Undergraduate students at the University of Chicago have found one of the oldest known stars in the universe — and they spotted it on the first night of their first observing run.
The star, catalogued as SDSSJ0715-7334, was discovered by ten students in a field course led by Alex Ji, a professor at UChicago and deputy Project Scientist for the Sloan Digital Sky Survey (SDSS-V). Their target emerged from a list of 77 candidates the students had flagged while searching SDSS data for unusually pristine stellar compositions. According to Mirage News, the plan was to spend 10 minutes per star when they reached the Magellan telescopes at Las Campanas Observatory in Chile. They spent three hours on SDSSJ0715-7334.
"We found it the first night, and it completely changed our plans for the course," Ji said.
The observation paid off. SDSSJ0715-7334 has 0.005 percent of the metal content found in the Sun — less than half the previous record holder, making it the lowest metallicity star ever observed, according to the Carnegie Science Center, which operates the Las Campanas Observatory. In astronomy, any element heavier than hydrogen or helium is classified as a metal. A star's metallicity is therefore a proxy for its age: the fewer metals it contains, the closer it is to the primordial composition left by the Big Bang. This star is from the second generation of stars in the cosmos, formed a few billion years after the universe began.
Analysis of the star's trajectory, using data from the European Space Agency's Gaia mission, shows it originated in the Large Magellanic Cloud and migrated into the Milky Way billions of years ago. Ji called it an "ancient immigrant" — a star that changed galactic addresses long before the Solar System existed. The discovery is published in Nature Astronomy, per Carnegie Science Center.
"This ancient immigrant gives us an unprecedented look at conditions in the early universe," Ji said. "Big data projects like SDSS make it possible for students to get directly involved in these important discoveries."
The metallicity finding matters because it sets a new empirical floor for stellar archaeology. Stars with extremely low metal content preserve the chemical fingerprint of the gas clouds from which they formed — gas that was enriched only by the first generation of stars, which died in supernovae before many subsequent stellar generations had a chance to form. SDSSJ0715-7334's carbon content was undetectable using current instruments, suggesting it formed via a formation pathway that has only been observed once before, according to Mirage News.
SDSS has been operating for 25 years, gathering spectroscopic data across the full sky through a collaboration of over 75 institutions. The survey's fifth generation, SDSS-V, deploys robots to acquire millions of spectra and is specifically designed to find rare objects like ancient stars buried in the stellar haystack. Juna Kollmeier, director of SDSS-V and a Carnegie astrophysicist, said the survey's statistical power is what makes discoveries like this possible.
"We have to look in our cosmic backyard to find these objects, because we can't yet observe individual stars at the dawn of star formation," Kollmeier said. "Since these stars are rare, surveys like SDSS-V are designed to have the statistical power to find these needles in the stellar haystack and test our theories of star formation and explosion."
For the students involved, the discovery appears to have been career-shaping. Natalie Orrantia, one of the undergraduates who found the star, monitored the instrument camera through the night. "I was looking at that camera the whole night to make sure it was working," she said. Ha Do, another student involved in the discovery, intends to pursue graduate studies in astronomy. Ji brought the class to Las Campanas for spring break in March 2025.
The students' experience is a real example of the kind of hands-on discovery that large survey datasets are increasingly enabling — not just for professional astronomers, but for people still learning the trade. Whether it generalizes to anything beyond itself is an open question. What the data shows is concrete: a star formed before most of the metals in the universe existed, drifted between galaxies, and has now been sitting in the Milky Way long enough for a group of undergraduates with a good telescope schedule to find it.
What to watch next: Kollmeier and Ji are continuing the search. More metal-poor stars mean more data points on how the first stellar generations seeded the cosmos with elements. A second Nature Astronomy paper, by a separate team, identified a different ancient star — called Pictor II — in roughly the same period. The two discoveries are unrelated and involve different objects; they should not be conflated.
