Science Enabled by a 30-Meter-Class Telescope in the Northern Hemisphere: Massive Stars at Low Metallicity

Massive stars are at the core of our observations of the Universe up to the reionization epoch, both through their intense ionizing fluxes and through the energetic end products that release fresh elements into the interstellar medium. Our interpretation of very high redshift galaxies and transient phenomena depends on knowledge derived from massive star populations in the Milky Way and nearby galaxies, with characteristics that only remotely resemble the conditions in the early Universe. However, the models supporting these interpretations have been tested in a narrow range of environments and carry significant uncertainties when extrapolated. Advancing in our understanding of the Universe beyond the Local Volume therefore requires extending massive star studies to conditions representative of the early Universe. The next generation of telescopes has the potential to accomplish this goal.

💡 Research Summary

The paper makes a compelling case that a 30‑meter class telescope in the Northern Hemisphere is essential for advancing our understanding of massive stars in low‑metallicity environments, which in turn is crucial for interpreting the early Universe. Massive stars dominate the ionizing photon budget and the chemical enrichment of galaxies from the epoch of reionization to the present day. However, current stellar evolution models have been calibrated only in a narrow metallicity range (≈0.2–0.5 Z⊙) represented by the Milky Way and the Magellanic Clouds. Extrapolating these models to the much lower metallicities (≈0.01–0.1 Z⊙) that characterized the bulk of star formation at redshifts z≈2–10 introduces large uncertainties.

The authors identify several key physical processes that are poorly constrained at low Z: (1) the efficiency of radiatively driven winds (RDW), which may break down for low‑luminosity, low‑metallicity stars, altering mass‑loss rates, wind opacity, and the emergent ionizing spectrum; (2) the prevalence of chemically homogeneous evolution (CHE), which could boost He II nebular emission and produce massive binary black‑hole progenitors, yet observational evidence for CHE at low Z remains scant; (3) the possible metallicity dependence of the upper end of the initial mass function (IMF), with current observations limited to ≈200 M⊙ stars in the LMC but lacking detections above ≈80 M⊙ in more metal‑poor systems; (4) the interplay between rotation, binarity, and magnetic fields, all of which shape the final fate of massive stars and may behave differently when metallicity is low; and (5) the occurrence of exotic end‑states such as pair‑instability supernovae and super‑luminous supernovae, which are highly sensitive to the core mass and wind history.

To break these degeneracies, the paper proposes a “metallicity ladder” that moves from the Milky Way and Magellanic Clouds to progressively more metal‑poor dwarf galaxies: Sex A (≈0.1 Z⊙, distance ≈1.4 Mpc), IC 10 (≈0.2 Z⊙, northern sky, high extinction), and the benchmark I Zw18 (≈0.02 Z⊙, SFR≈0.6 M⊙ yr⁻¹, distance ≈18 Mpc). Existing 8–10 m facilities and JWST lack the sensitivity, spatial resolution, or UV coverage to obtain high‑S/N, high‑resolution spectra of individual massive stars in these targets. A 30 m telescope equipped with high‑multiplex spectrographs, adaptive optics, and a UV integral‑field unit would enable the detection of stars down to V≈20 mag with S/N ≈ 30–50 in exposure times of order one hour, providing precise measurements of wind velocities, surface abundances, rotation rates, and binary signatures.

The authors stress that such observations must be coupled with space‑based assets (HST, JWST, and the upcoming Habitable Worlds Observatory) to obtain complementary UV imaging and spectroscopy. The synergy will allow population synthesis of unresolved high‑redshift galaxies to be anchored in empirically calibrated low‑Z stellar libraries, improving estimates of ionizing photon production, chemical yields, and the rates of gravitational‑wave progenitors.

In conclusion, the paper argues that without a 30‑meter class telescope in the Northern Hemisphere, the community will remain unable to test the fundamental assumptions underlying massive‑star physics at the metallicities that dominated cosmic star formation. The proposed observations will directly inform models of stellar winds, rotation, binarity, IMF upper limits, and exotic end‑states, thereby refining our interpretation of reionization, early chemical enrichment, and the origins of high‑energy transients across cosmic time.