Astronomers have discovered three “runaway” ancient stars on the outskirts of the Milky Way galaxy, racing in the wrong direction at speeds of hundreds of thousands of kilometers per hour.
Although alive for their age, the three stars are so old that they date back to the formation of the first galaxies. This is one to two billion years after the Big Bang.
Researchers from the Massachusetts Institute of Technology (MIT) found these stars in the Milky Way’s halo, a diffuse cloud of stars, gas and dust that surrounds our entire galaxy. The team called the stellar objects, whose ages are between 13 and 12 billion years old, “Small Accumulated Star System, or SASS stars.” This name suggests that each of these stars later formed in its own small, primitive galaxy. Cannibalized by our Milky Way.
Researchers believe there may be more ancient stellar rovers at the edges of the solar system, creating a kind of “fossil record” detailing how our galaxy grew by consuming others and adopting their stars. Such stars can also be used as analogs to study the oldest stars and galaxies of the 13.8 billion-year-old universe.
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“Given what we know about galaxy formation, these oldest stars should definitely be there,” team member and MIT physics professor Anna Frebel said in a statement. “They are part of our cosmic family tree. And now we have a new way to find them.”
Discovering more SASS stars would mean more analogues of stars called extremely faint dwarf galaxies, the oldest surviving galaxies in the universe. Although these galaxies remain intact, they are too distant and faint to be examined in detail. SASS stars, plucked from similar primitive galaxies and absorbed by the Milky Way, are therefore a more accessible way to understand how some of these very early galaxies evolved.
“We can now look for more analogs that are much brighter in the Milky Way and study their chemical evolution without having to chase these extremely dim stars,” Frebel added.
From classroom to cosmos
The search for ancient stars at the edge of the Milky Way begins in 2022 as part of Frebel’s new Observational Stellar Archaeology course. During these sessions, the MIT researcher detailed methods for studying older stars and discussed how these methods can be applied to unstudied stars to determine their origins.
“While most of our courses are taught from scratch, this course puts us right at the forefront of astrophysics research,” Hillary Andales, part of the Frebel laboratory at MIT’s Kavli Institute for Astrophysics and Space Research, said in a statement. she said.
Fredel’s students combed through years of collected data using the 6.5-meter Magellan-Clay telescope at Las Campanas Observatory to find stars of interest, 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 small amounts of heavier elements that astronomers call “metals.” As these stars lived, they forged metals in their cores and eventually exploded and dispersed the elements. These elements then become the building blocks of future generations of stars. This means that the first stars must have had “metal-poor” compositions, compared to later stars that were enriched by earlier stellar contributions to the heavy element manifest of the universe.
To detect ancient stars lurking in the Milky Way, students under Frebel’s tutelage focused specifically on stars that did not contain strontium and barium. This led them to three stars observed by the Magellan telescope in 2013 and 2014; They were objects that had not been studied very thoroughly by astronomers.
Not only did the three stars highlighted by the team lack strontium and barium, but the iron content of the objects was also quite low compared to more “modern” stars such as our 4.6 billion-year-old star, the Sun. In fact, the ratio of iron to helium for one of the stars is 10,000 times smaller than the ratio of the same elements for the sun.
And sure enough, the chemical composition of the stars not only revealed them to be between 12 and 13 billion years old, but also showed a remarkable similarity to the chemical composition of older, ultra-faint dwarf galaxies.
What’s more, to discover how these ancient stars became part of our galaxy, researchers looked at their orbits and paths across the sky. This revealed that the stars were located in three different locations of the Milky Way’s halo and were located about 30,000 light-years from Earth.
The origin of the stars that are part of the galaxies engulfed by the Milky Way is revealed not only by their metal-poor composition, but also by the fact that they orbit in a different direction from the Milky Way’s main disk and most of its halo. Stars also exhibited random angles and strange orbits that persisted for billions of years.
Investigating this retrograde motion, the team discovered 65 more stars showing the same pattern. These stars also had low strontium and barium contents, but strikingly they had something else in common with the SASS stars.
“They’re running away! Interestingly, they’re all pretty fast; they’re going in the wrong direction at hundreds of kilometers per second,” Frebel explained. “We don’t know why that was the case, but it was the piece of the puzzle that we needed and that I didn’t quite anticipate when we started.”
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There are about 400 billion stars in the Milky Way, and Frebel and his colleagues will now look for more SASS stars among them. They will do this by looking for metal-poor stellar compositions, then checking whether the selected objects have orbits that do not match the galactic flow. Additionally, Frebel’s Observational Stellar Archeology course will launch again next year, allowing more students to learn about his intriguing methodology. These results are, in a way, confirmation of it.
Frebel concluded: “It was amazing to work with three female college students. This is a first for me.” “This is really an example of the MIT way. We participate. Anyone who says ‘I want to participate’ can do it and good things will happen.”
The team’s results were published May 14 in the journal Monthly Notices of the Royal Astronomical Society.