Hubble’s TEMPOS survey measured ultraviolet spectra of 29 massive, extremely metal-poor O-type stars in nearby dwarf galaxies to model the kinds of stars that existed in the first few hundred million years after the Big Bang.
TEMPOS used Hubble’s Cosmic Origins Spectrograph to collect the largest UV dataset of extremely metal-poor massive stars to date.
These metal-poor O stars have much weaker stellar winds than similar-mass stars in the Milky Way, implying they lose less mass before exploding as supernovae.
Weaker winds and lower metallicity change how massive stars deposit energy and elements into their host galaxies, altering galaxy evolution and appearance in the early universe.
# Why early galaxies looked strange
# What TEMPOS measured
The survey targeted 29 massive, metal-poor O-type stars in low-mass dwarf galaxies whose gas contains far fewer elements heavier than hydrogen and helium than the Sun. The UV spectra reveal two crucial things: the stars' metal content and the properties of their stellar winds. Those winds are the main channel by which massive stars lose material over their short lifetimes.
# Why metallicity matters
In stellar physics, elements heavier than hydrogen and helium are called metals. Metal ions help couple a star's radiation to its outer layers, driving stronger stellar winds. At low metallicity, that coupling weakens, so winds are weaker and mass loss through winds declines.
TEMPOS shows a clear pattern: the lowest-metallicity stars in the sample have sharply reduced wind strengths compared with similarly massive stars in the Milky Way. That implies massive stars in early, metal-poor galaxies would retain more of their mass until they die in supernovae.
# How this reshapes models of early galaxies
Massive, metal-poor stars were common in the first few hundred million years after the Big Bang. They burned hot, bright, and fast and then exploded, returning newly forged elements to the interstellar medium. TEMPOS provides empirical data that help update models of those stars' lifecycles and feedback effects.
# Why surveying nearby dwarf galaxies works
The Milky Way and its massive stars are relatively metal-rich, so they are not good stand-ins for the first generations of stars. Low-mass dwarf galaxies in the local universe retain low metallicities that better match conditions in the infant cosmos. Observing massive stars in those systems therefore gives the best available direct constraints on how early massive stars actually behaved.
The TEMPOS dataset is the largest UV collection of extremely metal-poor massive stars so far. It identifies promising targets for deeper astrophysical study and supplies measured spectra that can be fed into stellar population and galaxy-evolution models. Those updated models will improve interpretations of current and future observations of high-redshift galaxies.
# What remains to investigate
New Hubble Space Telescope TEMPOS survey of 29 massive stars in extremely metal-poor galaxies finds unexpectedly weak stellar winds and delivers the largest-ever dataset of its kind, which could reshape our understanding of how galaxies formed in the early universe.
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