Astronomers have discovered three ancient stars "on the run" in the outskirts of the Milky Way galaxy, racing in the wrong direction at hundreds of thousands of kilometers per hour.
Despite being bright for their age, the three stars are so old that they date back to the time when the first galaxies formed. That is between one and two billion years after the Big Bang.
Researchers at the Massachusetts Institute of Technology (MIT) found these stars in the Milky Way's halo, a diffuse cloud made of stars, gas and dust that envelops our entire galaxy. The team has dubbed the stellar bodies, which are between 13 billion and 12 billion years old, 'Small Accreted Stellar System' or SASS stars. The name indicates that each of these stars formed in its own small, primitive galaxy. cannibalized by our Milky Way.
The researchers believe that even more ancient stellar stragglers may exist at the edges of the solar system, forming a kind of "fossil record" that details how our galaxy grew by consuming others and adopting their stars. Such stars could also be used as analogues to study the earliest stars and galaxies of the 13.8 billion-year-old universe.
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"These oldest stars should definitely be there, given what we know about galaxy formation," team member and MIT physics professor Anna Frebel said in a statement. "They are part of our cosmic family tree. And we now have a new way to find them."
Discovering more SASS stars would mean more analogues of stars in so-called ultra-faint dwarf galaxies, the oldest surviving galaxies in the universe. Although these galaxies remain intact, they are too distant and faint to examine in detail. SASS stars, torn from similar primordial galaxies and incorporated into the Milky Way, are therefore a more accessible way to understand how some of those very early galaxies evolved.
"Now we can look for more analogues in the Milky Way that are much brighter and study their chemical evolution without having to chase these extremely faint stars," Frebel added.
From the classroom to the cosmos
The search for old stars at the edge of the Milky Way began in 2022 as part of Frebel's new Observational Stellar Archeology course. During these sessions, the MIT researcher described methods for examining older stars and discussed how these methods could be applied to unstudied stars to determine their origins.
"Although most of our classes are taught from the ground up, this class immediately placed us on the frontier of research in astrophysics," said Hillary Andales, part of Frebel's lab at MIT's Kavli Institute for Astrophysics and Space Research, in the explanation.
Fredel's students sifted through years of data collected with the Las Campanas Observatory's 6.5-meter Magellan-Clay Telescope to find interesting stars, especially those with low concentrations of elements heavier than hydrogen and helium.
When the first stars formed, the universe was filled mainly with hydrogen, some helium, and only a few heavier elements, which astronomers call "metals." As these stars lived, they forged metals into their cores, eventually causing them to explode and scatter elements. These elements then become the building blocks of the next generation of stars. This means that the first stars should have a 'metal-poor' composition compared to later stars that are enriched by previous stellar contributions to the universe's heavy element manifesto.
To identify old stars lurking in the Milky Way, students under Frebel's tutelage focused primarily on stars lacking strontium and barium. This led them to three stars observed by the Magellan Telescope in 2013 and 2014; they were bodies that had not yet been examined very deeply by astronomers.
Not only did the three stars the team highlighted lack strontium and barium, but the objects' iron content was also quite low compared to more "modern" stars like our 4.6 billion-year-old star, the Sun. For one of the stars, the ratio of iron to helium is even 10,000 times smaller than the ratio of the same elements for the Sun.
Sure enough, the chemical composition of the stars not only revealed that they were between 12 billion and 13 billion years old, but also bore a remarkable similarity to the chemical composition of ancient, ultra-faint dwarf galaxies.
To discover how these old stars became part of our Milky Way, the researchers also looked at their orbits and paths across the sky. This showed that the stars are in three different locations in the Milky Way's halo, about 30,000 light-years away from Earth.
The origin of the stars as part of galaxies engulfed by the Milky Way was revealed not only by their metal-poor composition, but also by the fact that they rotate in a different direction than the Milky Way's main disk and most of its halo . The stars also showed random angles and strange trajectories that have existed for billions of years.
Investigating this retrograde motion, the team discovered another 65 stars that showed the same pattern. These stars also had low strontium and barium content, but surprisingly, they also had something else in common with the SASS stars.
"They're on the run! Interestingly enough, they're all quite fast - hundreds of kilometers per second, and they're going in the wrong direction," Frebel explained. "We don't know why that's the case, but it was the piece of the puzzle that we needed and that I didn't quite expect when we started."
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There are about 400 billion stars in the Milky Way, and Frebel and colleagues will now look for more SASS stars among them. They do this by looking for metal-poor stellar compositions and then checking whether the selected subjects have orbits that do not match the galactic flow. Additionally, Frebel's Observational Stellar Archeology course will return next year, allowing more students to learn more about her intriguing methodology. These results are, in a way, validating it.
"It was great to work with three female students. That's a first for me," concluded Frebel. "It's really an example of the MIT way. That's what we do. And whoever says, 'I want to participate,' they can do that and good things happen."
The team's results were published May 14 in the journal Monthly Notices of the Royal Astronomical Society.
