Oldest known stars in the universe found in the 'halo' of the Milky Way

A team from the Massachusetts Institute of Technology (MIT) has discovered three of the oldest stars in the universe. To their surprise, the stars were not in a distant galaxy that only the ultra-powerful James Webb Space Telescope can observe. They are located in our own galactic neighborhood, within the “halo” of the Milky Way, according to a study published May 14 in the journal Monthly notices of the Royal Astronomical Society.

The Milky Way's halo is a cloud of stars that encompasses the entire galactic disk of our galaxy. The team believes these three stars formed between 12 and 13 billion years ago, right around the time our universe began to take shape. They name the stars Small accumulated stars from the galaxy– or SASS – and each star may have belonged to its own small galaxy that was at some point absorbed by the larger and still growing Milky Way. These three stars are now all that remains of their former galaxies.

According to the team, they orbit the outskirts of the Milky Way, where more old and persistent stars may lurk.

“These oldest stars should definitely be there, given what we know about galaxy formation,” MIT astronomer and astrophysicist Anna Frebel said in a statement via MIT News. “They are part of our cosmic family tree. And we now have a new way to find them.”

[Related: Youth-stealing stars could explain ‘missing giants’ at the Milky Way’s center.]

This study grew out of a classroom concept that began during the Fall 2022 semester when Frebel was launched a course called Observational Stellar Archaeology. In the class, students learned the techniques needed to analyze old stars and then apply them to stars that had not yet been examined to determine their origins. It included several students and recent graduates who are now co-authors of this new study.

“While most of our classes are taught from the ground up, this class immediately put us at the frontier of research in astrophysics,” said Hillary Andales, co-author of the study and a member of MIT's Class of 2023, said in a statement.

The class was looking for ancient stars that formed shortly after the Big Bang, about 13.8 billion years ago. This was when the universe was mostly helium and hydrogen and did not contain large amounts of other chemical elements such as barium and strontium. The class sifted through the data Frebel had collected over the years from the Magellan-Clay Telescope at the Las Campanas Observatory in Chile. They were looking for stars with starlight measurements (or spectra) that indicated low amounts of strontium and barium.

They focused on three stars that the Magellan Telescope first observed between 2013 and 2014. Astronomers did not interpret their spectra or infer their origins, so they were good candidates for the class to study.

Their analysis showed that all three stars contained very low amounts of strontium and barium. They also contain few other elements, including iron, compared to our own sun. One of the stars has less than 1/10,000 the amount of iron in helium compared to today's Sun. The low chemical presence was a clear indication that the three stars originally formed about 12 to 13 billion years ago.

[Related: ‘Homemade’ Telescope Spots Seven Dwarves in Space.]

“It took many hours of staring at a computer, and a lot of debugging, frantic texting and emailing to figure this out,” MIT student Ananda Santos said in a statement. “It was a big learning curve and a special experience.”

Frebel plans to relaunch the class this fall semester. As this team continues to discover similar SASS stars, they hope to use them as proxies ultra-faint dwarf galaxies. These galaxies are believed to be among the first surviving galaxies in the universe. Although they are still alive, they are too distant for astronomers to study accurately and in depth. Because SASS stars once belonged to similar dwarf galaxies but are much closer and located in our own galaxy, they can help astronomers better understand how ultra-faint dwarf galaxies evolve.

“Now we can look for more analogues in the Milky Way, which are much brighter, and study their chemical evolution without having to chase these extremely faint stars,” says Frebel.

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