In a groundbreaking discovery, researchers at MIT have identified three of the universe’s oldest stars residing within the Milky Way’s halo. These stars, dubbed “SASS” for Small Accreted Stellar System stars, are estimated to be between 12 and 13 billion years old, dating back to the epoch when the first galaxies were forming. Their presence in the Milky Way’s halo, a diffuse cloud of stars enveloping the main galactic disk, suggests a fascinating origin story.
The researchers posit that these stars were once part of their own small, primitive galaxies, which were later absorbed by the burgeoning Milky Way. Today, these three stars are the sole remnants of their former galactic homes, orbiting the outskirts of our galaxy. This discovery opens a window into the early universe, offering insights into the formation and evolution of galaxies.
The discovery stemmed from a unique MIT course, 8.S30 (Observational Stellar Archaeology), designed by Professor Anna Frebel. Students in this class were tasked with analyzing starlight data from the Magellan-Clay telescope at the Las Campanas Observatory. Their focus was on identifying ancient stars formed shortly after the Big Bang, characterized by low abundances of heavier elements like strontium and barium.
After meticulous analysis, the students zeroed in on three stars whose spectra indicated remarkably low abundances of these elements, consistent with the chemical composition of the early universe. These stars exhibited less than 1/10,000 of the iron-to-helium ratio found in our sun, a clear sign of their ancient origins.
Further investigation revealed an intriguing characteristic of these stars – their retrograde motion. Unlike most stars in the Milky Way’s disk, which follow a predictable, uniform orbit, these ancient stars were moving in the opposite direction. This peculiar motion, termed “retrograde motion” in astronomy, suggests that these stars were captured from elsewhere, bolstering the theory that they originated from dwarf galaxies that merged with the Milky Way.
The combination of their low chemical abundances and retrograde motion provides strong evidence that these stars are indeed ancient relics from the early universe. Their discovery has prompted researchers to explore the possibility of other ancient stars residing in the Milky Way’s halo, offering a new avenue for studying the universe’s formative years.
Professor Frebel and her team have devised a strategy for identifying more SASS stars: look for stars with low chemical abundances and then analyze their orbital patterns for signs of retrograde motion. They believe this method will uncover more of the universe’s oldest stars among the Milky Way’s estimated 400 billion stars.
The discovery of these ancient stars in our galactic backyard offers a unique opportunity to study the early universe in unprecedented detail. These SASS stars serve as accessible analogs to ultrafaint dwarf galaxies, believed to be among the universe’s first galaxies, which are too distant and faint for detailed observation. By studying these nearby ancient stars, scientists can glean insights into the evolution of these primordial galaxies.
Professor Frebel expressed her enthusiasm for the discovery and the contribution of the undergraduate students involved in the research. Their findings, published in the Monthly Notices of the Royal Astronomical Society (MNRAS), highlight the power of collaborative research and the potential for uncovering remarkable secrets hidden within our own galaxy.
The research was supported, in part, by the National Science Foundation.
The image is courtesy of Serge Brunier; NASA.
The link to the original article can be accessed here.
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