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.