Researchers at MIT found three of the oldest stars in the universe orbiting around the outskirts of the Milky Way Galaxy, according to a study published in the Monthly Notices of the Royal Astronomical Society.
“This is the end of the line; this is as good as it gets,” said Anna Frebel, an author of the study and a professor of physics at MIT. “My career is built on finding second-generation stars, but I have never published a paper claiming these are the oldest ones.”
About 13 billion years old, the stars, given the “endearing” name SASS, Small Accreted Stellar System stars, formed in small dwarf galaxies and are located in the outer limits of the Milky Way, called the “halo,” which starts at about 10,000 light-years from Earth and extends to a million light-years away, Frebel said.
While the chemical compositions of the three stars indicate they are old, Frebel said the study, published on May 14, identified that the stars’ original dwarf galaxies were “eaten” — gobbled up through proximity and gravitational interactions — by the Milky Way. This process only happened during a short window of time 13 billion years ago, Frebel said, so the discovery pinpointed the stars’ old age that much more.
“This is really the most sure-fire way to say these are the oldest ones,” Frebel said. “It opens up this entire new field of looking at what is in the Milky Way right now but not of the Milky Way.”
Frebel, also the division head for astrophysics at MIT, has been researching what she calls “old stars,” or “13 billion-year-old stars,” for 20 years. In fall 2022, she started a class, Observational Stellar Archaeology, to help her undergraduate students gain research experience.
The research didn’t start by looking for anything “earth-shattering,” Frebel said. She asked her students to look for something “very small” — “old stars” that barely had any traces of the chemical element barium. The element is usually found in “significant amounts” in stars, Frebel said.
“I like to push the boundaries and ask the question, How can you make so little?” Frebel said with a smile. “I had no idea what we actually found that there was more to the story.”
Digging through a binder filled with 15 years of Frebel’s unanalyzed “old star” data from the 6.5-meter Magellan Telescope in Chile, the students started looking for small amounts of the element. Quickly, they discovered three stars with low amounts of barium and another element, strontium. Even better, the stars were orbiting in the wrong direction, indicating they must have come from a different galaxy, Frebel said.
“I had a hunch,” Frebel said. “I can read the tea leaves quite well.”
A freshman from the class continued working with Frebel, along with a second undergraduate and two other researchers. Over the last two years, the group has been “fine-tuning,” “fiddling,” and “revising” their findings, Frebel said.
The researchers published the study last week, but Frebel said this isn’t the end of the line for SASS stars; instead, it’s a new “framework” for other researchers. Already, the group has identified 65 other stars with chemical compositions similar to the first three.
“These three stars — they are special, but they’re not unique,” Frebel said. “One star is a discovery, two is a sample, and three is a population. When you find three stars doing the same thing in astronomy, it has meaning. It’s not random.”
Frebel plans to teach the class again in the fall, incorporating more “detective work.” The big question for not just Frebel, but most astronomers is: “What was the beginning?” Frebel said. Whether that be of the universe or the Milky Way Galaxy, the SASS stars might just provide an answer, Frebel said.
“[The stars] have preserved all this information from early on for 13 billion years for us because they’re just sitting there,” Frebel said. “Like the can of beans in the back of your cupboard, unless you crack it open or damage it somehow, it just keeps sitting there.”
Ava Berger can be reached at ava.berger@globe.com. Follow her @Ava_Berger_.