Astronomers have discovered one of the most chemically primitive stars ever identified — an ancient stellar relic that preserves the chemical imprint of the very first stars in the Universe. This star, named PicII-503, resides in the tiny, ultra-faint dwarf galaxy Pictor II. The discovery was enabled by the U.S. Department of Energy-fabricated Dark Energy Camera ( DECam ), mounted on the U.S. National Science Foundation Víctor M. Blanco 4-meter Telescope , at NSF Cerro Tololo Inter-American Observatory ( CTIO ) in Chile, a Program of NSF NOIRLab.
Pictor II is located in the constellation Pictor . It contains several thousand stars and is more than ten billion years old. PicII-503 lies on the outskirts of the galaxy, and it contains less iron than any other star ever measured outside of the Milky Way, while also having an extreme overabundance of carbon. These signatures unmistakably match those of carbon-enhanced stars found in the outer reaches of the Milky Way, whose origins have, until now, been a mystery.
The study was led by Anirudh Chiti, Brinson Prize Fellow at Stanford University, and the results are presented in a paper appearing in Nature Astronomy.
The first stars in the Universe formed from gas that contained only the simple elements, hydrogen and helium. Within their fiery cores, this first generation of stars created the first elements heavier than helium, such as carbon and iron, which astronomers refer to as "metals." When these stars exploded, they released their heavy elements into the interstellar medium to be recycled into the next generation of stars.
Second-generation stars are like time capsules, preserving the low amounts of heavy elements released during the explosive deaths of first-generation stars. By searching for these rare, low-metallicity stars and deriving their chemistry , scientists can better understand the mechanisms of initial element production in the Universe.
PicII-503 is the first unambiguous example of a second-generation star in an ultra-faint dwarf galaxy. It was uncovered in data from the DECam MAGIC (Mapping the Ancient Galaxy in CaHK) survey, a 54-night observing program designed to identify the oldest and most chemically primitive stars in the Milky Way and its dwarf galaxy companions. Using a specialized narrow-band filter sensitive to calcium absorption features, astronomers were able to estimate the metal content of thousands of stars from imaging data alone.
Among the hundreds of stars near Pictor II, MAGIC data singled out PicII-503 as an exceptionally metal-poor candidate, allowing researchers to target it for detailed follow-up study. "Without data from MAGIC, it would have been impossible to isolate this star among the hundreds of other stars in the vicinity of the Pictor II ultra-faint dwarf galaxy," says Chiti.
By combining data from MAGIC, the Magellan/Baade Telescope , and ESO's Very Large Telescope , the team found that PicII-503 has the lowest iron and calcium abundances ever measured outside of the Milky Way. This paucity of iron and calcium makes it the first object that clearly preserves enrichment from the first stars in a relic dwarf galaxy.
"Discovering a star that unambiguously preserves the heavy metals from the first stars was at the edge of what we thought possible, given the extreme rarity of these objects," says Chiti. "With the lowest iron abundance ever derived in any ultra-faint dwarf galaxy, PicII-503 provides a window into initial element production within a primordial system that is unprecedented."
Even more remarkably, the team discovered that PicII-503 has a carbon-to-iron ratio that is over 1500 times that of the Sun. This overabundance matches the distinct carbon signature of low-iron stars long observed in the Milky Way halo. These are known as carbon-enhanced metal-poor stars, and their origin has remained unknown until now.
One hypothesis is that carbon-enhanced metal-poor stars are second-generation stars that preserve the chemical elements produced by low-energy supernovae of first-generation stars. During this process, heavy elements that form close to the star's interior, like iron, fall back into the remnant compact object, while lighter elements that are near the star's outer regions, like carbon, are ejected into the interstellar medium to seed the formation of the next generation of stars.
PicII-503 supports the low-energy supernovae explanation because it is found in one of the smallest dwarf galaxies that we know of. If the supernova that produced the metals found in PicII-503 was high-energy, then the elements would have escaped the gravitational pull of the small Pictor II dwarf galaxy. PicII-503 also demonstrates that the carbon-enhanced metal-poor stars observed in the Milky Way halo likely originated from ancient relic dwarf galaxies that have, over time, merged with ours.
"What excites me the most is that we have observed an outcome of the very initial element production in a primordial galaxy, which is a fundamental observation!" says Chiti. "It also cleanly connects to the signature that we have seen in the lowest-metallicity Milky Way halo stars, tying together their origins and the first-star-enriched nature of these objects."
"Discoveries like this are cosmic archaeology, uncovering rare stellar fossils that preserve the fingerprints of the Universe's first stars," says Chris Davis, NSF Program Director for NOIRLab. "We look forward to many more discoveries with the start of the NSF–DOE Rubin Observatory's Legacy Survey of Space and Time later this year."
PicII-503 offers a rare, direct glimpse into the Universe's first chapter of chemical evolution, which is a foundational moment that ultimately set the stage for planets, chemistry, and life itself. It also connects long-standing mysteries about ancient stars in the Milky Way to their origins in primordial dwarf galaxies.
More information
This research was presented in a paper titled "Enrichment by the first stars in a relic dwarf galaxy" appearing in Nature Astronomy. DOI: 10.1038/s41550-026-02802-z
The team is composed of A. Chiti (Stanford University/University of Chicago/Brinson Prize Fellow, USA) , V. M. Placco (NSF NOIRLab, USA), A. B. Pace (University of Virginia, USA), A. P. Ji (University of Chicago/NSF-Simons AI Institute for the Sky, USA), D. S. Prabhu (University of Arizona, USA), W. Cerny (Yale University, USA), G. Limberg (University of Chicago, USA), G. S. Stringfellow (Yale University, USA), A. Drlica-Wagner (Fermi National Accelerator Laboratory/Stanford University/University of Chicago/NSF-Simons AI Institute for the Sky, USA), K. R. Atzberger (University of Virginia, USA), Y. Choi (NSF NOIRLab, USA), D. Crnojević (University of Tampa, USA), P. S. Ferguson (University of Washington, USA), N. Kallivayalil (University of Virginia, USA), N. E. D. Noël (University of Surrey, UK), A. H. Riley (Durham University, UK/Lund University, Sweden), D. J. Sand (University of Arizona, USA), J. D. Simon (Observatories of the Carnegie Institution for Science, USA), A. R. Walker (Cerro Tololo Inter-American Observatory/NSF NOIRLab, Chile), C. R. Bom (Centro Brasileiro de Pesquisas Físicas, Brazil), J. A. Carballo-Bello (Universidad de Tarapacá, Chile), D. J. James (ASTRAVEO LLC, Applied Materials Inc., USA), C. E. Martínez-Vázquez (NSF NOIRLab, USA), G. E. Medina (University of Toronto, Canada), K. Vivas (NSF NOIRLab, Chile).
NSF NOIRLab , the U.S. National Science Foundation center for ground-based optical-infrared astronomy, operates the International Gemini Observatory (a facility of NSF , NRC–Canada , ANID–Chile , MCTIC–Brazil , MINCyT–Argentina , and KASI–Republic of Korea ), NSF Kitt Peak National Observatory ( KPNO ), NSF Cerro Tololo Inter-American Observatory ( CTIO ), the Community Science and Data Center ( CSDC ), and NSF–DOE Vera C. Rubin Observatory (in cooperation with DOE 's SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy ( AURA ) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona.
The scientific community is honored to have the opportunity to conduct astronomical research on I'oligam Du'ag (Kitt Peak) in Arizona, on Maunakea in Hawai'i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence of I'oligam Du'ag to the Tohono O'odham Nation, and Maunakea to the Kanaka Maoli (Native Hawaiians) community.
Links
- Read the paper: Enrichment by the first stars in a relic dwarf galaxy
- Photos of the Víctor M. Blanco 4-meter Telescope
- Videos of the Víctor M. Blanco 4-meter Telescope
- Photos of DECam
- Check out other NOIRLab Science Releases
