Students Uncover an Ancient Metal-Poor Star Wandering Into the Milky Way

A team of students at the University of Chicago has identified an extraordinary ancient star, SDSS J0715-7334, that surprisingly resides within the Milky Way despite originating elsewhere. This star, nearly devoid of heavy elements, is believed to have formed in a distant galaxy before entering our own.

Guided by Professor Alex Ji, the students analyzed observations from the Sloan Digital Sky Survey (SDSS), a global effort collecting astronomical data to advance our understanding of stellar and galactic evolution, as well as black hole dynamics.

A Rare Relic Star Unearthed

The discovery began during an astronomy class at the University of Chicago. Professor Ji described how his students sifted through thousands of stellar observations from SDSS datasets. Right on their first observation night, one star immediately drew their attention.

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“We found it the first night, and it completely changed our plans for the course,” Ji said.

SDSS J0715-7334, a red giant star journeying from the Large Magellanic Cloud to the Milky Way. Credit: Carnegie Institution for Science

SDSS J0715-7334 is remarkable for its composition: primarily hydrogen and helium, with almost no metals. Such rarity makes it one of the universe’s oldest known stars. The minimal presence of heavy elements like carbon and oxygen suggests it formed well before the cosmic fireworks of supernova explosions that create heavier atoms.

“All of the heavier elements in the universe, which astronomers call metals, were produced by stellar processes—from fusion reactions occurring within stars to supernovae explosions to collisions between very dense stars,” added Ji in a statement released by the Carnegie Institution for Science. “So, finding a star with very little metal content in it told this group of students that they’d come across something very special.”

A Cosmic Traveler from Another Galaxy

SDSS J0715-7334 did not originally form in our galaxy but in the nearby Large Magellanic Cloud. By tracking its trajectory, researchers found that the star journeyed from its home galaxy and eventually merged into the Milky Way billions of years ago.

Because of this remarkable migration, Professor Ji described the star as an “ancient immigrant.”

“This ancient immigrant gives us an unprecedented look at conditions in the early universe,” he said. “Big data projects like SDSS make it possible for students to get directly involved in these important discoveries.”

Student-Led Breakthrough in Astronomy

The outstanding element of this discovery, which was published in Nature Astronomy, is its student-driven nature. Ha Do, one of the participants, expressed their exhilaration:

“To be able to actually contribute to something like this, it’s very exciting.” The student involvement in this work highlights how collaborative projects like SDSS offer young researchers access to advanced astronomical data.

Illustration depicting the orbit of SDSS J0715-7334 as it moves from the Large Magellanic Cloud (LMC) into our galaxy. Credit: Nature Astronomy

Natalie Orrantia, another team member, says this experience has inspired her to continue onto graduate study. Juna Kollmeier, director of SDSS-V, emphasized:

“When I was an undergraduate, I greatly preferred doing research to taking classes. I’m delighted that Alex’s course was transformed into a curriculum of discovery and I’d like to ensure surveys like SDSS-V and Gaia have the power to make that the norm and not the exception,” she noted. “These students have discovered more than just the most pristine star; they have discovered their inalienable right to physics.”