Astronomers at the Massachusetts Institute of Technology have discovered very old stars in the Milky Way’s halo, a cloud of stars that covers the entire disk of our galaxy. These objects formed between 12 and 13 billion years ago, when the first galaxies were beginning to form. Researchers believe that each star once belonged to its own dwarf galaxy, which was later absorbed into the larger but ever-growing Milky Way, making them known as small accreting star systems (SASS). It’s called a star.

“Given what we know about galaxy formation, these oldest stars should definitely exist,” says MIT professor Anna Froebel.

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

As they discover similar SASS stars, Professor Froebel and his colleagues hope to use them as analogues of ultrafaint dwarf galaxies, which are thought to be some of the first living galaxies in the universe.

These galaxies remain intact today, but they are too distant and faint for astronomers to study in detail.

SASS stars may once have belonged to similar primitive dwarf galaxies, but they are now located within the Milky Way and are much closer, making them more accessible for understanding the evolution of ultrafaint dwarf galaxies. This could be the key.

“Now we can look for more brighter analogs in the Milky Way and study their chemical evolution without chasing these very faint stars,” Professor Froebel said.

The low chemical abundances of these stars suggest that they first formed between 12 and 13 billion years ago.

In fact, their low chemical signature was similar to what astronomers had previously measured for several ancient, ultra-dark dwarf galaxies.

Are the team’s star players from similar galaxies? And how did they come to exist in the Milky Way?

Based on a hunch, scientists studied the orbital patterns of stars and how they move across the sky.

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

When astronomers used observations from ESA’s Gaia satellite to trace the movement of each star around the galaxy’s center, they noticed something strange. All three stars appeared to be in motion, compared to most of the stars in the main disk, which move like cars on a race track. Wrong way.

In astronomy, this is known as retrograde motion, and is information that the object was once accreted or pulled in from elsewhere.

“The only way to get a star wrong from other members is if you throw it the wrong way,” Professor Froebel says.

The fact that these three stars orbit in a completely different way than the rest of the galactic disk or halo, combined with the fact that their chemical abundances are low, suggests that these stars are actually It was strongly argued that it was ancient and once belonged to an earlier era, a small dwarf galaxy that fell into the Milky Way at a random angle and continued its stubborn orbit billions of years later.

The authors were interested in whether retrograde motion was a feature of other ancient stars in the halo that astronomers had previously analyzed, and they looked at the scientific literature and found similarly low strontium and barium contents, discovered 65 other stars that appear to be moving in retrograde motion as well. Galaxy flow.

“Interestingly, they are all traveling very fast, hundreds of kilometers per second, in opposite directions,” Professor Froebel said.

“They’re on the run! We don’t know why it happened, but this is the piece of the puzzle we need and we never expected it when we started.”

Researchers are keen to find other ancient SASS stars, and now have a relatively simple recipe for doing so. First, they look for stars with low chemical abundance, then track their orbital patterns for signs of retrograde motion.

Researchers hope this method will uncover a small but significant number of the universe’s oldest stars, out of the more than 400 billion stars in the Milky Way.

“I really enjoyed working with three female undergraduates. It was a first for me,” said Professor Froebel.

“This is just an example of the MIT way. It is. And anyone who says, ‘I want to participate,’ can do so, and good things happen.”

team’s paper Published in Royal Astronomical Society Monthly Notices.

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Hilary Diane Anders other. 2024. The oldest star with a small amount of neutron-capturing elements and originating from an ancient dwarf galaxy. MNRAS 530 (4): 4712-4729; doi: 10.1093/mnras/stae670