For roughly the first 200-300 million years after the Big Bang, the universe held no stars at all. Astronomers call this period the cosmic dark ages, when space was filled mostly with neutral hydrogen and helium left over from primordial nucleosynthesis. The moment darkness ended and the first stars ignited is known as cosmic dawn.
These stars, known as Population III or PopIII, are thought to have formed from gas containing no elements heavier than helium. Without metals to help cool collapsing gas clouds, the resulting stars could grow to enormous sizes — tens or hundreds of times more massive than the Sun, according to theory. Directly observing such objects at distances beyond 13 billion light-years has long been out of reach. A new analysis of JWST data is changing that.
The Hebe system near GN-z11
The study focuses on a system called Hebe — two compact components labeled C1 and C2, located close to the well-known distant galaxy GN-z11.
Both components show strong emission from ionized helium (HeII), a signature that only the hottest, most massive stars can produce, since generating the extreme ultraviolet radiation needed to ionize helium requires exceptional stellar temperatures. Their spectra show almost no trace of metal lines.
Using a locally calibrated model, the research team demonstrated that the only way to reproduce the measured upper limits on metal lines is if more than 50% of the stellar mass in these galaxies belongs to primordial PopIII stars. Component C1 turned out to be consistent with a purely pristine system, containing no detectable metals at all.
What the HeII-to-Hγ ratio reveals
The key result of the study goes beyond confirming the PopIII nature of the system — it places a quantitative constraint on the initial mass function (IMF) of these first stars, which describes how many stars of each mass form in a single star-formation episode.
The researchers used the ratio of the HeII emission line to the hydrogen Hγ line as an indicator. This ratio rules out steep IMFs, in which low-mass stars dominate. Instead, the data favor top-heavy distributions, where massive stars are the norm — particularly if the stellar population is younger than one million years.
Combining this constraint with the measured HeII luminosity, the authors estimate a total stellar mass for the system of between 20,000 and 600,000 solar masses. Adopting the lower masses predicted by star-formation simulations — below 100,000 solar masses — strengthens the preference for top-heavy IMFs even further.
Notably, degeneracies remain in the results: the shape of the IMF, the total stellar mass, and the age of the system are interrelated, and the current data cannot fully disentangle their individual contributions.
Near-field versus far-field constraints
Before this study, the only data-driven constraints on the IMF of the first stars came from near-field cosmology — the study of chemical fingerprints preserved in the oldest stars of our own Galaxy, remnants of elements forged and scattered by the very first stellar explosions.
Near-field data rule out the flattest IMFs, where low-mass stars would be overrepresented relative to massive ones. The Hebe observations, obtained at extreme redshift, work in the opposite direction, excluding the steepest distributions.
Together, these two independent approaches — chemical traces in ancient Milky Way stars and a direct observation of a pristine galaxy 13 billion light-years away — define a narrow, data-driven corridor of viable first-star IMFs, linking characteristic mass and slope.
The authors note that this marks the first time direct observations of a high-redshift PopIII system have provided independent quantitative constraints on the nature of the first stars. The approach opens a new observational path for testing theories of early star formation — and potentially for identifying similar pristine systems in future JWST observations.
