A Star from Another Galaxy: The Most Pristine Relic of the Early Universe
In a groundbreaking discovery, Alexander Ji and collaborators have found the most chemically pristine star ever observed, SDSS J0715−7334, a red giant that formed from nearly untouched material just after the Big Bang. This star, located in the Large Magellanic Cloud (LMC), contains fewer heavy elements than any other known star, providing a direct glimpse into how the first generations of stars shaped the universe.
Searching for the First Stellar Descendants
The study begins by framing the long-standing mystery of Population III stars, the first stars formed from hydrogen and helium. These stars lived short, explosive lives, enriching space with heavier elements, or “metals”, for later generations. Because none of those first stars survive today, astronomers instead look for their descendants: small, faint stars with extraordinarily low metallicities. Ji’s team used data from the SDSS-V Milky Way Mapper and confirmed SDSS J0715−7334’s composition with high-resolution spectroscopy from the Magellan/MIKE telescope, finding a metallicity of Z < 7.8 × 10⁻⁷, more than ten times lower than any known galaxy observed even with the James Webb Space Telescope.
An Extraordinary Chemical Signature
In the analysis, Ji and colleagues measured the star’s chemical fingerprint and found exceptionally low amounts of both iron and carbon, a rare combination. Most metal-poor stars known are unusually rich in carbon, but SDSS J0715−7334 is not. After carefully modeling the star’s atmosphere using three-dimensional simulations, they determined an upper limit on its carbon abundance that reinforces its status as the most metal-poor star known. This lack of metals implies that the gas from which the star formed had been enriched by just one or two supernovae from massive Population III stars, rather than by multiple generations of stellar explosions.
A Star from Another Galaxy
The team also studied SDSS J0715−7334’s motion through space and found that it did not originate in the Milky Way. Instead, it is likely a galactic immigrant from the Large Magellanic Cloud, a satellite galaxy currently orbiting our own. By simulating its orbit over billions of years, they showed that the star’s path aligns closely with that of the LMC, suggesting it was once part of that galaxy’s outer halo before being pulled into the Milky Way’s gravitational influence.
Tracing Its Ancestry to a Massive Supernova
From its chemical composition, Ji’s team inferred that SDSS J0715−7334 formed after a single supernova explosion of a 30-solar-mass Population III star with high energy. This contrasts with the previously known record-holder, J1029+1729, which likely came from a lower-mass, weaker supernova. The comparison between these two stars hints that the first stars may have formed differently depending on their environment, with those in the LMC forming from gas influenced by radiation from the early Milky Way, a process the authors suggest might represent an early form of “Population III.2” star formation.
Connecting Local and Distant Galaxies
In their final discussion, the authors connect this local discovery to observations of distant galaxies. Even the faintest galaxies seen by JWST have metallicities about ten times higher than that of SDSS J0715−7334. This implies that, despite their extreme youth, those galaxies are already enriched with metals from second-generation stars. The discovery of SDSS J0715−7334 therefore provides a nearby example of what a truly second-generation star looks like, and sets a new benchmark for identifying the universe’s first steps toward chemical complexity.
A Fossil from a Pristine Cosmos
Through a combination of precise observation and careful modeling, Ji and colleagues have shown that SDSS J0715−7334 is the closest thing we have to a surviving echo of the first stars, a fossil from a time when the cosmos was almost entirely pristine.
Source: Ji
