More than 13 billion years ago, before the universe had produced a single atom heavier than helium, the first stars ignited. Theory predicts they were giants — tens or hundreds of times more massive than the Sun — but direct observational evidence has remained elusive. Now, JWST is beginning to change that.
The focus of this study is Hebe: two compact galaxy components (C1 and C2) near the well-known galaxy GN-z11, located over 13 billion light-years away. Both components emit strongly in ionized helium — a signature of the extreme ultraviolet radiation produced only by the most massive and hottest stars. Their spectra show virtually no trace of metals.
Through detailed modeling, the researchers demonstrate that the observed properties can only be reproduced if more than 50% of the stellar mass in these galaxies belongs to Population III stars — the universe's first, metal-free generation. Component C1 is consistent with a purely pristine stellar system.
The key result is a constraint on the initial mass function (IMF) of the first stars. The HeII-to-Hγ ratio rules out steep IMFs dominated by low-mass stars, favoring instead top-heavy distributions where massive stars dominate — particularly if the system is younger than one million years. Combined with the measured HeII luminosity, this yields a total stellar mass between 20,000 and 600,000 solar masses.
Crucially, these constraints independently align with those from near-field cosmology — the chemical fingerprints left by ancient stars in our own Galaxy. Near-field data exclude the flattest IMFs; far-field data exclude the steepest. Together, they define a data-driven corridor of viable first-star IMFs, linking characteristic mass and slope.
This is the first time a direct observation of a high-redshift PopIII system has placed independent quantitative constraints on the nature of the first stars — opening a new observational window on how the universe lit its very first light.