Students Discover Ancient Star from the Early Universe Drifting Into the Milky Way

On the night of March 21, 2025, ten University of Chicago undergraduates sat at the controls of the Magellan telescopes at Carnegie Science's Las Campanas Observatory in Chile. They were there as part of a spring break field course in astrophysics, sifting through candidate stars flagged in the Sloan Digital Sky Survey (SDSS-V). The second star they observed that night — catalogued as SDSS J0715-7334 — turned out to be the most chemically pristine star ever found in the universe [1][2].

What was supposed to be a ten-minute observation stretched to three hours the following night, as the students and their professor realized they were looking at something without precedent: a red giant composed almost entirely of hydrogen and helium, carrying just 0.005% of the Sun's metal content [3]. Their findings, published in Nature Astronomy in April 2026, have implications that stretch from the physics of the first stars to fundamental questions about how galaxies like ours were assembled [4].

What the Star Is Made Of — and What It Isn't

In stellar astrophysics, "metals" refers to all elements heavier than hydrogen and helium. Because these heavier elements are forged in stellar interiors and expelled by supernovae, a star's metal content serves as a rough clock: the fewer metals, the closer to the beginning of cosmic time it must have formed.

SDSS J0715-7334 has a measured iron abundance of [Fe/H] = −4.3, meaning its iron content is roughly one ten-thousandth of a percent of the Sun's [5]. Its total metallicity is Z < 7.8 × 10⁻⁷ — approximately 0.8 parts per million, or about 20,000 times less than the Sun [5][6]. By this measure, it is twice as metal-poor as the previous record holder, SDSS J1029+1729, and 40 times more metal-poor than the most iron-poor star previously known, SMSS J0313-6708 [3][7].

What makes it especially unusual is its carbon content — or rather, the near-total lack of it. SDSS J0715-7334 has the lowest carbon abundance of any star on record, with an upper limit of [C/Fe] < −0.2 [5]. Most ultra-metal-poor stars discovered to date retain measurable amounts of carbon, which plays a critical role in the gas-cooling processes that allow small, long-lived stars to form. The absence of carbon in J0715-7334 points to a different formation pathway entirely.

Source: Nature Astronomy / ApJ compilation

Data as of Apr 3, 2026CSV

How It Compares to the Methuselah Star and Other Ancients

The best-known ancient star in the Milky Way is HD 140283, nicknamed the "Methuselah star," a metal-poor subgiant about 200 light-years away. Its age has been a subject of ongoing revision — early Hubble estimates placed it at 14.46 billion years (with 800-million-year uncertainty bars that uncomfortably overlapped the age of the universe), while a 2021 analysis brought the figure down to approximately 12 billion years [8][9]. HD 140283 has a metallicity of [Fe/H] ≈ −2.4, making it metal-poor by ordinary standards but far richer in heavy elements than J0715-7334 [8].

SDSS J0715-7334 is estimated to be 8 to 10 billion years old based on its current red giant status and inferred mass [10]. That makes it younger than the Methuselah star in absolute terms. But its metallicity tells a different story about its environment of origin: it formed from gas that had been enriched by far fewer supernova events — likely just one. Where HD 140283 was born in a primeval dwarf galaxy that had already undergone multiple generations of stellar enrichment, J0715-7334 appears to have formed in an environment where only a single massive star had previously lived and died [5][6].

Source: SDSS / Nature Astronomy

Data as of Apr 3, 2026CSV

A Classroom Discovery at the Magellan Telescopes

The discovery emerged from Professor Alexander Ji's "Field Course in Astrophysics" at the University of Chicago. Ji, who serves as deputy Project Scientist for SDSS-V, designed the course to give undergraduates hands-on experience with real survey data and professional-grade telescopes [1][2].

Over several weeks before the observing run, students worked directly with SDSS data, examining thousands of stellar spectra to identify candidates with unusually low metal signatures. At Las Campanas, they used the Magellan Inamori Kyocera Echelle (MIKE) spectrograph on the Magellan Clay telescope to obtain high-resolution spectra of their shortlisted targets [3][11].

Two students — Natalie Orrantia and Ha Do — led the analysis of J0715-7334's carbon abundance, a measurement that proved central to the paper's conclusions [1]. Graduate teaching assistants Hillary Andales and Pierre Thibodeaux supervised the observational work alongside Ji [2].

"These pristine stars are windows into the dawn of stars and galaxies in the universe," Ji said [3].

The discovery raises questions about the structure of professional astronomy. The students were not working on a purpose-built research project with years of planning; they were enrolled in a course. The fact that they identified the most metal-poor star ever observed, using publicly available SDSS-V data and a well-established spectrograph, speaks to the growing accessibility of large survey datasets. As SDSS-V, Gaia, and upcoming surveys like the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) continue to generate petabytes of stellar data, the bottleneck in galactic archaeology is shifting from data collection to analysis — a space where fresh eyes and systematic search strategies can produce results regardless of seniority [1][12].

An Extragalactic Origin: Tracing the Star's Journey

To determine where J0715-7334 came from, the team incorporated astrometric data from the European Space Agency's Gaia mission, which provides precise measurements of stellar positions, distances, and proper motions [3][11].

The star currently sits approximately 80,000 light-years from Earth, in the outer reaches of the Milky Way's halo [3]. By reconstructing its orbital trajectory backward through billions of years of gravitational dynamics, the researchers found that its path is consistent with an origin in or near the Large Magellanic Cloud (LMC), the Milky Way's largest satellite galaxy [1][5].

Ji described J0715-7334 as an "ancient immigrant" — a star born in a companion galaxy that was gravitationally captured by the Milky Way over cosmic time [1]. The LMC, located about 160,000 light-years away, is actively interacting with the Milky Way and is expected to merge fully with it in roughly 2.4 billion years. Stars stripped from the LMC's outer halo during earlier close passages would follow orbital paths similar to what the team observed for J0715-7334 [5][6].

This makes J0715-7334 one of a small number of stars with both extreme metal poverty and a confirmed extragalactic origin. Most ultra-metal-poor stars found to date reside in the Milky Way's halo, and the majority are thought to have formed in situ — either in the proto-Milky Way itself or in small dwarf galaxies that were absorbed early in our galaxy's history. Finding such a star with kinematic ties to the LMC specifically adds a new data point to models of galactic assembly, suggesting that even the Milky Way's most recent large satellite harbored conditions capable of forming second-generation stars from nearly pristine gas [5][7].

What the Abundance Pattern Reveals About the First Stars

The chemical fingerprint of SDSS J0715-7334 constrains the properties of the Population III star — the first-generation star — whose supernova provided the heavy elements from which J0715-7334 formed.

Population III stars have never been directly observed. They are predicted to have been massive (tens to hundreds of solar masses), short-lived, and composed entirely of hydrogen and helium from the Big Bang. When they exploded, their supernovae seeded the surrounding gas with the first heavy elements, enabling the formation of the second generation of stars — Population II — which includes objects like J0715-7334 [13][14].

The abundance pattern in J0715-7334 is best explained by enrichment from a single supernova of a star with an initial mass of approximately 30 solar masses and an explosion energy of roughly 5 × 10⁵¹ ergs — several times more energetic than a typical core-collapse supernova [5][6]. The very low carbon abundance, in particular, rules out enrichment by lower-mass Population III supernovae, which tend to produce more carbon relative to iron.

The near-absence of carbon also has implications for how J0715-7334 itself formed. In standard models of early star formation, carbon and oxygen atoms in cooling gas radiate away thermal energy, allowing gas clouds to collapse into small, low-mass stars. Without sufficient carbon, an alternative cooling mechanism is required. The research team concluded that dust cooling — where solid particles of silicates or other minerals radiate heat — must have been operating at the time of J0715-7334's formation [5][6]. This pushes the onset of dust production in the universe to an earlier epoch than some models had assumed, or at least demonstrates that even in extremely metal-poor environments, enough dust could form from a single supernova's ejecta to enable low-mass star formation.

The Census of Ultra-Metal-Poor Stars

The field of stellar archaeology — using the chemical compositions of old stars to reconstruct conditions in the early universe — has grown substantially over the past two decades. Research publications on ultra-metal-poor stars peaked at 742 papers in 2023 and have averaged over 400 per year since 2017, according to OpenAlex data [12].

Source: OpenAlex

Data as of Jan 1, 2026CSV

The number of known stars with [Fe/H] ≤ −4.0 remains small — on the order of a few dozen. The earliest systematic surveys, such as the Hamburg/ESO Survey, identified 146 stars with [Fe/H] ≤ −2.5 by 2005 [14]. More recent efforts using SDSS, LAMOST, and the Pristine survey have expanded the census considerably, but the most extreme objects remain rare. Stars below [Fe/H] = −5.0, like HE 0107-5240 ([Fe/H] = −5.4) and HE 1327-2326 ([Fe/H] = −5.7), can be counted on one hand [14][7].

Of the ultra-metal-poor stars identified so far, the vast majority reside in the Milky Way's halo and are generally assumed to have formed in situ or in early-accreted dwarf galaxies whose identities have been erased by tidal disruption. Stars with clear kinematic evidence of extragalactic origin — like J0715-7334's connection to the LMC — are rarer still, and each one provides a test of whether chemical enrichment histories differed between the proto-Milky Way and its satellite galaxies [5][7][14].

A 2024 study by MIT researchers identified three of the universe's oldest stars within the Milky Way, each with ages exceeding 12 billion years, further demonstrating that the galactic halo contains a fossil record of the earliest cosmic epochs [15]. J0715-7334 extends this record by providing the first clear link between an ultra-metal-poor star and a specific surviving satellite galaxy.

The Case for Skepticism

Several aspects of the J0715-7334 analysis rest on model-dependent assumptions that warrant scrutiny.

Age estimation. The star's age of 8 to 10 billion years is inferred from its position on the red giant branch and its estimated mass, not from a direct chronometric measurement. As astronomer Matt Bothwell noted in an interview, "the actual age range spans approximately 2 billion years, reflecting model-dependent assumptions about stellar evolution timescales" [10]. Stellar evolution models are well-tested for solar-metallicity stars but less so at the extreme low-metallicity end, where opacity tables and nuclear reaction rates carry larger uncertainties.

Extragalactic origin. The claim that J0715-7334 originated in the LMC depends on backward orbital integration over billions of years. Such calculations require assumptions about the Milky Way's gravitational potential, the LMC's mass and past trajectory, and the effects of dynamical friction — all of which carry uncertainties. An orbit consistent with LMC origin does not constitute proof; it establishes plausibility. Alternative origins — a tidally disrupted dwarf galaxy other than the LMC, or the Milky Way's own outer halo — cannot be excluded on kinematic grounds alone [5][10].

Single-progenitor model. The inference that J0715-7334's metals came from a single 30-solar-mass supernova is a best-fit result, not a unique solution. Different supernova models (varying progenitor mass, explosion energy, mixing, and fallback) can produce overlapping abundance patterns, and the number of measured elemental abundances in such a metal-poor star is necessarily limited [5][6].

Independent verification. As of April 2026, no independent group has published a reanalysis of the MIKE spectra or attempted to reproduce the abundance measurements. Bothwell expressed confidence that "the models will get updated based on this," but also framed revision as "part of doing good science" — an implicit acknowledgment that the current interpretation may evolve [10]. The Nature Astronomy publication has undergone peer review, but replication by separate teams using independent observations would strengthen the claims considerably.

What Comes Next

SDSS J0715-7334 represents a single data point, but a powerful one. If its extragalactic origin holds up under further scrutiny, it demonstrates that the Large Magellanic Cloud — a galaxy still in the process of merging with the Milky Way — contained pockets of nearly primordial gas that formed stars within the first few billion years of cosmic history. That has direct implications for models of how galaxies grow through accretion and what chemical conditions prevailed in satellite galaxies during the epoch of reionization.

The discovery also validates the approach of putting survey data directly into students' hands. The next generation of astronomical surveys will produce datasets orders of magnitude larger than SDSS-V. Whether through formal coursework or citizen science programs, the analytical workforce needed to mine those datasets for rare objects extends well beyond the traditional professoriate.

"This ancient immigrant gives us an unprecedented look at conditions in the early universe," Ji said [1]. The star has been drifting for billions of years. It took a classroom of undergraduates to notice it.