In a remarkable achievement, a group of ten undergraduate students from the University of Chicago has discovered one of the oldest stars in the Universe residing in the Milky Way. This star, designated SDSS J0715-7334, is a red giant with a mass 29 times that of our Sun and is located approximately 79,256 light-years away.
What makes this discovery particularly intriguing is that SDSS J0715-7334 did not originate in the Milky Way; instead, it migrated here from another galaxy. The student team, guided by Professor Alex Ji, who is also the deputy Project Scientist for the Sloan Digital Sky Survey (SDSS-V), utilized data from the SDSS, which has been operational since 2020. This program is part of a 25-year initiative aimed at acquiring spectra from millions of celestial objects to enhance our understanding of stellar and galactic evolution.
During their Field Course in Astrophysics, the students analyzed data from the SDSS, examining thousands of candidates before identifying 77 for further observation. On March 21, 2025, they utilized the Magellan Inamori Kyocera Echelle (MIKE) instrument on the Magellan telescopes at Las Campanas Observatory to observe SDSS J0715-7334 for three hours.
The composition of SDSS J0715-7334 is strikingly primitive, consisting almost entirely of hydrogen and helium, with a metallicity of only 0.005% compared to stars like our Sun. This low metallicity suggests that the star formed in the early Universe, following the death of the first generation of stars (Population III) that produced heavier elements through supernovae. As a Population II star, SDSS J0715-7334 is currently in the Red Giant Branch (RGB) phase of its lifecycle.
Further analysis of its orbit confirmed that SDSS J0715-7334 originated in the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way, and migrated here billions of years ago. This led Ji and his students to affectionately nickname it the “Ancient Immigrant.” Ji remarked, “We found it the first night, and it completely changed our plans for the course.”
Using data from the ESA’s Gaia mission, the team also determined the star’s distance from the Sun and its motion through the galaxy. The analysis revealed an exceptionally low carbon content, which Ji noted suggests a unique formation pathway that has only been observed once before in another ancient star, HD 140283, also known as the “Methuselah star.”
Ji emphasized the significance of this discovery, stating, “This ancient immigrant gives us an unprecedented look at conditions in the early universe.” The findings underscore the potential for student involvement in groundbreaking astronomical research, facilitated by large data projects like SDSS and Gaia.
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