A group of undergraduate students at the University of Chicago has identified one of the oldest known stars in the universe using data from the Sloan Digital Sky Survey (SDSS). The star did not originate in our galaxy. Instead, it formed in a nearby companion galaxy and later moved into the Milky Way.
The discovery was made by ten students enrolled in the university’s “Field Course in Astrophysics,” taught by Professor Alex Ji, deputy Project Scientist for SDSS-V, along with graduate teaching assistants Hillary Andales and Pierre Thibodeaux.
How Big Data Led to a Major Discovery
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 so researchers and students alike can explore them. In its current phase, the project uses robotic instruments to collect spectra from millions of objects across the sky, helping scientists study how stars, black holes, and galaxies evolve over time.
In Ji’s class, students worked directly with SDSS data. Over several weeks, they examined thousands of stars from the latest survey results, searching for unusual candidates. From this effort, they selected 77 stars for closer study during a planned observing trip.
A Spring Break Trip That Changed Everything
The group traveled to Carnegie Science’s Las Campanas Observatory in Chile during Spring Break, where they used the Magellan Inamori Kyocera Echelle (MIKE) instrument on the Magellan telescopes. Their first observing session took place on March 21st, 2025. The second star they examined that night, labeled SDSSJ0715-7334, quickly stood out.
“We found it the first night, and it completely changed our plans for the course,” Ji said.
Originally, the plan was to observe each target for about 10 minutes. After realizing how unusual this star was, the students spent three hours studying it the following night.
“I was looking at that camera the whole night to make sure it was working,” said Natalie Orrantia, one of the students involved in the discovery.
An “Ancient Immigrant” Star From Another Galaxy
The star turned out to be extraordinarily pure, consisting almost entirely of hydrogen and helium. This chemical makeup indicates it formed very early in cosmic history, making it one of the oldest stars ever observed.
Further analysis revealed that the star was not born in the Milky Way. Instead, it formed in the Large Magellanic Cloud, the Milky Way’s largest companion galaxy, and later 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 an unprecedented look at conditions in the early universe,” said Ji. “Big data projects like SDSS make it possible for students to get directly involved in these important discoveries.”
Record-Low Metallicity Reveals Early Universe Origins
Astronomers use the term “metals” to describe elements heavier than hydrogen and helium. A star’s “metallicity” refers to how much of these heavier elements it contains. SDSSJ0715-7334 has just 0.005 percent of the metals found in the Sun, making it the most metal-poor star ever observed, more than twice as metal-poor as the previous record holder.
“We analyzed the star for a large swath of elements, and the abundances are quite low for all of them,” said Ha Do, another student on the team.
Low metallicity is a key indicator of age. Elements heavier than hydrogen and helium are created in supernova explosions. A star with very few of these elements must have formed before most supernovae occurred, meaning it likely belongs to the earliest generations of stars in the universe.
Tracing the Star’s Journey Across Galaxies
To better understand the star’s history, the team combined their observations with data from the European Space Agency’s Gaia mission. This allowed them to calculate both its distance and its motion through the Milky Way.
By tracing its path backward over billions of years, they determined that the star originated in the Large Magellanic Cloud before eventually being pulled into the Milky Way.
A Rare Chemical Signature and Unexpected Clues
Further analysis revealed another surprising feature. Ji divided the class into groups to study different aspects of the star. Orrantia and Do led the team examining its carbon content, which turned out to be so low that it could not be detected.
“The star has so little carbon that it suggests an early sprinkling of cosmic dust is responsible for making it,” said Ji. “This formation pathway has only been seen once before.”
A Discovery That Shapes Future Careers
Taking part in such a significant discovery early in their academic careers has influenced the students’ future plans. Both Orrantia and Do now intend to pursue graduate studies in astronomy.
“To be able to actually contribute to something like this, it’s very exciting,” Do said.
“These students have discovered more than just the most pristine star.” said Juna Kollmeier, the Director of SDSS-V. “They have discovered their inalienable right to physics. Surveys like SDSS and Gaia make that possible for students of all ages everywhere on Earth and this example shows that there is still plenty of room for discovery.”