Astronomers Find 12- to 13-Billion-Year-Old Stars in Milky Way’s Halo

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Astronomers at MIT have discovered thee extremely ancient stars in the Milky Way’s halo, a cloud of stars that envelopes the entire main Galactic disk. These objects formed between 12 and 13 billion years ago, the time when the very first galaxies were taking shape. The researchers label these stars as Small Accreted Stellar System (SASS) stars, as they believe each star once belonged to its own dwarf galaxy that was later absorbed by the larger but still growing Milky Way.

An artist’s conception of the Milky Way Galaxy. Image credit: Pablo Carlos Budassi / CC BY-SA 4.0.

“These oldest stars should definitely be there, given what we know of galaxy formation,” said MIT’s Professor Anna Frebel.

“They are part of our cosmic family tree. And we now have a new way to find them.”

As they uncover similar SASS stars, Professor Frebel and colleagues hope to use them as analogs of ultrafaint dwarf galaxies, which are thought to be some of the Universe’s surviving first galaxies.

Such galaxies are still intact today but are too distant and faint for astronomers to study in depth.

As SASS stars may have once belonged to similarly primitive dwarf galaxies but are in the Milky Way and as such much closer, they could be an accessible key to understanding the evolution of ultrafaint dwarf 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 said.

The stars’ low chemical abundance did hint 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, ultrafaint dwarf galaxies.

Did the team’s stars originate in similar galaxies? And how did they come to be in the Milky Way?

On a hunch, the scientists checked out the stars’ orbital patterns and how they move across the sky.

The three stars are in different locations throughout the Milky Way’s halo and are estimated to be about 30,000 light-years from Earth.

As they retraced each star’s motion about the Galactic center using observations from ESA’s Gaia satellite, the astronomers noticed a curious thing: relative to most of the stars in the main disk, which move like cars on a racetrack, all three stars seemed to be going the wrong way.

In astronomy, this is known as retrograde motion and is a tipoff that an object was once accreted, or drawn in from elsewhere.

“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 said.

The fact that these three stars were orbiting in completely different ways from the rest of the Galactic disk and even the halo, combined with the fact that they held low chemical abundances, made a strong case that the stars were indeed ancient and once belonged to older, smaller dwarf galaxies that fell into the Milky Way at random angles and continued their stubborn trajectories billions of years later.

The authors, curious as to whether retrograde motion was a feature of other ancient 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 appeared to also be going against the Galactic flow.

“Interestingly they’re all quite fast — hundreds of km per second, going the wrong way,” Professor Frebel said.

“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.”

The researchers are eager to search out other ancient SASS stars, and they now have a relatively simple recipe to do so: first, look for stars with low chemical abundances, and then track their orbital patterns for signs of retrograde motion.

Of the more than 400 billion stars in the Milky Way, they anticipate that the method will turn up a small but significant number of the universe’s oldest stars.

“It’s been awesome to work with three women undergrads. That’s a first for me,” Professor Frebel said.

“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.”

The team’s paper was published in the Monthly Notices of the Royal Astronomical Society.

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Hillary Diane Andales et al. 2024. The oldest stars with low neutron-capture element abundances and origins in ancient dwarf galaxies. MNRAS 530 (4): 4712-4729; doi: 10.1093/mnras/stae670

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