“Stellar archeology”; reveals a stellar stream composed of the remains of an ancient globular cluster that has been torn apart Milky Wayis gravity 2 billion years ago.
A team of astronomers, including Carnegie Ting Li and Alexander Ji, discovered a stellar current composed of the remnants of an ancient globular cluster that was torn apart by the gravity of the Milky Way 2 billion years ago, when the most complex life forms on Earth were single-celled organisms. This surprising finding, published in nature, highlights the conventional wisdom of how these celestial objects are formed.
Imagine a sphere composed of a million stars bound by gravity and orbiting the galactic nucleus. This is a spherical cluster. The Milky Way is home to about 150 of them, which form a delicate halo that surrounds our galaxy.
But the spherical cluster that created this newly discovered stellar current had a life cycle that was very different from the spherical clusters we see today.
“It’s stellar archeology that reveals the remnants of something ancient, exaggerated in the newer phenomenon,” Ji explained.
Using an Anglo-Australian telescope, the flux was detected using S5, Spectroscopic Survey Spectorroscope Survey Collaboration. The aim of the initiative, led by Li, is to map the motion and chemistry of stellar currents in the southern hemisphere.
In this study, the team focused on a stream of stars in the constellation Phoenix.
“The remnants of the globular clusters that make up Phoenix Stream were disrupted many billions of years ago, but fortunately they retain the memory of its formation in a very early universe, which we can read from the chemical composition of its stars,” Li said.
The team measured a number of heavier elements – what astronomers call the metallization of the star.
Stellar makeup reflects the cloud of galactic gas from which it was born. Previous generations of stars inoculated this material with heavy elements that they created during their lifetime, making the stars enriched or metallic. Therefore, a very ancient primitive star will have almost no heavy elements.
“We were really surprised to find that Phoenix Stream is significantly different from all other Milky Way spherical clusters,” explained lead author Zhen Wan of the University of Sydney. “Even though the cluster was destroyed billions of years ago, we can still say that it formed in the early universe.”
Because other known spherical clusters are enriched by the presence of heavy elements forged by old generations of stars, it has been assumed that there is a minimum number of heavier elements needed to form a spherical cluster.
However, the Phoenix Stream progenitor is well below this predicted minimal metallicity, which poses a significant problem for previous notions of how spherical clusters are born. “One possible explanation is that Phoenix Stream is the last of its kind, the rest of the ball cluster population, born in a radically different environment than the one we see today,” Li said.
Scientists have suggested that these globular clusters, which no longer existed, be constantly depleted by the gravitational forces of the Milky Way, which tore them to pieces. The remnants of other ancient globular clusters can also live as faint streams that can still be discovered before they disperse over time.
“There is still a lot of theoretical work to be done and we now have a lot of new questions to examine how galaxies and spherical clusters form,” said co-author Geraint Lewis, also of the University of Sydney.
Reference: 29 July 2020, nature.
DOI: 10,1038 / s41586-020-2483-6
This study was part of the Southern Stellar Streams Spectroscopic Survey, or S5 for short, an international collaboration using the 2dF / AAOmega instrument at the Anglo-Australian Telescope in Coonabarabran, NSW, to detect star currents uncovered during the Dark Energy Survey (DES).
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