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The first stars in the Universe, known as Population III or Pop. III stars, are thought to have formed from pristine gas made almost entirely of hydrogen and helium. Unlike stars today, they contained no heavy elements. Because of this, they may have been very massive, very hot, and very bright in ultraviolet radiation. Finding them would give us a direct glimpse of the earliest stages of the cosmic history, when the first galaxies began to form and the Universe started to become chemically enriched.

However, no Pop. III stars have been directly observed so far. They are expected to be short-lived, distant, and rare. One possible way to search for them is by indirect observations, for example by detecting the light emitted by the gas around them. Massive Pop. III stars produce many ionising photons, which let the surrounding hydrogen gas glow in characteristic emission lines. In this work, we studied whether the hydrogen Balmer lines — especially Hα — could be detected by the James Webb Space Telescope. To answer this, we used the semi-analytical model A-SLOTH, which follows the formation of early dark matter halos, gas reservoirs, Population III and Population II star formation, and feedback from radiation and supernova explosions. This is important because previous estimates often asked whether JWST could detect  a Pop. III star cluster of a given mass. Our approach instead asks a more fundamental physical question: do such massive Pop. III star clusters form at all in a realistic model of early galaxy formation?

The answer from our simulations is: usually, no. Pop. III star formation occurs in short bursts. Once the first massive stars form, their radiation heats and ionises the surrounding gas. Soon after, supernova explosions eject gas and enrich the halo with metals. This feedback rapidly shuts down further metal-free star formation and triggers the transition toward Population II stars. As a result, the Pop. III stellar populations in our models typically remain small, with young Pop. III stellar masses per halo of only about 10–10,000 solar masses. This has direct consequences for observability. The predicted Hα fluxes from Pop. III star-forming halos are well below the JWST/NIRSpec detection threshold for a 10,000-second exposure. The higher Balmer lines, such as Hβ, Hγ, and Hδ, are even fainter. The Figure illustrates that even the most massive Pop. III systems produced in the model remain far below the flux needed for a detection with JWST. Detectable Balmer emission would require Pop. III stellar masses roughly above ${10}^5{\text{M}}_{\odot }$. However, such massive system do not form in our simulation.