The survey, called the Treasury of Extremely Metal-Poor O Stars, or TEMPOS, used ultraviolet observations from the Hubble Space Telescope’s Cosmic Origins Spectrograph to study massive stars in nearby dwarf galaxies that serve as the best available analogs for stars in the early universe. Published on September 21, 2026, in The Astrophysical Journal Supplement Series, the project assembled the largest dataset of its kind ever collected, covering 29 massive stars spread across six local dwarf galaxies, each with a metallicity below one-fifth of the sun’s. Astronomers use the term metallicity to describe the abundance of elements heavier than hydrogen and helium, and the universe’s first galaxies contained far fewer of these heavy elements than mature galaxies do today.

“Webb opened up a whole bunch of new questions about the evolution of these early galaxies—they’re weird,” said Grace Telford, assistant professor in the Department of Physics & Astronomy at the University of Utah and lead author of the study. “That’s the scientific motivation behind the TEMPOS program: to help understand what is going on in these early galaxies.” The survey’s unprecedented scale matters because individual massive stars in galaxies beyond the Milky Way are extraordinarily faint targets. Each star required up to 35 hours of precious Hubble observing time, making the construction of a statistically meaningful sample an expensive and painstaking undertaking that previous efforts had never achieved.

The scientific case for studying these stars rests on their outsized influence. Massive stars, defined as those with masses more than ten times greater than the sun, are rare but powerful engines of galactic change. They burn hot, bright and fast, flood their surroundings with intense radiation, shed material through stellar winds, and ultimately die as supernova explosions that deposit enormous amounts of energy and chemical enrichment into the surrounding gas. “They govern the evolution of their host galaxies by heating and essentially regulating the gas that’s then available to cool and form into new stars,” Telford explained. In a young galaxy, this feedback helps determine how efficiently the galaxy converts its gas reservoir into new generations of stars.

The stars at the center of the survey belong to a particularly consequential class. O-type stars are among the biggest and brightest stars in the universe, and they live relatively short lives by stellar standards. Near the end of their lifetimes, the most massive of them evolve into Wolf-Rayet stars, which release huge quantities of mass into space through powerful stellar winds that strip away their outer layers and expose their hot inner interiors. Understanding how these winds behave at low metallicity is therefore essential for modeling how the first generations of massive stars enriched the cosmos and sculpted the environments in which later stars formed.

The physics of stellar winds is intimately tied to metallicity. Metal ions within a star’s atmosphere couple the outgoing radiation to the surrounding material, effectively allowing light to push gas off the stellar surface. Astronomers have long expected that stars born with fewer heavy elements should drive weaker winds and lose less mass over their lifetimes. The TEMPOS observations confirmed the expected overall trend: as metallicity decreases, the maximum speed of the stellar winds decreases as well. But the data also delivered a surprise. At the lowest metallicities, in stars with metallicity below about ten percent of the sun’s, the wind speeds declined much more sharply than extrapolations from higher-metallicity trends would predict.

“There’s sort of a smooth trend and then suddenly for lowest-metallicity stars, the wind speed really drops off,” Telford said. “I was so excited to find that fun surprise in the data.” The discovery carries significant implications. If extremely metal-poor stars lose less mass through weaker winds, they retain more of their original mass as they age, which alters how they evolve, how they die, and how much energy and material they inject into their host galaxies. Because massive stars regulate the gas around them, changes in their mass-loss behavior could cascade outward, affecting star formation rates, gas temperatures and the ionizing radiation output that shapes a galaxy’s observable signature in the early universe.

Beyond wind speeds, the survey probed the chemistry of stellar atmospheres in a way no previous study had managed. Iron may be the single most important element in massive star physics: it plays a key role in launching stellar winds, determining how a star evolves throughout its lifetime, and triggering the supernova explosions that end massive stars’ lives. Yet iron abundance is notoriously difficult to measure in metal-poor environments. Instead, astronomers often use oxygen in a galaxy’s gas as a proxy for overall metallicity, because oxygen ions produce bright, easily observed emission lines when illuminated by massive stars. The convenience comes with an assumption—that iron and oxygen abundances track each other perfectly—which is not guaranteed to hold.

To test that assumption, the TEMPOS team measured the strengths of hard-to-detect iron absorption features in the ultraviolet spectra, assessing how much light the iron removed from what would otherwise be a flat level of ultraviolet emission. The results showed that massive stars in more oxygen-rich galaxies tend to have much stronger iron absorption in their UV spectra than stars in oxygen-poor galaxies, and the variation in absorption strengths across the sample suggests that these metal-poor stars span a wide range of iron abundances. “This is the first time we’ve had the statistical power to see that trend across a large sample of stars in six galaxies, all with different chemical compositions,” Telford said. “TEMPOS gives us the foundation for determining how massive-star physics changes as iron abundance changes in the very low-metallicity regime.”

The work marks a decisive break from a long-standing bottleneck in the field. Telford had previously modeled three of the stars in the TEMPOS sample in detail, but such small numbers could never reveal population-level trends. “With only three, you don’t see these trends,” she said. “We’ve always just been stuck in this low number statistics regime, so this is our very best attempt to build a big enough sample to do something more useful.” The researchers are now combining the Hubble ultraviolet spectra with visible-light observations from the Keck Observatory in Hawaii, a pairing that will allow them to model the stars in greater detail and measure properties such as chemical abundances and wind-driven mass-loss rates. Meanwhile, the science-ready ultraviolet spectra will be made publicly available through the Mikulski Archive for Space Telescopes, giving the broader research community a resource for pursuing additional questions about massive stars and their role in galaxy evolution. As Webb continues to deliver puzzling portraits of the universe’s first galaxies, TEMPOS offers astronomers the stellar physics they will need to finally interpret what they are seeing.

Subject of Research: Ultraviolet spectroscopy of extremely metal-poor massive stars to model stellar winds and early galaxy evolution

Article Title: Galaxies in the early universe are weird. These stars may explain why

Article References: Galaxies in the early universe are weird. These stars may explain why. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: massive stars, stellar winds, metallicity, early galaxies, Hubble Space Telescope, James Webb Space Telescope, O-type stars, Wolf-Rayet stars, dwarf galaxies, iron abundance, supernovae, galaxy evolution