Astronomers have discovered a “shredded globular cluster” – the remnants of a collection of stars torn apart by the Milky Way more than two billion years ago.
What’s surprising, they say, is that the stars have much lower quantities of heavier elements than in other such clusters, suggesting that the original structure was the last of its kind, with a lifecycle very different to those remaining today.
And that would upend conventional wisdom about how these celestial objects form.
The work was led by Zhen Wan and Geraint Lewis from Australia’s University of Sydney, as part of the international Southern Stellar Stream Spectroscopic Survey (S5) collaboration, and is described in a paper in the journal Nature.
There are about 150 globular clusters in the Milky Way, each a ball of a million or so stars that orbit in the stellar halo.
Using the Anglo-Australian Telescope in New South Wales, the researchers focussed on a stream of stars in the Phoenix Constellation, measuring the abundances of heavier elements – what astronomers call a star’s metallicity.
A star’s makeup mirrors that of the cloud of galactic gas from which it is born. The more prior generations of stars have seeded this material with heavy elements that they produced during their lifetimes, the more enriched, or metallic, the stars are said to be. Therefore, a very ancient, primitive star, will have almost no heavy elements.
“We were really surprised to find that the Phoenix Stream is distinctly different to all of the other globular clusters in the Milky Way,” says Wan. “Even though the cluster was destroyed billions of years ago, we can still tell it formed in the early Universe.”
Observations of other globular clusters have found that their stars are enriched with heavier elements forged in earlier generations of stars.
Current formation theories suggest that this dependence on previous stars means that no globular cluster should be found unenriched and that there is a minimum metallicity “floor” below which no cluster can form. The metallicity of the Phoenix Stream progenitor sits well below this minimum, however.
“This stream comes from a cluster that, by our understanding, shouldn’t have existed,” says co-author Daniel Zucker from Australia’s Macquarie University.
One possible explanation, says S5 team leader Ting Li from Carnegie Observatories, US, 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”.
The researchers suggest that these globular clusters no longer with us were steadily depleted by the Milky Way’s gravitational forces, which tore them to pieces. The remnants of other ancient globular clusters may also live on as faint streams that can still be discovered before they dissipate over time.
“There is plenty of theoretical work left to do, and there are now many new questions for us to explore about how galaxies and globular clusters form,” says Lewis.
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