Stellar wind speeds fall off sharply below a tenth of solar metallicity in Hubble's 29-star survey

In a hot star's wind, metal ions absorb the star's radiation and pass that momentum to the gas around them, so a star with fewer metals drives a weaker wind and sheds less mass over its life ^[7]. That was the prediction going in, and TEMPOS saw the trend: as metallicity falls, the maximum wind speed falls with it ^[8]. The break came at the bottom end. Below about 10% of solar metallicity, the speeds drop much faster than the higher-metallicity trend puts them ^[9].

"There's sort of a smooth trend and then suddenly for lowest-metallicity stars, the wind speed really drops off," said Grace Telford, the University of Utah astronomer who led the survey ^[10][18]. "I was so excited to find that fun surprise in the data," she said ^[11].

TEMPOS measured the maximum speed of the wind. It did not measure the rate at which mass leaves the star. The paper's consequence is stated conditionally: if extremely metal-poor stars lose less mass through weaker winds, they keep more of their original mass, and that changes how they evolve and how they die ^[12].

The denominator is 29 stars in six dwarf galaxies, all below a fifth of the sun's metallicity ^[3]. Twelve are new Hubble observations and the other 17 come from data already collected ^[4][19]. Individual massive stars in galaxies outside the Milky Way are faint, and each needs many hours on a large telescope ^[16]. "It's a sample of 29 stars, which doesn't sound like a lot, but when each one costs up to 35 hours of Hubble time to observe, it gets really expensive," Telford said ^[5]. At that ceiling the sample is about 1,015 hours of Hubble time, roughly 42 days ^[20]. The survey is published in The Astrophysical Journal Supplement Series ^[2].

The route from these spectra to JWST runs through stellar models. This paper reanalyzes no Webb data. Telford said the program exists "to help understand what is going on in these early galaxies" ^[14], and phys.org reports that many of the surprising properties of the earliest galaxies may be explained by differences between their massive stars and those in galaxies like the Milky Way ^[17]. Milky Way massive stars are too metal-rich to stand in for the job, which is why the survey went to local dwarfs at all ^[15]. Whether the wind-speed break changes a published early-galaxy interpretation depends on the model grids adopting it first.

So the survey tests wind physics in nearby analogs ^[1]. No early galaxy was measured, and the paper does not say how many of the 29 stars sit below the 10% metallicity threshold where the decline steepens ^[21].