Astronomers have discovered three ancient stars surrounding the Milky Way’s halo, which formed 12-13 billion years ago. MIT researchers have discovered three of the oldest stars in the universe, and they live in our galactic neighbors.

The team, including several students, detected stars in the Milky Way’s “halo”, the cloud of stars surrounding the entire main galactic disk. According to the team’s analysis, the three stars formed between 12 and 13 billion years ago, when the oldest galaxies were forming.

Researchers coined the so-called “SASS” stars for the stars of the Small Aggregate Star System because they believe that each star once belonged to its own small, primitive galaxy and was later swallowed by the larger but still growing Milky Way. Today, only three stars remain from their galaxy. The team has been circling the outskirts of the Milky Way, where they suspect more such ancient stars may survive.

A new method to study ancient stars

“Given what we know about galaxy formation, these oldest stars should definitely be there,” says MIT physics professor Anna Froebel. “They are part of our cosmic family tree. Now there’s a new way to find them.”

By discovering similar SASS stars, researchers hope to use them as analogs of extremely faint dwarf galaxies believed to be among the oldest surviving galaxies in the Universe. Such galaxies remain intact today, but are too distant and faint for astronomers to study them in depth. Since SASS stars may once have belonged to the same primitive dwarf galaxies, but are in the Milky Way and are therefore much closer together, they may be an accessible key to understanding the evolution of extremely faint dwarf galaxies.

“We can now look for more similar, much brighter stars in the Milky Way and study their chemical evolution without going after these extremely dim stars,” says Froebel.

He and his colleagues published their findings on May 14. Monthly Notices of the Royal Astronomical Society (MNRAS). Co-authors of the study are Mohammad Mardini from Zarqa University, Jordan; Hillary Andales ’23; and current MIT students Ananda Santos and Casey Fienberg.

Classroom concept leads to great discoveries

The expedition team was born from the concept of the classroom. In the fall 2022 semester, Froebel launched a new course called 8.S30 (Observational Stellar Archaeology), in which students learn techniques for analyzing ancient stars and then apply those tools to previously unstudied stars to determine their origins.

“Although most of our courses are taught from scratch, this course instantly puts us at the forefront of astrophysics research,” says Andales.

Students worked from stellar data Froebel had collected over the years from the 6.5-meter Magellan-Clay Telescope at Las Campanas Observatory. He keeps hard copies of the data in a large folder in his office; students scan these folders to find stars of interest.

They specifically looked for ancient stars that formed shortly after the Big Bang, which occurred 13.8 billion years ago. At that time, the universe consisted mostly of hydrogen and helium, with very small amounts of other chemical elements such as strontium and barium. So the students looked through the Froebel file to find stars with spectra indicating low strontium and barium, or starlight measurements.

Analysis of ancient stars

Their research was limited to three stars observed by the Magellan telescope between 2013 and 2014. Astronomers have never studied these stars to interpret their spectra and determine their origin. They were ideal candidates for students in Froebel’s class at the time.

Students learned how to characterize a star in preparation for analyzing the spectra of each of three stars. Using different star models, they were able to determine the chemical composition of each. The intensity of a particular feature in the stellar spectrum corresponding to a particular wavelength of light corresponds to a particular amount of a particular element.

After completing their analysis, the students were able to confidently conclude that these three stars contained very low levels of other elements, such as strontium, barium, and iron, compared to their reference star, our own sun. In fact, one star contained less than 1/10,000 the amount of iron in helium compared to the modern sun.

“We spent hours looking at the computer and debugging a lot, frantically texting and emailing,” Santos recalls. “It was a great learning curve and a special experience.”

“On the road”

The low chemical content of the stars suggested that they first formed 12-13 billion years ago. In fact, their low chemical signatures were similar to those that astronomers had previously measured for some ancient extremely faint dwarf galaxies. Were the stars of the team born from similar galaxies? So how did they reach the Milky Way?

According to the hypothesis, scientists controlled the orbital patterns of stars and how they moved across the sky. The three stars are located in different locations in the Milky Way’s halo and are estimated to be about 30,000 light-years away from Earth. (For reference, the Milky Way’s disk spans 100,000 light-years.)

Tracking each star’s movement around the galactic center using observations from the Gaia astrometry satellite, the team noticed something interesting: all three stars appeared to go the wrong way, with most of the stars in the main disk moving like cars on a racetrack. In astronomy, this is known as “backward motion” and is a clue that the object was once “accelerated” or pulled from another direction.

“The only way to get the stars to go in the wrong direction from the rest of the group is if you throw them in the wrong direction,” says Froebel.

Future perspectives and research

The fact that these three stars rotate quite differently from the rest of the galactic disk and even the halo, combined with the fact that their chemical composition is low, supported the fact that the stars are very old indeed and were once among the oldest. Smaller dwarf galaxies that fall into the Milky Way at random angles and continue their stubborn orbits billions of years later.

Interested in whether retrograde motion was a feature of other ancient stars in halos that astronomers had previously analyzed, Froebel examined the scientific literature and found 65 other stars low in strontium and barium that also defied the galactic flow.

“The interesting thing is that they’re all pretty fast; they’re going in the wrong direction at hundreds of kilometers per second,” Froebel says. “They’re running away! We don’t know why, but it was a piece of the puzzle that we needed and that I wasn’t expecting when we started.”

The team is keen to look for other former SASS stars, and they now have a relatively simple recipe for doing so: First look for stars with low chemical content, then monitor their orbital patterns for signs of retrograde motion. Of the more than 400 billion stars in the Milky Way, they expect the method to reveal a small but significant fraction of the universe’s oldest stars.

Froebel plans to continue the course this fall and looks back with admiration and gratitude at the first course and the three students who contributed their results to the publication.

“It was great working with three female students. This is the first time for me,” she says. “This really exemplifies MIT’s method. We do. “Whoever says ‘I want to participate too’ can do it and good things will happen.”