Hidden in the Halo: MIT researchers discover the universe’s oldest stars

Stars in the Milky Way “Halo”

MIT astronomers discovered three of the oldest stars in the universe, and they live in our own galactic neighborhood. The stars are located in the Milky Way’s “halo” – the cloud of stars surrounding the main galactic disk – and they appear to have formed between 12 and 13 billion years ago, when the very first galaxies took shape. Credit: Serge Brunier; NASA

Astronomers discovered three ancient stars orbiting Earth Milky Way‘s halo, formed 12-13 billion years ago.

MIT Researchers have discovered three of the oldest stars in the universe, and they happen to live in our own galactic neighborhood.

The team, including a number of students, spotted the stars in the Milky Way’s ‘halo’: the cloud of stars that surrounds the entire main disk of the Milky Way. Based on the team’s analysis, the three stars formed between 12 and 13 billion years ago, the time when the very first galaxies were forming.

The researchers coined the stars ‘SASS’, which stands for Small Accreted Stellar System stars, because they believe that each star once belonged to its own small, primitive galaxy that was later absorbed by the larger but still growing Milky Way. Today, the three stars are all that remain of their respective galaxies. They orbit the outskirts of the Milky Way, where the team suspects there may be more such ancient stellar survivors.

A new method to study old stars

“These oldest stars should definitely be there, given what we know about galaxy formation,” said Anna Frebel, a professor of physics at MIT. “They are part of our cosmic family tree. And we now have a new way to find them.”

As they discover similar SASS stars, the researchers hope to use them as analogs of ultra-faint dwarf galaxies, which are believed to be some of the remaining first galaxies in the universe. Such galaxies are still intact, but are too distant and faint for astronomers to study in depth. Because SASS stars may once have belonged to similar primitive dwarf galaxies, but are located in the Milky Way and as such are much closer, they may provide an accessible key to understanding the evolution of ultra-faint dwarf galaxies.

Ananda Santos, Casey Fienberg and Anna Frebel

Researchers have a folder full of data about stars that they have collected over the years, including the brightness of stars over time. From left to right: Ananda Santos, Casey Fienberg and Anna Frebel. Credit: Courtesy of the researchers

“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.

She and her colleagues published their findings on May 14 in the Monthly Notices of the Royal Astronomical Society (MNRAS). The study’s co-authors are Mohammad Mardini, at Zarqa University in Jordan; Hillary Andales ’23; and current MIT students Ananda Santos and Casey Fienberg.

Classroom concept leads to great discoveries

The team’s discoveries emerged from a classroom concept. During the fall 2022 semester, Frebel launched a new course, 8.S30 (Observational Stellar Archaeology), in which students learned techniques for analyzing ancient stars and then applied those tools to stars that had never been studied before to determine their origins.

“Although most of our classes are taught from the ground up, this class immediately placed us on the frontier of astrophysics research,” Andales says.

The students worked on star data that Frebel had collected over the years with the 6.5-meter Magellan-Clay telescope at the Las Campanas Observatory. She keeps paper copies of the data in a large folder in her office, which the students combed through to look for interesting stars.

In particular, they looked for old stars that formed shortly afterward Big bang, which occurred 13.8 billion years ago. At that time, the universe consisted mainly of hydrogen and helium and very low amounts of other chemical elements, such as strontium and barium. So the students looked through Frebel’s folder for stars with spectra, or measurements of starlight, that indicated low amounts of strontium and barium.

Analyzing old stars

Their search was limited to three stars originally observed by the Magellan telescope between 2013 and 2014. Astronomers have never tracked these specific stars to interpret their spectra and infer their origins. So they were perfect candidates for the students in Frebel’s class.

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

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

“It took a lot of hours staring at a computer, and a lot of debugging, frantically texting and emailing each other to figure this out,” Santos remembers. “It was a big learning curve and a special experience.”

“On the run”

The stars’ low chemical presence indicated that they originally formed 12 to 13 billion years ago. In fact, their low chemical signatures were similar to what astronomers had previously measured for some ancient, ultra-faint dwarf galaxies. Do the team’s stars come from similar galaxies? And how did they end up in the Milky Way?

Acting on a hunch, the scientists checked the orbit patterns of the stars and how they move through 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 from Earth. (For reference, the Milky Way’s disk is 100,000 light-years across.)

As they tracked the motion of each star around the galactic center using observations from the astrometric Gaia satellite, the team noticed something curious: compared to most stars in the main disk, which move like cars on a race track, all three seemed the stars are going in the wrong direction. In astronomy, this is known as ‘retrograde motion’ and is an indication that an object was once ‘accreted’, or moved in from elsewhere.

“The only way you can get stars to go the wrong way with the rest of the gang is to throw them the wrong way,” says Frebel.

Future perspectives and research

The fact that these three stars orbited in completely different ways than the rest of the galactic disk and even the halo, combined with the fact that they contained few chemicals, was strong evidence that the stars were indeed old and once older stars belonged. , smaller dwarf galaxies that fell into the Milky Way at random angles and continued their persistent trajectories billions of years later.

Frebel, curious whether retrograde motion was a feature of other old stars in the halo that astronomers previously analyzed, looked through the scientific literature and found 65 other stars, also with low strontium and barium abundances, that also seemed contrary to expectations to go. galactic flow.

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

The team is eager to look for other old SASS stars, and they now have a relatively simple recipe for doing so: first look for stars with low chemical concentration, 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 yield a small but significant number of the universe’s oldest stars.

Frebel plans to relaunch the course this fall and looks back with admiration and gratitude on that first course and the three students who published their results.

“It was great to work with three female students. That is a first for me,” she says. “It really exemplifies the MIT way. We do. And anyone who says, ‘I want to participate,’ can do so and good things happen.”

Reference: “The Oldest Stars with Low Neutron Capture Elements and Their Origins in Ancient Dwarf Galaxies” by Hillary Diane Andales, Ananda Santos Figueiredo, Casey Gordon Fienberg, Mohammad K Mardini and Anna Frebel, May 14, 2024, Monthly notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stae670

This research was supported in part by the National Science Foundation.

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