Remains of stars ripped apart by our galaxy two billion years ago found by astronomers

Scientists have found the leftovers of an ancient cluster of stars that was ripped apart by our own Milky Way, more than two billion years ago.

The remnants offer a way of looking back to the universe when it was much younger, and has fundamentally changed our understanding of how such celestial objects formed.

A globular cluster is a round collection of a million stars, stuck together by gravity and all orbiting around a galactic core. There are about 150 of them in our Milky Way, together enveloping our galaxy.

But the newly discovered cluster is like none ever found before. Its time was very different from those others formed today – and its death happened in a spectacular clash that saw it torn apart by the gravity from our Milky Way, billions of years ago.

The newly discovered star stream spawned by that cluster is like the artefacts found deep beneath the earth, offering a similar look at a time before ours.

"This is stellar archeology, uncovering the remnants of something ancient, swept along in a more-recent phenomenon," said Alexander Ji, from the Carnegie Institution for Science, who was one of the team of astronomers who discovered the cluster.

Though the cluster was torn apart by our galaxy, and has been disrupted for billions of years, the galaxy is said to be the last of its kind, and retains a memory of a kind of celestial structure that was once found elsewhere in the universe.

"The globular cluster remnants that make up the Phoenix Stream were disrupted many billion years ago, but luckily retain the memory of its formation in the very early universe, which we can read from the chemical composition of its stars," said Ting Li, also from Carnegie.

The cluster is unusual when compared with other stars in its nearby Phoenix consellation, which allowed researchers both to identify that it appeared to be different from the clusters surrounding it, as well as offering an insight into the conditions in which it was formed.

"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," said Zhen Wan, a PhD student at the University of Sydney, who led the new study.

"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."

The metallicity of the cluster is so low that astronomers would previously not have expected it to be able to actually form. Researchers had previously oibserved that other globular clusters' stars are enriched with heavier elements, originally former in earlier generations that went before them, and that therefore such clusters relied on previous stars, and would not form if they were below a given metallicity floor.

But the new cluster sits comfortably beneath that minimum, and so should not have been able to form, according to our current understanding. That suggests that our account of how such clusters form could be significantly wrong.

"This stream comes from a cluster that, by our understanding, shouldn't have existed," said co-author Associate Professor Daniel Zucker from Macquarie University.

That makes it difficult for astronomers to place where it could have come from, and to re-write our stories of how galaxies form. We were lucky to catch the cluster at all, the researchers said – while there was once many of them, they were gradually torn apart by our Milky Way, and then absorbed into the system before fading away.

"We found the remains of this cluster before it faded forever into the Galaxy's halo," Mr Wan said.

Researchers now hope to discover whether there are more examples of this stream.

"The next question to ask is whether there are more ancient remnants out there, the leftovers of a population that no longer exists. Finding more such streams will give us a new view of what was going on in the early Universe," said Dr Li.