A group of undergraduate students at the University of Chicago used data from the Sloan Digital Sky Survey (SDSS) to identify one of the oldest known stars in the universe. This star was not born in our galaxy. Instead, it formed in a nearby companion galaxy and later migrated to the Milky Way.
The discovery was made by 10 students enrolled in the university’s Astrophysics Field Course, led by Professor Alex Gee, deputy project scientist at SDSS-V, and graduate teaching assistants Hilary Anderes and Pierre Thibodeau.
How big data led to major discoveries
SDSS is a global collaboration involving more than 75 scientific institutions and has been operating for 25 years. Its mission is to make large astronomical datasets publicly available for exploration by researchers and students alike. In its current stage, the project uses robotic instruments to collect spectra from millions of objects in the sky, helping scientists study how stars, black holes and galaxies evolve over time.
In Ji’s class, students worked directly with SDSS data. For several weeks, they examined thousands of stars from the latest findings, looking for unusual candidates. From this effort, they selected 77 stars for detailed study during a planned observation trip.
The spring break trip that changed everything
The group visited Carnegie Science’s Las Campanas Observatory in Chile during spring break, where they used the Magellan Telescope’s Magellan Inamori Kyocera Echelle (MIKE) instrument. Their first observation session took place on March 21, 2025. The second star they studied that night (SDSSJ0715-7334) immediately stood out.
“I found it on the first night and it completely changed my plan for the course,” Ji said.
The original plan was to observe each target for about 10 minutes. Realizing how rare this star was, the students spent three hours studying it the next night.
“I was staring at that camera all night, making sure it was working,” said Natalie Orrantia, one of the students involved in the discovery.
An “ancient immigrant” star from another galaxy
The star was found to be extremely pure, consisting almost entirely of hydrogen and helium. This chemical composition indicates that the star formed very early in the history of the universe, making it one of the oldest stars ever observed.
Further analysis revealed that this star was not born in the Milky Way. Instead, it formed in the Large Magellanic Cloud, the Milky Way’s largest companion galaxy, and then migrated into our galaxy billions of years ago. Because of its origin and age, Ji described it as an “ancient immigrant.”
“This ancient immigrant gives us unprecedented observations about conditions in the early universe,” Gee said. “Big data projects like SDSS allow students to be directly involved in these important discoveries.”
Record-low metal content reveals the origin of the early universe
Astronomers use the term “metal” to describe elements heavier than hydrogen or helium. The “metallicity” of a star refers to how much of these heavy elements it contains. SDSSJ0715-7334 contains only 0.005 percent of the metals in the Sun, making it the most metal-poor star ever observed and more than twice as metal-poor as the previous record holder.
“We analyzed a wide range of elements in this star, and the abundances were very low for all elements,” said Ha Do, another student on the team.
Low metal content is an important indicator of age. Supernova explosions produce elements heavier than hydrogen and helium. Stars containing few of these elements would have formed before most supernovae, meaning they probably belong to the earliest generation of stars in the universe.
Follow a star’s journey across the galaxy
To better understand the star’s history, the team combined their observations with data from the European Space Agency’s Gaia mission. This allowed us to calculate both the distance and movement of the Milky Way.
By retracing its path over billions of years, they determined that the star started in the Large Magellanic Cloud and was eventually pulled into the Milky Way.
Unusual chemical features and unexpected clues
Further analysis revealed another surprising feature. Mr. Gee divided the class into groups to study different aspects of stars. Orrantia and Do led a team to investigate its carbon content, which was found to be too low to be detected.
“This star has very little carbon, suggesting that early cosmic dust played a role in its formation,” Gee said. “This formation pathway has only been confirmed once before.”
Discoveries that shape your future career
Participating in such an important discovery so early in their academic career influenced the students’ future plans. Orrantia and Dou now plan to pursue graduate studies in astronomy.
“It’s really exciting to actually be able to contribute to something like this,” Do said.
“These students have discovered more than just the most primitive stars,” said SDSS-V Director Djuna Kollmeyer. “They have discovered an inalienable right to physics, and studies like SDSS and Gaia make it possible for students of all ages, anywhere on the planet. This example shows that there is still much room for discovery.”
