Astronomers find a rare globular cluster in the Milky Way

Hyperaxion Jul 30, 2020

Named Phoenix Stream, the globular cluster is the last of its kind and contains fewer metals than predicted by scientific models.

Astronomers from Australian, North American, and European institutions have discovered a stellar stream formed by the debris of a globular cluster of stars that was destroyed by the Milky Way’s gravity 2 billion years ago.

Astronomers find a rare globular cluster in the Milky Way
Artist’s impression shows the Phoenix Stream surrounding the Milky Way. (Credit: James Josephides (Swinburne Astronomy Productions) and the S5 Collaboration).

The article on the discovery, published this week in the journal Nature, is the result of studies on the chemical composition of the Phoenix constellation.

Our galaxy has about 150 globular clusters, spheres composed of millions of stars that remain connected due to gravity and orbit a galactic core, forming a “halo” around it.

Other similar globular clusters have been observed by scientists, but none have shown a lifecycle similar to Phoenix Stream.

According to current parameters, the metallicity (proportion of elements more massive than helium) in stars grows over time – and since most of the existing globular clusters are enriched with heavy elements, it is believed that globular clusters must have at least a certain amount of heavy elements.

However, looking at the chemical composition of the Phoenix Stream, the researchers found that it contains less heavy elements than expected.

“We were really surprised to find that the Phoenix Stream is distinctly different to all of the other globular clusters in the Milky Way,” said Zhen Wan, lead author of the study and a researcher at the University of Sydney, Australia. “Even though the cluster was destroyed billions of years ago, we can still tell it formed in the early universe.”

(Credit: Geraint F. Lewis and the S5 collaboration).

The study casts doubt on previous theories that describe how globular clusters are born. According to Ting Li, co-author of the article and scientist at the Carnegie Institution for Science, in the United States, a possible explanation for the phenomenon is that the Phoenix Stream represents “the last of its kind”, it is part of a class of globular clusters that no longer exists.

The researchers theorize that, probably, other clusters of this type were destroyed by the gravitational forces of the Milky Way long ago. “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,” said Geraint Lewis, also a co-author of the study and a researcher at the University of Sydney.


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