.

"This is about two times more metal-poor than the previous record holder, J1029+1729 (Z < 1.4×10^-6). It is over ten times more metal-poor than the most iron-poor star known, SMSS J0313-6708," the study authors say.

But it's not just iron that this star is deficient in. SDSS J0715-7334 also contains a surprisingly low amount of carbon. Even the other stars documented to have very low iron content still contained a fair amount of carbon, making this newly found star even rarer.

The group says SDSS J0715-7334's chemical pattern suggests it formed from gas that originated from a 30 solar mass Population III star's supernova—"Population III" stars being the first stars to form after the Big Bang.

"The detailed chemical abundances of the most metal-poor stars can be linked back to the properties of metal-free Population III stars through supernova nucleosynthesis models. J0715−7334 is an especially clean probe of Population III, as its distant halo orbit completely precludes significant surface contamination from the interstellar medium and its large convective envelope removes any diffusive settling effects," the study authors explain.

The researchers used kinematic analysis to trace the star's origin to the Large Magellanic Cloud (LMC) using Gaia data and orbital modeling. The analysis indicates that it was originally a part of the LMC and later migrated into the Milky Way.

While this discovery offers a glimpse into the universe's earliest stars and the origins of heavier elements, the data on J0715−7334 also provides some answers about how stars cool. The team notes that J0715−7334 is now the second star below something called the "fine structure cooling threshold," which describes how some gas clouds cool faster with the help of heavier elements releasing more energy. The team says this work indicates that cooling with the help of cosmic dust—or dust cooling—is required to cool gas clouds enough to form low metallicity stars at this threshold, and also occurs in galaxies beyond the Milky Way.

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