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Hubble UV spectra show stellar winds weakening sharply in 29 metal-poor O stars
University of Utah astronomers observed 29 massive stars in six dwarf galaxies with Hubble and found winds weaken sharply below a fifth of solar metallicity. None of the stars sits in an early galaxy, so the link to Webb's early galaxies remains an inference.
The Scientist · Science desk
What happened
- A University of Utah survey called TEMPOS used ultraviolet measurements from Hubble's Cosmic Origins Spectrograph to examine massive stars in nearby galaxies that resemble early-universe environments.
- The survey covers 29 massive stars spread across six local dwarf galaxies.
- Every galaxy in the sample has a metallicity below one-fifth of the sun's, which is why its stars are being used as stand-ins for conditions common much earlier in cosmic history.
- The researchers found stellar winds weaken sharply at extremely low metallicities, which would let massive stars retain more material and evolve in ways current models do not anticipate.
- They also measured large star-to-star differences in iron abundance, an element that feeds both the winds and the eventual supernovae.
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Why it matters
- capability Wind strength can now be read off individual stars in low-metallicity galaxies, a quantity nobody can measure star by star at high redshift.
- constraint Early-galaxy models tuned on metal-rich Milky Way stars have a low-metallicity comparison set to answer to, and the lead author says the Milky Way cannot stand in for it.
- decision Modellers have to choose whether one metallicity value per galaxy is still an adequate input, given how much the iron abundances differ between stars.
- cost Deepening the sample is expensive: each of these stars is faint enough to need many hours of ultraviolet spectroscopy, so per-galaxy counts grow slowly.
A massive star's wind sets how much of the star survives to the end, and ultraviolet light is where that shows up. The spectra TEMPOS collected carry the fingerprints of elements in the stars' atmospheres along with details of the material leaving their surfaces [11]. Stars more than ten times the sun's mass are uncommon, and the influence of one can extend across an entire galaxy [10]. The survey was published on Sept. 21, 2026, in The Astrophysical Journal Supplement Series [2].
The iron result may be the more useful one. The team found large differences in iron abundance among the stars, and iron is a key ingredient in both stellar winds and supernovae [6]. If iron does much of the driving, two stars sharing the same overall metallicity need not lose mass at the same rate. A model that carries one chemical number for an entire galaxy would then be wrong about individual stars in both directions.
Twenty-nine stars in six galaxies averages fewer than five per galaxy [15]. That supports a trend across the sample better than a claim about any single system. The University of Utah calls the dataset unusually large [20], and for these targets it is: individual massive stars beyond the Milky Way are extremely faint, and the measurements can require many hours each [13].
No early galaxy was observed here. Astronomers cannot examine individual massive stars in the distant early universe in enough detail to measure many of these properties directly [12], so the survey used nearby dwarfs whose chemistry resembles conditions common much earlier in cosmic history [4]. The Utah announcement describes the weakening qualitatively and does not report mass-loss rates or the metallicity at which the winds fall away [19]. Whether the effect is large enough to change what a model predicts for the galaxies Webb has been turning up since its 2021 launch [14] turns on those numbers.
The dataset is meant to improve models of massive stars and of how they influenced young galaxies [20]. "Webb opened up a whole bunch of new questions about the evolution of these early galaxies -- they're weird," said Grace Telford, the Utah assistant professor who led the work [7]. On whether the Milky Way's metal-rich stars could be used as the reference, she said "we can't just study how metal-rich massive stars in the Milky Way behave to interpret those observations" [8].
What to watch
- The mass-loss rates and uncertainties in the published paper, and whether the weakening looks like a threshold at some metallicity or a continuous trend.
- Whether groups fitting Webb's early-galaxy spectra adopt the TEMPOS wind rates, and whether the fits actually move.
- Whether the iron scatter is galaxy-to-galaxy or star-to-star within a single galaxy.