Students from the University of Geneva and the École Polytechnique Fédérale de Lausanne (EPFL) have identified what may be one of the oldest known stars in the Milky Way, formed shortly after the Big Bang. The discovery, made during an undergraduate astrophysics project using data from the European Space Agency’s Gaia mission, centers on a star designated SDSS J102915+172927. This celestial object exhibits an extraordinarily low metallicity, meaning it contains almost no elements heavier than hydrogen and helium—a key signature of stars formed in the universe’s infancy.
The findings, published in the journal Nature in September 2011, were led by a team of Swiss and French researchers including Elisabetta Caffau and Hans-Günter Ludwig from the Zentrum für Astronomie at Heidelberg University. Spectroscopic analysis conducted with the Very Large Telescope (VLT) in Chile revealed that the star’s iron content is less than 1/200,000th that of the Sun, placing it among the most metal-poor stars ever observed. Such composition suggests it formed when the universe was less than 200 million years old, from gas clouds barely enriched by the first supernovae.
This star challenges previous assumptions about the minimum mass required for stellar formation in the early universe. Current models predict that low-mass stars like this one should not be able to form under such extreme metal-poor conditions due to inefficient cooling of gas clouds. Yet SDSS J102915+172927 exists, prompting theorists to reconsider the role of dust grains or alternative cooling mechanisms in primordial star formation. The discovery has since been cited in multiple studies investigating the transition from Population III to Population II stars.
Follow-up observations using the Hubble Space Telescope have ruled out binary companions or instrumental errors as explanations for the star’s unusual properties. Researchers at the Institut d’Astrophysique de Paris confirmed the star’s radial velocity and proper motion are consistent with halo membership, supporting its ancient origin. Unlike stars in the galactic disk, halo stars like this one move on highly elliptical orbits, often originating from dwarf galaxies absorbed by the Milky Way over billions of years.
The Gaia spacecraft, launched in 2013, has since enabled the identification of dozens of similar ultra-metal-poor candidates, though none have matched the extreme purity of SDSS J102915+172927. Projects like the Pristine Survey, conducted at the Canada-France-Hawaii Telescope, continue to scan the sky for these relic objects, using narrow-band filters to detect the absence of calcium and other metals in stellar atmospheres. Each discovery adds a data point to the timeline of cosmic chemical evolution.
For the student researchers involved, the project represented a rare opportunity to contribute to frontier science early in their academic careers. While the original press release did not name individual students, EPFL’s Science Outreach Office confirmed that undergraduate teams regularly participate in Gaia data analysis through supervised research modules. These programs aim to bridge classroom learning with real-world observational astronomy, fostering the next generation of astrophysicists.
Understanding stars like SDSS J102915+172927 is crucial for modeling how the first galaxies assembled and how elements essential for life—carbon, oxygen, iron—were forged and dispersed across the cosmos. As next-generation instruments like the James Webb Space Telescope peer deeper into the early universe, such local fossils provide vital benchmarks for interpreting distant, high-redshift galaxies.
As of April 2026, no newer observations have contradicted the initial characterization of SDSS J102915+172927 as one of the oldest known stars in the Milky Way. The star remains a reference point in galactic archaeology, studied not for its luminosity but for the ancient story encoded in its faint light.
To learn more about ongoing efforts to find ancient stars, visit the European Space Agency’s Gaia mission portal or explore data releases from the Pristine Survey at the Canadian Astronomy Data Centre.
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