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01/18/2021 An artistic representation of generic stellar currents in the Milky Way. RESEARCH AND TECHNOLOGY POLICY NASA / JPL-CALTECH / R. HURT, SSC & CALTECH


The 468 stars that make up one of the thousands of recently discovered stellar streams in the Milky Way were born at the same time and travel in the same direction across the sky.

The Milky Way is home to 8,292 stellar streams – a rare linear pattern, rather than a cluster, of stars – all called Theia. But Theia 456 is special, as revealed by the multiple combination of data sets captured by the Gaia space telescope.

“Most star clusters form together,” Jeff Andrews, an astrophysicist at Northwestern University and a member of the team that created the find, said in a statement. “The exciting thing about Theia 456 is that it is not a small group of stars together. It is long and stretched. There are relatively few nearby streams, young and so scattered.”

Andrews presented this research during a virtual press conference at the 237th meeting of the American Astronomical Society.

While researchers have long known that stars form in groups, most of the known clusters are spherical in shape. Only recently have astrophysicists started to find new patterns in the sky. They believe that long rows of stars were once tightly packed clusters, gradually ripped apart and stretched by tidal forces.

“As we become more advanced in our instrumentation, our technology and our ability to extract data, we have discovered that stars exist in more structures than groups,” Andrews said. “These streams often form across the sky. Although we’ve known them for decades, we’re starting to find some hidden ones.”


Spanning more than 500 light years, Theia 456 is one of those undercurrents. Because it inhabits the galactic plane of the Milky Way, it is easily lost within the backdrop of the galaxy of 400 billion stars.

“We tend to focus our telescopes in other directions because things are easier to find,” Andrews said. “Now we are beginning to find these currents in the galaxy itself. It is like finding a needle in a haystack.”

Identifying and examining these structures is a data science challenge. Artificial intelligence algorithms combined huge data sets of stellar data to find these structures. Then Andrews developed algorithms to cross-reference that data with pre-existing catalogs of iron abundances from documented stars.

Andrews and his team found that the 468 stars within Theia 456 had similar iron abundances, meaning that, 100 million years ago, the stars likely formed together. Adding more evidence to this finding, the researchers examined a dataset of light curves, which captures how the brightness of stars changes over time.

“This can be used to measure how fast the stars are spinning,” Agüeros said. “Stars of the same age should show a different pattern in their spin speeds.”

With the help of data from NASA’s Transiting Exoplanet Reconnaissance Satellite and the Zwicky Transient Facility, which produced light curves for the stars in Theia 456, Andrews and his colleagues were able to determine that the stars in the stream share a common age.

The team also discovered that the stars move together in the same direction.

“If you know how stars move, you can go back to find where the stars come from,” Andrews said. “As we turned the clock backwards, the stars got closer and closer. So we believe that all these stars were born together and have a common origin.”

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