A metal-free star has been identified: a cosmic relic that dates back to the early stages of the history of the universe. How and when this ancient gem was formed.
Not all stars are the same. Although they may appear similar at first glance, their chemical composition reveals very different stories. Astronomers believe that the first stars born after the Big Bang were composed almost exclusively of hydrogen and helium, with traces of lithium: the only elements present in the early universe.
Heavier elements — such as iron, carbon and oxygen — were formed only later, in the hearts of stars, through nuclear fusion processes. When these ancient stars exhausted their fuel and exploded as supernovas, they scattered the elements they had created into space, thus enriching the interstellar gas from which new stars formed. Each subsequent generation was therefore born a little “richer” in heavy elements, called “metals” in astronomical jargon.
Pristine stars. Today, almost all stars still contain mainly hydrogen and helium, but with an increasing proportion of heavier elements, visible in spectroscopic analyzes of their light. However, there are very rare cases of “pristine” stars, almost completely devoid of metals: true cosmic relics that date back to the early stages of the history of the universe.
A primordial star. A team of astronomers led by Alexander Ji of the University of Chicago has identified what may be the most pristine star ever observed: SDSS J0715−7334. The research results were published on the arXiv preprint server. SDSS J0715−7334 is a red giant with an extraordinarily primitive composition. The team’s spectroscopic and chemical analysis reveals that the star has a total metallicity (Z) of less than 7.8 × 10⁻⁷, the lowest value ever recorded.
For comparison, the previous record holder — a star in the Milky Way — had a metallicity of about 1.4 × 10⁻⁶. “SDSS J0715−7334 is approximately twice as metal-poor as the star J1029+1729 (Z < 1.4×10⁻⁶) and more than ten times poorer than the star with the lowest known iron content, SMSS J0313−6708,” explain the authors of the study. But it's not just the iron that's missing. This star also shows an unusually low amount of carbon: an even rarer anomaly, because even the stars with very little iron known so far still showed a fair amount of carbon.
A fragment of the first stars. According to scholars, the particular composition of SDSS J0715−7334 suggests that it was formed from gas produced by the explosion of a Population III star — the very first stars born after the Big Bang — with a mass about 30 times that of the Sun.
«The fact that the stars are extremely poor in metals can be traced back to the properties of Population III stars, thanks to models of supernova nucleosynthesis», explain the researchers, «J0715−7334 is an exceptionally pure probe of that primordial generation, since its orbit, located in the distant galactic halo, protects it from any contamination by the interstellar medium. Furthermore, its large convective envelope eliminates any internal sedimentation effects.”
From the Large Magellanic Cloud. Through kinematic analysis and data provided by the Gaia satellite, the team traced the origin of SDSS J0715−7334, discovering that the star comes from the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way. Only later would it migrate to our galaxy, bringing with it the testimony of a very distant era.
