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A stream of nearly 500 stars in the Milky Way is actually a family

Posted January 15, 2021 1:02 p.m. EST

— Our Milky Way galaxy is filled with streams of stars, but one of them appears to be a family that includes nearly 500 relatives, according to a new study.

Astronomers have discovered 8,292 stellar streams in our galaxy. Rather than clusters of stars, streams form linear patterns.

Each stream is named Theia for the Greek titan goddess of sight and heavenly light.

When astronomers used data gathered by the European Space Agency's Gaia space telescope to study Theia 456, they discovered that all 468 stars in this stream were born simultaneously. This elongated stream of stars is also moving in the same direction together across the sky.

The discovery was presented Friday at the 237th meeting of the American Astronomical Society, which is occurring virtually due to the pandemic.

"Most stellar clusters are formed together," said study author Jeff Andrews, a postdoctoral fellow at Northwestern University, in a statement. "What's exciting about Theia 456 is that it's not a small clump of stars together. It's long and stretched out. There are relatively few streams that are nearby, young and so widely dispersed."

Stars are often formed in spherical groups, which are known as clusters. More recent data, however, has revealed other patterns, like these long streams, which astronomers think occurred when clusters of stars were ripped apart or stretched out.

"As we've started to become more advanced in our instrumentation, our technology and our ability to mine data, we've found that stars exist in more structures than clumps," Andrews said. "They often form these streams across the sky. Although we've known about these for decades, we're starting to find hidden ones."

Theia 456 stretches for 570 light-years across the Milky Way.

This stellar stream remained hidden from astronomers for a long time because it lives in the galactic plane, where the stream can easily be cloaked by the Milky Way's 400 billion stars. The galactic plane is where most of a galaxy's mass exists.

Typically, stellar streams have been found outside of our galaxy by telescopes that point away from the Milky Way.

"We tend to focus our telescopes in other directions because it's easier to find things," Andrews said. "Now we're starting to find these streams in the galaxy itself. It's like finding a needle in a haystack. Or, in this case, finding a ripple in an ocean."

The stars within Theia 456 have a similar composition in that they all contain about the same amount of iron. This suggests that the stars all likely formed together about 100 million years ago.

Astronomers also looked at how the brightness of these stars has changed over time and determined that the stars spin at similar rates. This is further proof that they are the same age.

"If you know how the stars are moving, then you can backtrack to find where the stars came from," Andrews said. "As we rolled the clock backwards, the stars became closer and closer together. So, we think all these stars were born together and have a common origin."

Uncovering more about star formation in galaxies could lead to a greater understanding of the universe and how it came to be filled with galaxies and stars.

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