A group of undergraduate students at the University of Chicago has identified one of the oldest stars ever discovered while analyzing massive datasets for an astrophysics course. The star, designated SDSSJ0715-7334, formed near the dawn of the universe and currently resides within the Milky Way after migrating from the Large Magellanic Cloud.
The discovery occurred during the university's "Field Course in Astrophysics," led by Professor Alex Ji, who serves as the deputy Project Scientist for the Sloan Digital Sky Survey (SDSS). Ten students spent weeks combing through survey data to identify unusual candidates before traveling to Carnegie Science’s Las Campanas Observatory in Chile for hands-on observation.
A cosmic time capsule in the data
On the first night of observations in March 2025, the team used the Magellan telescopes to analyze their second target. The star immediately stood out for its chemical composition, which consists almost entirely of hydrogen and helium.
"We found it the first night, and it completely changed our plans for the course," Professor Ji said. What was intended to be a series of 10-minute observations turned into a three-hour intensive study of the object.
Student Natalie Orrantia, who participated in the discovery, noted the intensity of the process. "I was looking at that camera the whole night to make sure it was working," she said.
The star possesses just 0.005 percent of the metals found in the Sun, making it the most metal-poor star ever observed. In astronomy, elements heavier than hydrogen and helium are classified as metals. Because these elements are forged in supernova explosions, a star with such low metallic content must have formed before the universe was populated by widespread stellar deaths.
By combining their observations with data from the European Space Agency’s Gaia mission, the researchers mapped the star’s movement through space. This data confirmed that the star was not born in the Milky Way. Instead, it originated in the Large Magellanic Cloud and was pulled into our galaxy's gravitational reach billions of years ago, earning it the label of an "ancient immigrant."
Student Ha Do confirmed the findings after a thorough analysis of the star's chemical signature. "We analyzed the star for a large swath of elements, and the abundances are quite low for all of them," Do said.
Professor Ji emphasized that the discovery highlights the value of public data initiatives like SDSS, which has operated for 25 years. By providing access to robotic survey data, the project allows students to contribute directly to modern understanding of how stars and galaxies evolve.
