A new University of Utah study uses the Hubble Space Telescope to investigate how massive stars evolve in chemically primitive environments. The project, known as the Extremely Metal-Poor O Star Trove (TEMPOS), uses ultraviolet (UV) observations from Hubble’s Cosmic Origins Spectrograph (COS) to study massive stars in nearby dwarf galaxies that resemble conditions found in the early Universe.

The unusually large TEMPOS data set could improve models of massive stars and help researchers understand how these powerful objects influenced young galaxies. That work is becoming increasingly important as the James Webb Space Telescope, launched in 2021, continues to reveal unexpectedly complex galaxies from the Universe’s early history.

“Webb has uncovered entirely new questions about the evolution of these early galaxies. Galaxies are strange,” said Grace Telford, assistant professor in the University of Utah’s Department of Physics and Astronomy and lead author of the study. “This is the scientific motivation behind the TEMPOS program: to help us understand what is happening in these early galaxies.”

This study was completed on September 21, 2026. Astrophysical Journal Appendix Series.

Why massive stars shape entire galaxies

Stars more than 10 times the mass of the Sun are rare, but their influence extends across their host galaxies. They emit enormous amounts of radiation, continuously lose material through stellar winds, and eventually end their short lives in supernova explosions.

“They burn very hot, bright, and fast, and end their short lives as supernova explosions that deposit large amounts of energy and material into the surrounding gas,” Telford said. “They control the evolution of their host galaxies by essentially controlling the gas available to heat and cool and form new stars.”

One of the most important differences between early galaxies and modern galaxies is their chemical composition. Astronomers use the term metallicity to describe the abundance of elements heavier than hydrogen and helium in stars and galaxies.

Early galaxies produced far fewer heavy elements than galaxies such as today’s Milky Way. As a result, the massive stars that formed in those environments may have had physical properties that differ significantly from those of the massive stars astronomers can study nearby.

“Massive stars with low metallicity are particularly important for building accurate models of early galaxies,” Telford said. “And we can’t just study how the massive, metal-rich stars in the Milky Way behave and interpret those observations.”

Nearby dwarf galaxies provide a window into the early Universe

Astronomers cannot study individual massive stars in the distant early Universe in enough detail to directly measure many of their properties. TEMPOS therefore focused on relatively nearby dwarf galaxies with chemical compositions that provide useful comparisons.

The study examined 29 massive stars across six local dwarf galaxies. Every galaxy in the sample contains less than one-fifth of the Sun’s metallicity, making these stars useful stand-ins for studying conditions that were common much earlier in cosmic history.

Ultraviolet light provides especially valuable information about massive stars. Their UV spectra contain clues about chemical elements in their atmospheres and reveal details about the stellar winds that carry material away from their surfaces.

Obtaining these measurements is difficult because individual massive stars beyond the Milky Way are extremely faint. Studying them can require hours of observing time with some of the world’s most powerful telescopes.

“This is a sample of 29 stars, which doesn’t seem like a lot, but when you consider that each observation takes up to 35 Hubble hours, it becomes very expensive,” Telford said.

TEMPOS added new observations of 12 stars to previously collected measurements, creating a larger and more consistent data set for comparison.

Stellar winds slow dramatically at very low metallicity

Massive stars gradually eject material through powerful stellar winds. The strength and speed of those winds are closely related to a star’s metallicity.

Metal ions help transfer energy from a star’s radiation to surrounding material and push that material outward. Scientists therefore expect stars with fewer heavy elements to have weaker winds and lose less mass during their lifetimes.

TEMPOS confirmed the general pattern predicted by astronomers: as metallicity decreases, the maximum velocity of a stellar wind also tends to decrease.

However, the most metal-poor stars revealed an unexpected result.

For stars with less than about 10% of the Sun’s metallicity, wind speeds fell much more sharply than predicted by the trends measured at higher metallicities.

“There’s a sort of smooth trend, and then all of a sudden the wind speeds actually drop off for the least metallic stars,” Telford said. “I was so excited to find that fun surprise in the data.”

This rapid decline could significantly affect how massive stars live and die. If extremely metal-poor stars lose less material through stellar winds, they could retain more of their original mass throughout their lifetimes.

That difference could alter their later evolution, eventual deaths, and the amount of energy and matter they return to their surroundings. Because massive stars help regulate the gas available for future star formation, changes within individual stars can ultimately affect the development of entire galaxies.

Iron may hold the key to understanding massive-star winds

Among the heavy elements found in massive stars, iron may be particularly important. It helps propel stellar winds, influences how massive stars evolve, and plays a role in the processes leading to their eventual supernova explosions.

However, measuring iron in extremely metal-poor environments is notoriously difficult.

Astronomers often estimate metallicity by measuring the amount of oxygen in a galaxy’s gas. Oxygen ions are relatively easy to detect because they produce bright emission lines when illuminated by massive stars.

Researchers often assume that iron abundance closely follows oxygen abundance. However, the two elements do not necessarily increase or decrease in exactly the same way.

TEMPOS allows researchers to investigate that relationship more directly by examining the extremely faint absorption signature of iron in ultraviolet spectra. The team measured how much UV light was absorbed by iron compared with the amount of light that would otherwise be present.

The researchers found that massive stars in more oxygen-rich, metal-rich galaxies generally show much stronger iron absorption than stars in oxygen-poor, metal-poor galaxies.

At the same time, the range of iron absorption measured by TEMPOS indicates that metal-poor stars can contain surprisingly different amounts of iron.

“This is the first time that we can statistically confirm the trend across a large sample of stars in six galaxies with different chemical compositions,” Telford said. “TEMPOS provides the basis for determining how the physics of massive stars changes as iron abundance changes at very low metallicities.”

Larger samples reveal hidden trends in massive stars

Before TEMPOS, Telford carried out detailed modeling of three stars that are now included in the larger survey.

That small data set was not large enough to reveal the broader patterns emerging from observations of dozens of stars.

“You don’t see these trends with just three,” she said. “We’ve always been stuck with this small number of statistical regimes, so this is our best attempt at building a large enough sample to do more useful things.”

The researchers now plan to expand their analysis by combining Hubble’s ultraviolet measurements with visible-light observations collected at the Keck Observatory in Hawaii.

Using both data sets, scientists will be able to build more detailed models of massive stars and calculate properties such as chemical abundance and the rate at which stellar winds strip away mass.

These measurements could ultimately improve the models used to interpret the unusual early galaxies being observed by the James Webb Space Telescope.

TEMPOS’ science-ready UV spectra will also be made publicly available through the Mikulski Archive for Space Telescopes, allowing other researchers to investigate additional questions about massive stars and their influence on galaxy evolution.

Collaborators on the study are Christiana Erba and the Dowing Planetarium at California State University, Fresno; 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 the Christian-Albrecht University Kiel and the Astronomical Research Institute (ARI); John Chisholm and Daniel 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.

This research is based on observations with the NASA/ESA Hubble Space Telescope and was supported by NASA through grant numbers GO-16767, GO-16920 and GO-17491.