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Astronomers investigating the earliest phases of chemical enrichment in nearby dwarf galaxies have identified one of the most ancient stars known beyond the Milky Way. The object, PicII-503, lies inside the ultra-faint dwarf galaxy Pictor II and shows an extreme deficiency of iron together with strong carbon enhancement. Researchers first detected the star through wide-field imaging obtained with the Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory using the Dark Energy Camera (DECam) as part of the Mapping the Ancient Galaxy in CaHK survey. Follow-up spectroscopy later confirmed that its atmosphere preserves the chemical signature of enrichment by only one or a few early supernova explosions.
Because Pictor II experienced very limited chemical evolution after its formation, the abundance pattern recorded in PicII-503 likely reflects conditions that existed shortly after the first stellar generations ended their lives. The discovery provides an observational window into the transition between primordial star formation and the emergence of long-lived stellar populations that still survive today.
Chemical evidence from a second-generation stellar population
The first stars formed from gas composed almost entirely of hydrogen and helium. Nuclear fusion inside their cores produced heavier elements for the first time. These elements later entered the surrounding interstellar medium during supernova explosions. Gas enriched in this way then formed the next generation of stars. PicII-503 belongs to this early population.
Spectroscopic measurements show that the star contains only about one-forty-thousandth of the Sun’s iron abundance. This extremely low metallicity places it among the most chemically ancient stars known outside the Milky Way. At the same time, its atmosphere displays strong carbon enhancement relative to iron. This chemical combination provides clear evidence that the gas from which the star formed experienced only limited enrichment before the star itself formed.
The ultra-faint dwarf galaxy Pictor II
The location of PicII-503 inside the ultra-faint dwarf galaxy Pictor II plays a crucial role in interpreting its chemical properties. This galaxy lies roughly 150,000 light-years from Earth and contains only a small stellar population. Observations show that it formed early and then evolved slowly.
Large galaxies such as the Milky Way experienced repeated cycles of star formation over billions of years. During those cycles, stellar ejecta mixed with interstellar gas and gradually increased the overall metallicity of the system. As a result, the earliest enrichment signatures often became difficult to identify.
By contrast, ultra-faint dwarf galaxies followed much simpler evolutionary histories. Their gas reservoirs remained relatively undisturbed after their earliest star-formation episodes. Consequently, their stellar populations preserve clearer records of early chemical enrichment. The presence of PicII-503 inside Pictor II therefore strengthens the interpretation that its abundance pattern reflects only one or a few early enrichment events rather than many later ones.
Blanco Telescope and the Dark Energy Camera
Astronomers identified PicII-503 through imaging obtained with the Víctor M. Blanco 4-meter Telescope at the Cerro Tololo Inter-American Observatory in Chile. This telescope operates as one of the most important survey facilities for wide-field optical astronomy in the southern hemisphere. It played a central role in locating chemically ancient stars across nearby dwarf galaxies.
Researchers carried out the initial detection as part of the Mapping the Ancient Galaxy in CaHK survey, also known as the MAGIC survey. This program searches for extremely metal-poor stars by measuring absorption features linked to calcium in stellar atmospheres. Because calcium abundance tracks overall metallicity, this method enables astronomers to efficiently identify promising candidates across large sky areas.
The observations relied on the Dark Energy Camera (DECam) mounted on the Blanco Telescope. DECam remains one of the most powerful wide-field optical cameras currently operating on a ground-based telescope. It contains a mosaic of high-sensitivity CCD detectors that, together, produce a field of view approximately two degrees across. This coverage allows astronomers to observe large regions of the sky in a single exposure.
Equally important, DECam provides excellent sensitivity in the blue part of the spectrum. That capability makes it especially suitable for detecting the calcium absorption features targeted by the MAGIC survey. As a result, researchers could identify candidate metal-poor stars inside faint dwarf galaxies such as Pictor II with high confidence.
Observational Strategy and Spectroscopic Confirmation
Astronomers first identified PicII-503 through wide-field imaging obtained as part of the Mapping the Ancient Galaxy in CaHK survey. This survey searches for extremely metal-poor stars by measuring absorption features associated with calcium in stellar spectra. Because calcium abundance correlates with overall metallicity, this method efficiently identifies promising candidates for detailed study.
After selecting PicII-503 as a candidate metal-poor star, researchers carried out high-resolution spectroscopic observations using large ground-based telescopes. These observations measured the strengths of absorption lines produced by elements such as iron, calcium, and carbon in the stellar atmosphere.
The resulting data confirmed the star’s extremely low iron abundance. At the same time, the observations revealed strong carbon enhancement relative to iron. This chemical pattern provided essential evidence for interpreting the enrichment history of the gas from which the star formed.
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