The more astronomers learn about the universe’s earliest galaxies, the stranger they seem. Many of their surprising properties may be explained by differences between their massive stars and those in galaxies like our own Milky Way. A new University of Utah-led survey with the Hubble Space Telescope is shedding light on the stellar astrophysics operating in early galaxies.
The survey, called the Treasury of Extremely Metal-Poor O Stars ( TEMPOS ), uses ultraviolet (UV) observations from Hubble’s Cosmic Origins Spectrograph (COS) to study massive stars in nearby galaxies that are the best available analogs of stars in the early universe.
The unprecedently large dataset from TEMPOS could help astronomers build better models of massive stars to understand how they shaped galaxies when the universe was young. Such models are essential to interpret observations of early galaxies now coming from the James Webb Space Telescope, which launched in 2021.
“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 published on Sept. 21, 2026, in The Astrophysical Journal Supplement Series .
Engines of the galaxies
Massive stars—those with masses more than 10 times greater than the sun—are rare but powerful engines of galactic change. They produce intense radiation, shed material through stellar winds and eventually explode as supernovae.
“They burn very hot, bright and fast and they end their short lives as supernova explosions that deposit a lot of energy and material into the surrounding gas,” said Telford. “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.”
Astronomers use the term metallicity to describe the abundance of elements heavier than hydrogen and helium. The universe’s earliest galaxies contained far fewer heavy elements than galaxies like the Milky Way do today. The massive stars forming in those galaxies likely also had different physical properties.
“Massive stars at low metallicity are particularly important for building accurate models of early galaxies,” Telford said. “And we can’t just study how metal-rich massive stars in the Milky Way behave to interpret those observations.”
Looking closer to home
Instead, TEMPOS looked to nearby, low-mass dwarf galaxies, which have low metallicities more typical of galaxies near the dawn of the universe. TEMPOS surveyed 29 massive stars across six local dwarf galaxies that all have metallicities below one-fifth of that of the sun.
The UV spectrum contains detailed signatures of elements in the stars’ atmospheres and reveals information about the stellar winds that continually blow material away from their surfaces. Individual massive stars in these galaxies outside the Milky Way are very faint, requiring many hours of observation with some of the most powerful telescopes available.
“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.
TEMPOS combined new observations of 12 stars with previously collected data into a larger, more uniform dataset.
Fewer metals, slower winds
Massive stars lose mass through stellar winds, and the strength of the winds depends on metallicity. Metal ions couple the star's radiation to the surrounding material, so astronomers expect lower-metallicity stars to drive weaker winds and lose less mass over their lifetimes.
The TEMPOS observations showed the expected overall trend: as metallicity decreases, the maximum speed of the stellar winds decreases as well.
But at the lowest metallicities—stars with metallicity below about 10% of the sun's—the wind speeds decline much more sharply than expected from trends observed at higher metallicities.
“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.”
If extremely metal-poor stars lose less mass through weaker winds, they may retain more of their original mass, affecting how they evolve, die and shape their host galaxies. Because massive stars influence the gas around them, changes in their evolution could ripple outward to affect their host galaxies.
Stellar chemistry: oxygen and iron
Iron may be the most important element in massive star physics. It plays a key role in launching stellar winds, determining how a star will evolve throughout its lifetime, and triggering the supernova explosions that end massive stars’ lives.
Despite its crucial role, iron abundance is notoriously difficult to measure in metal-poor environments. Instead, astronomers often use oxygen in a galaxy’s gas to estimate its metallicity because oxygen ions produce bright, easily observed emission lines when illuminated by massive stars. They assume the iron abundance matches the oxygen—but it’s not guaranteed that iron and oxygen would track each other perfectly.
The TEMPOS team measured the strengths of hard-to-detect iron absorption features in the UV spectra. Basically, they assessed how much light the iron was removing from what would otherwise be a flat level of ultraviolet light.
They found that massive stars in more oxygen-rich (high metallicity) galaxies tend to have much stronger iron absorption in their UV spectra than stars in oxygen-poor (low metallicity) galaxies. The variation in iron absorption strengths in the TEMPOS dataset 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.”
Sample size matters
Telford had previously modeled three of the stars in the TEMPOS sample in detail.
“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.”
This work is just the beginning. The researchers are now combining the Hubble UV spectra with visible-light observations from the Keck Observatory in Hawaii. Together, the datasets will allow them to model the stars in greater detail and measure properties such as chemical abundances and wind-driven mass-loss rates—information that could ultimately help astronomers interpret what Webb is seeing in the early universe.
The TEMPOS science-ready UV spectra will also be made publicly available through the Mikulski Archive for Space Telescopes, providing a resource for researchers to pursue additional questions about massive stars and their role in galaxy evolution.
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The survey’s collaborators are Christiana Erba of California State University, Fresno and the Dowing Planetarium; Kristen McQuinn of the Space Telescope Science Institute (STScI) and Rutgers University; Calum Hawcroft, Julia Roman-Duval and Claus Leitherer of STScI; Andreas Sander of Christian-Albrechts-Universität zu Kiel and the Astronomisches Rechen-Institut (ARI); John Chisholm and Danielle Berg of The University of Texas at Austin and the Cosmic Frontier Center; Varsha Ramachandran of ARI; Yong Zheng of Rensselaer Polytechnic Institute; Abby Mintz of Princeton University; and Evan Kirby of the University of Notre Dame.
The work was based on observations with the NASA/ESA Hubble Space Telescope and was supported by NASA (grant numbers GO-16767, GO-16920, and GO-17491).
Survey, The treasury of extremely metal-poor O stars, published on Sept. 21, 2026 in The Astrophysical Journal Supplement Series . DOI: 10.3847/1538-4365/ae95f6
The Astrophysical Journal Supplement Series
Observational study
Not applicable
The treasury of extremely metal-poor O stars
21-Sep-2026