How did TEMPOS collect its data?

Individual massive stars beyond the Milky Way are extremely faint. Each measurement in TEMPOS cost up to 35 hours of Hubble time, according to Telford. The program added 12 new stars to measurements gathered earlier, producing a uniform sample across six galaxies with diverse chemical compositions.

The team targeted ultraviolet light because a star's UV spectrum carries fingerprints of the elements in its atmosphere and reveals how fast its stellar wind moves. Metal ions help transfer energy from a star's radiation into surrounding material, pushing it outward. Scientists expect stars with fewer metals to drive weaker winds.

TEMPOS confirmed the broad trend. As metallicity drops, the maximum wind speed tends to fall.

What changes below the lowest metallicities?

Beneath that smooth decline, the data held a sharper break. For stars with metallicity below roughly 10% of the sun's, wind speeds fell much faster than the higher-metallicity trend would predict.

"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 such stars lose less mass to their winds, they may keep more of their original mass through their lives. That could change their later evolution, the way they explode, and how much energy they seed into their surroundings, with downstream effects on how their host galaxies form new stars.

Why does iron abundance matter?

The TEMPOS team also measured iron in the same stars by looking for faint ultraviolet absorption features — wavelengths of light that iron atoms have removed from the stellar spectrum. Stars in galaxies richer in oxygen generally showed stronger iron absorption than stars in oxygen-poor galaxies. Even within the metal-poor sample, the spread of iron values looked larger than researchers had anticipated.

"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."

The result matters because astronomers usually infer iron content from oxygen content, an easier measurement that relies on bright emission lines in surrounding gas. The TEMPOS data suggest that assumption can mislead at low metallicity, where iron and oxygen appear to decouple.

What comes next?

The collaborators plan to combine Hubble's ultraviolet spectra with visible-light observations from the W. M. Keck Observatory in Hawaii. Using both should let them build fuller models of each star, calculating wind mass-loss rates and detailed chemical abundances. TEMPOS is also releasing its science-ready UV spectra through the Mikulski Archive for Space Telescopes, opening the data to outside researchers.

The work was based on Hubble observations and supported by NASA grants GO-16767, GO-16920, and GO-17491. Collaborators include Christiana Erba of California State University, Fresno and Downing 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; 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.