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 was 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.