A team of undergraduates from 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 a ring of stars near it and later moved to the Milky Way.
The discovery was made by ten students enrolled in the university’s “Field Course in Astrophysics”, taught by Professor Alex Ji, assistant Project Scientist for SDSS-V, along with graduate teaching assistants Hillary Andales and Pierre Thibodeaux.
How Big Data Has Led to Big Data
SDSS is a global collaboration involving more than 75 scientific institutions and has been active for 25 years. Its mission is to make large astronomical databases publicly available so that researchers and students alike can explore them. In its current phase, the project uses robotics to collect observations from millions of objects across the sky, helping scientists study how stars, black holes and galaxies change over time.
In Ji’s class, students worked directly with SDSS data. For several weeks, they examined thousands of stars from the latest survey results, looking for unusual individuals. From this effort, they selected 77 stars for close study during the planned observing trip.
The Ultimate Spring Break Trip
The team visited Carnegie Science’s Las Campanas Observatory in Chile during Spring Break, where they used the Magellan Inamori Kyocera Echelle (MIKE) instrument on the Magellan telescope. Their first observation session took place on March 21, 2025. The second star they examined that night, labeled SDSSJ0715-7334, quickly became prominent.
“We found it on the first night, and it completely changed our course plans,” Ji said.
Initially, 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 watching that camera all night to make sure it was working,” said Natalie Orrantia, one of the students involved in the discovery.
“Ancient Alien” Star From Another Neck
The star appeared to be unusually pure, consisting almost entirely of hydrogen and helium. This chemical structure indicates that it started very early in the history of the universe, 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 largest galaxy in the Milky Way, and then migrated into our own galaxy billions of years ago. Because of its origin and age, Ji described it as an “old alien.”
“This ancient alien gives us a unique view of the conditions in the early universe,” said Ji. “Big data projects like SDSS make it possible for students to directly engage with these important discoveries.”
Record-Low Metallicity Reveals Early Universe
Astronomers use the term “metals” to describe elements heavier than hydrogen and 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 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 have examined the star for many features, and most of them are very weak,” said Ha Do, another student in the group.
A low iron mass is an important 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, meaning it may belong to the oldest generations of stars in the universe.
Following Star Trek Across the Neck
To better understand the star’s history, the team combined what they saw with data from the European Space Agency’s Gaia mission. This enabled them to calculate its location and its movement down the Milky Way.
By tracing its path back billions of years, they discovered that the star originated in the Large Magellanic Cloud before eventually being pulled into the Milky Way.
Unusual Chemical Signatures and Unexpected Radiation
Further analysis revealed another surprising feature. Ji divided the group into groups to study different aspects of the star. Orrantia and Do led a team that analyzed its carbon, which turned out to be too low to be detected.
“The star has so little carbon that it suggests a sprinkling of cosmic dust is responsible,” Ji said. “This formation has only been seen once before.”
Discovery That Shapes Future Jobs
Participating in such an important discovery early in their academic careers has influenced students’ future plans. Orrantia and Do currently intend to pursue studies in astrology.
“To be able to participate in something like this, it’s really cool,” Do said.
“These students have discovered more than just the cleanest star.” said Juna Kollmeier, Director of SDSS-V. “They’ve earned their unalienable right to physics. Surveys like SDSS and Gaia make that possible for students of all ages around the world and this example shows that there’s still a lot of room for discovery.”
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