Astronomers are investigating why early galaxies behave differently than current models predict, with a focus on how massive stars may have evolved under conditions unlike those in the modern Milky Way. The Treasury of Extremely Metal-Poor O Stars (TEMPOS) project, led by researchers at the University of Utah, uses ultraviolet measurements from the Hubble Space Telescope to study massive stars in nearby dwarf galaxies with chemical compositions resembling the early universe. The research was published on Sept. 21, 2026, in The Astrophysical Journal Supplement Series.
Massive stars—those more than ten times as heavy as the sun—wield outsized influence on their host galaxies despite their rarity. These objects emit intense radiation, shed material through stellar winds, and eventually explode as supernovae, processes that regulate the availability of gas for forming new stars. A key difference between early and modern galaxies lies in metallicity, the abundance of elements heavier than hydrogen and helium. The earliest galaxies contained far fewer such elements, meaning massive stars that formed then likely possessed different physical characteristics than their metal-rich counterparts studied in the Milky Way today.
Because individual massive stars in the distant early universe cannot be studied in sufficient detail, the TEMPOS survey examined 29 massive stars distributed across six nearby dwarf galaxies with metallicity values below one-fifth of the sun’s. Each star required up to 35 hours of Hubble observing time, making the dataset unusually large and valuable for comparison. Ultraviolet spectra proved particularly informative, revealing chemical fingerprints in stellar atmospheres and characteristics of stellar winds ejecting material from stellar surfaces.
The survey confirmed an anticipated pattern: as metallicity decreases, stellar wind speeds also tend to decline since metal ions facilitate energy transfer that drives outflows. However, an unexpected finding emerged at the lowest metallicity levels. For stars with metallicity below roughly 10 percent of the sun’s value, wind speeds dropped far more dramatically than models based on higher-metallicity trends would predict. This sharp decline could significantly affect how extremely metal-poor stars evolve, explode, and redistribute energy and material to their surroundings.
Iron emerged as a potentially crucial element in understanding these phenomena. While astronomers typically estimate metallicity by measuring oxygen, which produces easily detectable emission lines, iron and oxygen do not necessarily vary in lockstep. Iron plays important roles in driving stellar winds and influencing the processes leading to supernovae, yet measuring it in extremely metal-poor environments remains particularly challenging for researchers.
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