Astronomers are finding that the first generations of galaxies do not always behave the way existing models predict. One possible reason is that their most massive stars may have evolved very differently from massive stars in the modern Milky Way. A new survey led by the University of Utah is using the Hubble Space Telescope to investigate those differences.
The project, known as the Treasury of Extremely Metal-Poor O Stars (TEMPOS), relies on ultraviolet (UV) measurements from Hubble's Cosmic Origins Spectrograph (COS) to examine massive stars in nearby galaxies that resemble the environments found in the early universe. The unusually large TEMPOS dataset could allow researchers to improve models of massive stars and better understand how these powerful objects influenced young galaxies. That work is becoming increasingly important as the James Webb Space Telescope, which launched in 2021, continues to uncover unexpectedly complex galaxies from the universe's early history.
"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 was published on Sept. 21, 2026, in The Astrophysical Journal Supplement Series. Stars more than 10 times as massive as the sun are uncommon, but their influence can extend across an entire galaxy.
They emit enormous amounts of radiation, continuously lose material through stellar winds and eventually end their lives in supernova explosions. "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." One major difference between early galaxies and galaxies today is their chemical composition.
Astronomers use metallicity to describe the amount of elements heavier than hydrogen and helium present in a star or galaxy. The earliest galaxies had produced far fewer of these heavier elements than galaxies such as the Milky Way contain today. As a result, massive stars born during that era may have had physical properties that differ substantially from massive stars astronomers can study closer to home.
"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." Nearby Dwarf Galaxies Offer a Window Into the Past Astronomers cannot examine individual massive stars in the distant early universe in enough detail to measure many of these properties directly. TEMPOS therefore turned to relatively nearby dwarf galaxies whose chemical compositions provide a useful comparison.
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