Stellar Fossil Offers Insight into the Early Universe

Astronomers at the University of Virginia have identified one of the most iron-poor stars ever observed outside the Milky Way, a discovery that offers rare insight into how the very first stars lived and died. 

The star is located in a tiny, ancient galaxy called Pictor II containing extraordinarily small amounts of heavy elements such as iron — elements that are forged inside stars and dispersed when they explode. That scarcity suggests the star formed very early in cosmic history, after only one of the universe’s first stars had ended its life in a supernova. 

“These are the kinds of stars that preserve the chemical fingerprints of the first stars. They’re essentially fossils from the early universe,” said Andrew Pace,  a  postdoctoral research associate and Galaxy Evolution and Cosmology fellow with the College and Graduate School of Arts & Sciences’ Department of Astronomy, one of an international team of researchers that includes professor of astronomy and dean’s research fellow Nitya Kallivaylil; UVA graduate student Kaia Atzberger, who contributed to the analysis of the system. 

What makes this star especially striking is its combination of extremely low iron and unusually high carbon. According to Pace, that pattern supports the idea that some of the very first stars ended their lives in relatively low-energy explosions. In those events, lighter elements such as carbon would have been expelled, while much of the heavier material fell back into the collapsed remnant. 

“This is exactly the kind of abundance pattern we expect from a faint supernova,” Pace said. “It tells us that a single early explosion likely enriched the gas that formed this star.” 

Ultra-faint dwarf galaxies like Pictor II are thought to be relics of the earliest building blocks of larger galaxies, including the Milky Way. Because they experienced very little subsequent star formation, they can preserve the chemical record of just one or a few early stellar events. 

“Systems like this are some of the smallest, most chemically pristine galaxies we know,” Pace said. “They give us a window into what galaxy formation looked like at the very beginning.” 

Atzberger’s work helped confirm the star’s membership in Pictor II and interpret what its chemical makeup reveals about early enrichment. By studying the motions and composition of stars in these faint systems, the team was able to reconstruct events that occurred more than 10 billion years ago. 

“These ultra-faint dwarf galaxies are basically time capsules. They haven’t formed many stars, so they preserve the chemical record of just one or a few early supernovae,” Atzberger said. 

The discovery was made using precision imaging and follow-up spectroscopy from some of the world’s largest telescopes. Researchers say additional stars like this may still be hidden in the outskirts of faint galaxies, preserving further evidence of the universe’s first generation of stars. 

For the Arts & Sciences, the findings highlight UVA’s role in uncovering the universe’s earliest history — using nearby stellar fossils to illuminate how the first stars seeded the cosmos with the elements that would eventually make planets, and life, possible. 

“Finding a star like this outside the Milky Way helps us understand what the very first stars were actually doing,” Atzberger said.