Invisible Firstborns: How to Find the Universe's First Stars express
Immediately after the Big Bang, an idea championed by Georges Lemaître, the cosmos was a dark broth of hydrogen and helium. Then the first stars ignited — enormous, dazzlingly bright and hot, like sparks from a cosmic bonfire. They lived furiously and died young, scattering the first heavy elements. Most have long since faded, but as Edwin Hubble showed, the Universe is expanding, and in distant galaxies, pockets of pristine gas still give birth to these giants. The James Webb Space Telescope hunts for their light.
The key lies in the spectral fingerprint. Young clusters of such stars emit a powerful glow in the ionized helium line. This “helium stamp” is so bright that Webb’s instruments can capture it from over 13 billion light-years away. Ordinary stars, tainted by heavy elements, can't produce such a flare — like damp logs that won’t ignite. Even in galaxies dominated by mature stars, Webb can spot isolated stellar nurseries where primordial sparks reignite.
Detecting this helium afterglow means witnessing the Universe's journey from darkness to light.
Original abstract
as written by the authors arXiv:2603.27582
Finding the first generation of (Population III or Pop III) stars is one of the most ambitious and exciting challenges of astrophysics. JWST opened concrete prospects for their detection during the Epoch of Reionization (EoR), where increasing evidence suggests that residual Pop III formation may persist, even within pristine pockets of high-mass halos, due to inhomogeneous enrichment. However, the identification of Pop III stars within globally enriched environments will be challenging. We investigate the detectability of a subdominant Pop III component in/around massive ($$M_\star \gtrsim 10^9 ~\mathrm{M_\odot}$$) galaxies at $$z \approx 6.5 - 9$$ from the dustyGadget cosmological simulation suite, and the confusion arising from second-generation (Pop II) stars in their surroundings. We find that young ($$\lesssim 1$$ Myr), massive ($$M_\mathrm{III} \sim 6 \times 10^5 ~\mathrm{M_\odot}$$) Pop III clusters forming within these galaxy environments are responsible for strong HeII1640 line emission ($$L_\mathrm{HeII1640} \gtrsim 10^{41} ~\mathrm{erg \, s^{-1}}$$), which would be detectable with $$\approx 10 (50)$$ h of medium-resolution observations with NIRSpec/IFU at $$z \approx 6 (10)$$. These bright luminosities cannot be produced by standard Pop II populations alone. On the other hand, the dominant Pop II component within massive ``hybrid'' Pop III hosts powers strong metal line emission ($$L_\mathrm{[OIII]5007} \gtrsim 10^{42} ~\mathrm{erg \, s^{-1}}$$), indicating that the detection of metal lines alone cannot exclude the presence of Pop IIIs in high-$$z$$ galaxy environments. We further discuss candidate selection strategies based on Ly$$\alpha$$, H$$\alpha$$ and H$$\beta$$ emission, and how spatially resolved observations may enable the detection of isolated, pristine pockets in the outskirts of massive halos.
If such a primordial giant star ignited nearby, it would shine tens of millions of times brighter than the Sun and vaporize all the oceans in seconds.