The Milky Way Has Ripped Apart A Unique Collection Of Stars

Artist’s impression of the thin stream of stars torn from the Phoenix globular cluster, wrapping around the Milky Way (left). Astronomers targeted bright red giant stars (artist’s impression, right) to measure the chemical composition of the disrupted Phoenix globular cluster. James Josephides, Swinburne Astronomy

Globular clusters are spherical collections of stars that orbit galaxies. More than 150 of them have been discovered around the Milky Way alone. Comprehensive studies of these and many others orbiting distant galaxies suggest they formed from the earliest generation of stars, but a new discovery might append this scenario.

An international team of researchers has discovered a structure around the Milky Way, dubbed the Phoenix Stream. This is a spread out ribbon of stars that's believed to be what remains of an ancient globular cluster ripped apart by the Milky Way roughly 2 billion years ago.

What is truly unique about the Phoenix Stream is its composition of stars. As reported in the journal Nature, the researchers likely found what remains of the oldest known globular cluster, a relic of a bygone age.  

“Once we knew which stars belonged to the stream, we measured their abundance of elements heavier than hydrogen and helium; something astronomers refer to as metallicity. We were really surprised to find that the Phoenix Stream has a very low metallicity, making it distinctly different to all of the other globular clusters in the Galaxy,” lead author Zhen Wan, a graduate researcher at the University of Sydney, said in a statement. “Even though the cluster was destroyed billions of years ago, we can still tell it formed in the early Universe from the composition of its stars.”

Globular clusters are believed to have a “metallicity floor”, below which they can't form in the way we expect them to. This minimum value comes from the expected enrichment from the earliest stars that were made exclusively of hydrogen and helium, and their explosive demise that spread heavier elements into the cosmos. If the Phoenix Stream was indeed a globular cluster, then maybe the universe has more than one way to form these compact collections of stars.

“This stream comes from a cluster that, by our understanding, shouldn’t have existed,” explained co-author Associate Professor Daniel Zucker from 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 Dr Ting Li from Carnegie Observatories and leader of the S5 international collaboration, which studies stellar streams.

There are many unanswered questions. The team remains unsure of how the Phoenix Stream might have formed and if it is unique, but they plan to study more stellar streams to find out.

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