911, I wanna report a robbery. Hundreds of thousands of stars stolen from a cluster. I think it was the Milky Way

Ancient ball of suns smeared across space by our galaxy two billion years ago

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The Milky Way galaxy ripped apart an ancient star cluster born in the early universe and stole its suns two billion years ago, according to a study published in Nature on Wednesday.

These globular clusters are fairly common, and typically found on the fringes of galaxies. The Milky Way is orbited by about 150 such clusters each containing about a million stars. These spherical structures can fall apart as a nearby galaxy’s gravitational forces tug on the stars, like a ball of yarn unraveling.

That process, known as tidal stripping, resulted in our galaxy snaffling hundreds of thousands of stars from a globular cluster billions of years ago, astrophysicists now reckon. The scientists came to that conclusion after studying the Milky Way's Phoenix constellation and observing it had been smeared across space from a ball of stars to a long streak, a telltale sign it was once a kidnapped globular cluster.

“The longer a globular cluster orbits, the more the forces of the galaxy can strip stars off, with the entire cluster dissolving into a long stream before completely dispersing,” Geraint Lewis, lead author of a paper describing the study, and a physics professor at Australia's University of Sydney, explained to The Register.

“This is the stage we find Phoenix at. It is 27,000 light years long, but only 150 light years across, and so has been drawn from a sphere into a long piece of spaghetti, the unmistakable sign that it is being torn apart by the Milky Way.”

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What’s surprising about this particular case of galactic cannibalism, however, is that the globular cluster that the Phoenix stars once belonged to appears to be much older than the other clusters in the Milky Way.

"This stream comes from a cluster that, by our understanding, shouldn't have existed," said paper co-author Daniel Zucker, an associate professor of astronomy at Australia's Macquarie University.

"One possible explanation is that the Phoenix Stream represents the last of its kind, the remnant of a population of globular clusters that was born in radically different environments to those we see today," added Ting Li, co-author of the paper and a postdoctoral fellow at Carnegie Observatories.

It’s difficult to find out the exact age of these stars, though their lack of elements heavier than hydrogen and helium are a sign of their primordial nature. After the Big Bang, the universe contained only the simplest atoms, like hydrogen and helium. Heavier elements, like calcium, oxygen, and various metals are only found in later generations of stars.

The lack of complex elements in the Phoenix stream suggest that they came from an ancient population of globular clusters. “Aging stars in a globular cluster is tricky, but we know that the ones we see orbiting the Milky Way are old, formed in the first few billion years after the Big Bang. The fact that the Phoenix Stream appears to lack the quantity of elements seen in the existing globular clusters suggests that it is at least as old and quite probably older than these,” Lewis said.

The team believes that studying the globular cluster could unlock secrets to how the Milky Way formed over time. “Like all good research, we are left with more questions than answers and we really need to work out how Phoenix fits into the overall picture of the life of the Milky Way,” he added. Li told El Reg that it’s possible that the Phoenix stars were once in a globular cluster orbiting a satellite galaxy that was torn apart by the Milky Way.

“The ultimate goal is to understand how the Milky Way arose from the featureless universe that existed just [after] the Big Bang,” Lewis concluded. ®

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