Astronomers Discover Oldest Known Stars In Universe From 13 Billion Years Ago

A team of astronomers from the Massachusetts Institute of Technology has identified a unique population of ancient stars that are providing unprecedented insights into the early universe and the formation of the first galaxies. These three stars are believed to have formed between 12 and 13 billion years ago when the very first galaxies were taking shape. The exciting findings have been published in the journal Monthly Notices of the Royal Astronomical Society.

These stars, which MIT researchers have dubbed “Small Accreted Stellar System” stars, or SASS stars for short, are extremely old and metal-poor, meaning they contain very few elements heavier than hydrogen and helium. This characteristic suggests that SASS stars formed very early in the history of the universe, before successive generations of stars had enriched the cosmos with heavier elements through supernova explosions.

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

What makes SASS stars particularly intriguing is their chemical similarity to stars found in the Milky Way’s smallest satellite galaxies, known as ultra-faint dwarf galaxies (UFDs). UFDs are thought to be some of the first galaxies that formed in the universe, and their stars retain chemical signatures of the very first generations of stars.

However, studying UFDs is challenging because they are extremely distant and faint. That’s where SASS stars come in — they are much closer to Earth, residing in the halo of our own Milky Way galaxy. This proximity makes them much easier to observe and study in detail.

“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,” notes Frebel.

The research team, led by astronomers at MIT and the University of Liège in Belgium, analyzed high-resolution spectra of six metal-poor stars taken with the Magellan Telescope in Chile. They found that half of these stars had chemical abundances nearly identical to those seen in UFDs, particularly when it came to elements like strontium and barium, which are created through neutron-capture processes in dying stars.

The SASS stars also showed a much wider scatter in the abundances of lightweight elements compared to typical Milky Way halo stars. This scatter is similar to what is seen in the most metal-poor stars and suggests that the SASS stars formed from gas enriched by the explosions of the very first stars in the universe, which are thought to have been massive, short-lived, and chemically distinct from later stellar generations.

Researchers also conducted a kinematic analysis, tracing the motions of the SASS stars, and found that they are all moving in retrograde orbits — that is, orbiting the Milky Way in the opposite direction of the galaxy’s rotation. This retrograde motion is a telltale sign that the SASS stars did not originate within the Milky Way, but were instead captured from smaller galaxies that collided with our galaxy in the distant past.

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

To expand their sample, the team searched astronomical databases and found an additional 61 stars in the Milky Way halo that fit the SASS criteria. This larger sample reinforced their findings and showed that a significant fraction of the most metal-poor stars in our galaxy are actually SASS stars.

The discovery of SASS stars is exciting because it opens up a new window into the early universe. By studying these ancient stars in detail, astronomers can learn about the first galaxies that formed after the Big Bang, the first generations of stars that lit up the cosmos, and the early assembly of our own Milky Way.


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