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Remains of an old globular cluster that is “the last of its kind”



Discovery: Remnant of an old globular cluster that is

Representation of a Phoenix Stellar Stream artist. Originally a ball of stars, known as a globular cluster, Phoenix has been stretched into a stream of stars by the gravitational pull of the Milky Way. In a few billion years, Phoenix will be completely destroyed and absorbed into our galaxy. Credit: Geraint F. Lewis and S5 collaboration.

A team of astronomers including Teg Li of Carnegie and Alexander Ji have discovered a stellar current made up of the remnants of an ancient globular cluster that was cut by the gravity of the Milky Way 2 billion years ago, when most Earth̵

7;s complex life were single-celled organisms. This surprising finding, published in Nature, reinforcing conventional wisdom on how to form these celestial objects.

Imagine a sphere made up of a million gravity-bound stars orbiting a galactic core. That’s a globular group. The Milky Way is home to about 150 of them, forming a tenuous halo that achieves our galaxy.

But the globular group that generated this newly discovered stellar flux had a life cycle that was very different from the globular groups we are seeing today.

“This is stellar archeology, revealing the remnants of something ancient, swept away in the more recent phenomenon,” Ji explained.

Using the Anglo-Australian Telescope, the stream was revealed by S5, the Southern Stellar Stream Spectroscopic Survey Collaboration. Led by Li, the initiative aims to portray the motion and chemistry of stellar flows in the Southern Hemisphere.

In this study, the collaboration focused on a stream of stars in the constellation of Phoenix.

“The remnants of the globular cluster that make up the Phoenix Flow were disrupted many billion years ago, but fortunately they retain the memory of its formation in the early universe, which we can read from the chemical composition of its stars,” she said. Li

Discovery: Remnant of an old globular cluster that is

An artist’s impression of the thin stream of stars cut by the globular group Phoenix, wrapped around our Milky Way (left). For the study, astronomers targeted a bright Red Giant star, to measure the chemical composition of the disrupted globular cluster of Phoenix (right). Credit: James Josephides (Swinburne Astronomy Productions) and S5 Collaboration.

The team measured the abundances of heavier elements — what astronomers call the metallicity of a star.

The shape of a star reflects that of the cloud of galactic gas from which it is born. The more previous generations of stars sow this material with heavy elements that they produced during their lifetime, the more tired, or metallic, they are said to be the stars. Thus, a very old, primitive star, has almost no heavy elements.

“We were really surprised to find that the Phoenix Stream is different from all of the other globular clusters in the Milky Way,” explained lead author Zhen Wan of the University of Sydney. “Even though the cluster was destroyed millions of years ago, we can still say that it was formed in the early universe.”

Because other known globular clusters are enriched by the presence of forged heavy elements from previous stellar generations, it has been theorized that there was a minimal abundance of heavier elements needed to form a globular cluster.






How a group of ancient globes lacking heavy elements was destroyed by the gravitational field of Christmas Milk billions of years ago. Credit: Sebastian Zentilomo, University of Sydney

But the progenitor of Phoenix Stream is well below this predicted minimum metallicity, which poses a significant problem for earlier ideas about how globular clusters are born. “One possible explanation is that the Phoenix Stream represents the last of its kind, the remnant of a population of globular clusters born in radically different environments to the ones we see today,” Li said.

The researchers proposed that these globular clusters that are no longer with us were weakened by the gravitational forces of the Milky Way, which cut them into pieces. The remnants of other ancient globular clusters can also live on bad streams that can still be discovered before they are released over time.

“There is still a lot of theoretical work left, and now there are a lot of new questions for us to explore about how galaxies and globular clusters form,” said co-author Geraint Lewis, also of the University of Sydney.


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More information:
The tidal residue of a poor globular group is not the usual metal, Nature (2020). DOI: 10.1038 / s41586-020-2483-6, www.nature.com/articles/s41586-020-2483-6

Provided by the Carnegie Institution for Science



Citation: Discovered: Remnant of an old globular cluster that is “the last of its kind” (2020, July 29) obtained on July 29, 2020 from https://phys.org/news/2020-07 -remnant-ancient-globular-cluster-kind .html

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