MIT researchers and students have found that some of the universe's oldest celestial history can be found in our Milky Way. Formed anywhere from 12 to 13 billion years ago, a trio of stars within the Milky Way's halo have been deemed the longest-surviving stars ever studied. Their lengthy existence could offer us insight into the development of the universe—and our own galaxy—immediately following the Big Bang.
MIT professor of physics Anna Frebel has spent a significant part of her career collecting data from the Magellan-Clay telescope at the Las Campanas Observatory in Chile's Atacama Desert. In late 2022, Frebel started a new class focused on "observational star archaeology." The class would teach students the skills necessary to research the origins of ancient stars, but first, they'd need to find stars to study.
Using Frebel's stockpiles of Magellan-Clay data, students explored stars whose spectra, or light measurements, pointed toward low quantities of strontium and barium. Because both of these elements were very scarce immediately following the Big Bang, low levels of strontium and barium indicated that those stars may have formed during the universe's early history. The class eventually landed on three stars initially observed in 2013 and 2014. Using extensive analysis, students confirmed that all three stars contained extremely low abundances of strontium, barium, and even iron when compared with newer reference stars, including our Sun.
These low chemical abundances reminded Frebel and her students of previous research, which found that ancient dwarf galaxies contained low levels of strontium and barium due to their relatively scarce star formation cycles. This pushed the team to investigate the stars' orbital patterns. Quickly, Frebel and her students discovered that their trio of stars engaged in "retrograde motion," meaning they traveled in the opposite direction from the rest of the stars in the Milky Way's halo.
Retrograde motion is often a sign that a celestial object isn't native to its current environment, but was rather "accreted," or gathered up from another source. This pattern of movement, combined with the stars' low chemical abundances, led Frebel to believe that the stars once existed within their own small, early galaxies. These galaxies would have been absorbed by our own galaxy as the Milky Way grew, leaving only these ancient stars behind.
The class's findings have since been termed Small Accreted Stellar System stars, or SASS stars. According to a paper published Tuesday in Monthly Notices of the Royal Astronomical Society, Frebel, her colleagues, and her former students believe there could be 61 additional SASS stars hidden throughout the Milky Way.
“These oldest stars should definitely be there, given what we know of galaxy formation,” Frebel said in a statement. “They are part of our cosmic family tree. And we now have a new way to find them.”