The early universe, existing approximately 400 million years after the Big Bang, was characterized by its chemical simplicity, consisting entirely of hydrogen, helium, and trace amounts of lithium, while completely lacking the heavier elements astronomers classify as "metals." For decades, scientists have hypothesized that the first stars, known as Population III, formed from this primordial gas; however, detecting them remained extremely difficult due to vast distances and the effects of redshift prior to the launch of the James Webb Space Telescope (JWST).
Analysis of JWST data, initially presented in 2024 and corroborated by studies in 2026, has provided the first direct evidence for the existence of Population III stars. Researchers identified a source of radiation, designated "Hebe," within the halo of the high-redshift galaxy GN-z11, observed at a time approximately 430 million years after the Big Bang. The Hebe object is located about three kiloparsecs (kpc) from the center of GN-z11, a galaxy exhibiting a redshift of z≈10.6. An international team of scientists utilized the JWST’s Near-Infrared Spectrograph (NIRSpec-IFU) to conduct a detailed analysis of the light emanating from the Hebe region.
The resulting spectrum unequivocally confirmed the total absence of emission lines for carbon, neon, oxygen, and other heavy elements, proving the chemical purity of the gas. Despite the zero-metallicity environment, the object displayed an intense signal in the spectral line of doubly ionized helium (HeII) at a wavelength of 1640 Å. Generating HeII requires ultraviolet photons with energies exceeding 54.4 electron volts, a threshold that rules out ordinary stars like the Sun. The high equivalent width of this emission (over 20 Å) aligns with models predicting an upper limit for the initial mass function (IMF) of such stars of at least 500 solar masses (M⊙).
The research team meticulously tested and largely dismissed alternative explanations. The hypothesis of an accreting supermassive black hole (AGN) was rejected because the narrow HeII line lacked the kinetic line broadening typically predicted by high gas velocities. The possibility of Wolf-Rayet stars was also excluded, as their stellar winds depend on heavy elements that are absent from the metal-free Hebe region. The exclusion of these models led researchers to conclude that the ionizing source must be a cluster of Population III stars. Based on the luminosity of the HeII line, a total stellar mass of approximately 2 x 10^5 M⊙ was derived for this burst of star formation.
Theoretical modeling led by Elke Roest suggests that these primitive stars had an IMF skewed toward very high-mass objects, ranging from ten to one hundred times the mass of the Sun. This observation supports theories that the absence of heavy elements resulted in a hotter collapse, favoring the formation of extremely massive stars in the early universe. These massive stars ended their short lives as pair-instability supernovae, dispersing the first heavy elements and thereby enabling subsequent cosmic evolution. A separate study led by Professor Roberto Maiolino of the University of Cambridge initially found signs of this primordial gas cloud in 2024, with independent confirmation later provided by the detection of the Hγ line at the same location and redshift, as reported in a companion paper.
