“Ghost Stars of the Milky Way” –Reveal Secrets of Ancient Globular Clusters

Milky Way's Phoenix Stream


“In less than a hundred years, we have found a new way to think of ourselves,” observed Stephen Hawking. “From sitting at the center of the universe, we now find ourselves orbiting an average-sized sun, which is just one of millions of stars in our own Milky Way galaxy.” Some of the oldest stars in the universe are found in ancient globular clusters that orbit around the plane, the nucleus, of our home galaxy, which is circled by 150 globular clusters, harboring hundreds of thousands, and sometimes millions of stars, formed very early in the vast halo surrounding the embryonic Milky Way before it flattened to form the spiral disc.

A team of astronomers have discovered a stellar stream composed of the remnants of an ancient globular cluster that was ripped asunder by the Milky Way’s gravity 2 billion years ago that had a lifecycle that was very different from the globular clusters we see today.. This surprising finding, reports the journal Nature, upends conventional wisdom about how these celestial objects form.

“This is stellar archeology, uncovering the remnants of something ancient, swept along in a more-recent phenomenon,” explained Cargenie’s Alexander Ji.

Stream of Stars in the Phoenix Constellation

Using the Anglo-Australian Telescope, the stream was revealed by S5, the Southern Stellar Stream Spectroscopic Survey Collaboration. Led by the Carnegie Institutes’ Carnegie’s Ting Li, the initiative aims to map the motion and chemistry of stellar streams in the Southern Hemisphere focusing on a stream of stars in the Phoenix constellation.

“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 Li.

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An artist’s impression above shows the thin stream of stars torn from the Phoenix globular cluster, wrapping around our Milky Way (left). For the study, the astronomers targeted bright Red Giant stars, to measure the chemical composition of the disrupted Phoenix globular cluster (right). Top right image: the thin stream of stars torn from the Phoenix globular cluster, wrapping around our Milky Way (left). For the study, the astronomers targeted bright Red Giant stars, to measure the chemical composition of the disrupted Phoenix globular cluster (right). Illustration is courtesy of James Josephides (Swinburne Astronomy Productions) and the S5 Collaboration.

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.

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“We were really surprised to find that the Phoenix Stream is distinctly different to 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 billions of years ago, we can still tell it formed in the early universe.”

Because other known globular clusters are enriched by the presence of heavy elements forged by stellar earlier generations, it was theorized that there was a minimum abundance of heavier elements required for a globular cluster to form.

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But the Phoenix Stream progenitor is well below this predicted minimum metallicity, posing a significant problem for previous 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 that was born in radically different environments to those we see today,” Li said.

Ghosts of Milky Way’s Gravitational Forces

The researchers proposed that these no-longer-with-us globular clusters 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,” said co-author Geraint Lewis, also of the University of Sydney.

Source information: The tidal remnant of an unusually metal-poor globular cluster, Nature (2020). DOI: 10.1038/s41586-020-2483-6 , www.nature.com/articles/s41586-020-2483-6

The Daily Galaxy, Max Goldberg, via Carnegie Institution for Science and Nature

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