Newly discovered stars in the Milky Way are as old as the universe

Have you ever looked up at the night sky and wondered about the vastness of the universe? What if some of the universe’s oldest stars are right in our galactic backyard?

A team of MIT researchers, including undergraduate students, has discovered three stars right here in the Milky Way that date back 12 to 13 billion years. That’s almost as old as the universe itself, which is estimated to be 13.8 billion years old.

Milky Way’s past

How did they find these ancient stellar relics? It all started in a classroom. Professor Anna Frebel, a physics professor at MIT, launched a new course in the fall of 2022 called “Observational Stellar Archaeology.”

In this course, students learned how to analyze the light emitted by stars to determine their age and composition.

The students then applied their newfound knowledge to a treasure trove of star data that Professor Frebel had collected over the years using the Magellan-Clay telescope. They were particularly interested in finding stars that formed shortly after the Big Bang when the universe was still in its infancy.

These early stars would have a very different chemical composition than younger stars like our sun. They would contain lower amounts of heavier elements like strontium and barium because these elements were not yet abundant in the early universe.

SASS stars in the universe

The students’ search paid off. They identified three stars with spectra indicating low abundances of heavier elements, suggesting they were indeed ancient. These stars have been dubbed “SASS” stars, which stands for “Small Accreted Stellar System” stars.

“These oldest stars should definitely be there, given what we know of galaxy formation,” says Professor Frebel. “They are part of our cosmic family tree. And we now have a new way to find them.”

Why “Small Accreted Stellar System”? The researchers believe these stars once belonged to their own small galaxies that were later absorbed by the Milky Way. In other words, they are cosmic immigrants that have found a new home in our galaxy.

Clue to the stars’ origins in the universe

But how did these stars end up in the Milky Way? The answer lies in their unusual motion.

Most stars in the Milky Way’s disk orbit in the same direction, like cars on a racetrack. However, the SASS stars are moving in the opposite direction, a phenomenon known as “retrograde motion.” This suggests they were captured from another galaxy that was moving in a different direction.

“The only way you can have stars going the wrong way from the rest of the gang is if you threw them in the wrong way,” Professor Frebel explains.

This retrograde motion, combined with their ancient chemical composition, provides strong evidence that the SASS stars are indeed cosmic relics from a time when the universe was much younger.

“Interestingly they’re all quite fast — hundreds of kilometers per second, going the wrong way,” Frebel says. “They’re on the run! We don’t know why that’s the case, but it was the piece to the puzzle that we needed, and that I didn’t quite anticipate when we started.”

New window into the early universe

The discovery of the SASS stars opens up exciting possibilities for studying the early universe.

These stars are remnants from a period when galaxies were just beginning to form. By analyzing their composition and motion, we can gain insights into galaxy evolution. This research helps us understand how the universe has transformed over billions of years.

The chemical makeup of these stars provides information about the conditions of the early universe. Their movements offer clues about the history of their host galaxies. This can shed light on the processes that shaped the Milky Way and other galaxies.

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

Future of stellar archaeology

This discovery is just the beginning. The researchers plan to continue their search for more SASS stars, and they hope to inspire a new generation of scientists to explore the mysteries of the cosmos.

“It’s been awesome to work with three women undergrads. That’s a first for me… It’s really an example of the MIT way. We do. And whoever says, ‘I want to participate,’ they can do that, and good things happen,” Professor Frebel notes,

So, the next time you look up at the night sky, remember that some of the universe’s oldest stars are closer than you think. They are a testament to the vastness of time and the enduring mysteries of the cosmos.

The study is published in the Monthly Notices of the Royal Astronomical Society.

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