Astronomers Discover Record-Breaking 'Ancient Immigrant' Star
An international team of astronomers, including University of Chicago undergraduates, has identified a star with the lowest known abundance of heavy elements. Designated SDSS J0715-7334, the star is characterized by its extreme chemical purity and an origin traced to outside the Milky Way galaxy. The findings are published in Nature Astronomy.
The star possesses a metallicity of approximately 0.005 percent that of the Sun, establishing it as the star with the lowest known metal content.
The Discovery
The star was identified using data from the Sloan Digital Sky Survey-V (SDSS-V), a project that makes large astronomical datasets publicly available. A University of Chicago undergraduate astrophysics field course, led by Professor Alexander Ji, analyzed this data to select candidate stars for further study.
On March 21, 2025, the student group used the Magellan telescopes at Carnegie Science's Las Campanas Observatory in Chile to observe selected targets. SDSS J0715-7334 was the second star observed that night.
Recognizing its unusual properties, the team dedicated approximately three hours to its study on a subsequent night, deviating from the initial plan of shorter observations per target.
A Star of Extreme Purity
Analysis of the star's spectrum revealed a composition almost entirely of hydrogen and helium. In astronomy, elements heavier than these are called "metals."
- Record-Breaking Purity: SDSS J0715-7334's metal content is less than half that of the previous record holder.
- Primordial Origin: This extreme lack of heavy elements indicates it formed from material that had undergone very few prior cycles of stellar nucleosynthesis, suggesting it is among the earliest generations of stars.
Further examination, led in part by students Natalie Orrantia and Ha Do, found the star's carbon content to be undetectably low. Researchers note this specific chemical signature suggests a rare formation pathway involving an early distribution of cosmic dust.
An Ancient Immigrant
By combining their data with information from the European Space Agency's Gaia mission, researchers calculated the star's distance and motion.
- Location: Approximately 80,000 light-years from Earth.
- Origin: Tracing its trajectory backward indicates it did not form in the Milky Way. The analysis suggests it originated in the Large Magellanic Cloud, a companion galaxy, and was later gravitationally captured by our galaxy billions of years ago.
Professor Alexander Ji has referred to SDSS J0715-7334 as an "ancient immigrant."
Impact and Context
The Sloan Digital Sky Survey has operated for 25 years. Its current phase uses robotic instruments to collect spectra from millions of objects to study stellar, black hole, and galactic evolution.
Juna Kollmeier, Director of SDSS-V, stated that large-scale public data projects like SDSS and Gaia enable direct student involvement in scientific discovery.
Following their participation, students Natalie Orrantia and Ha Do have stated their intention to pursue graduate studies in astronomy.