Artist rendering of the Phoenix Stellar Stream. Originally a ball of stars, known as a globular cluster, Phoenix has been converted to 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: Illustration courtesy of Geraint F. Lewis and collaboration S5.
“Stellar archeology” reveals a stellar stream made up of the remains of an ancient globular cluster that was destroyed by the Milky WayThe gravity of 2 billion years ago.
A team of astronomers, including Ting Li and Alexander Ji of Carnegie, discovered a stellar stream made up of the remains of an ancient globular cluster that was destroyed by gravity in the Milky Way galaxy 2 billion years ago, when life forms most Earth complexes were single-celled organisms. This surprising finding, published in Nature, it changes the conventional wisdom on how these celestial objects are formed.
Imagine a sphere made up of a million stars bound by gravity and orbiting a galactic core. That is a globular cluster. The Milky Way is home to about 150 of them, which form a faint halo that surrounds our galaxy.
But the globular cluster that generated this newly discovered star current had a life cycle that was very different from the globular clusters that we see today.
“This is stellar archeology, uncovering the remains of something ancient, carried away by a more recent phenomenon,” Ji explained.
Using the Anglo-Australian Telescope, the current was revealed by S5, the Southern Star Stream Spectroscopic Study Collaboration. Led by Li, the initiative aims to map the motion and chemistry of star currents in the southern hemisphere.
An artist’s impression of the thin stream of stars ripped from the Phoenix globular cluster, enveloping our Milky Way galaxy (left). For the study, astronomers targeted bright Red Giant stars to measure the chemical composition of the disrupted Phoenix globular cluster (right). Credit: Illustration courtesy of James Josephides (Swinburne Astronomy Productions) and the S5 Collaboration.
In this study, the collaboration focused on a stream of stars in the Phoenix constellation.
“The remnants of the globular cluster that make up the Phoenix Current were disrupted many billions of years ago, but fortunately they retain the memory of their formation in the early universe, which we can read of the chemical composition of their stars,” Li said.
The team measured the abundance of heavier elements, what astronomers call the metallicity of a star.
The makeup of a star reflects that of the galactic gas cloud from which it is born. The more previous generations of stars have seeded this material with heavy elements that they produced during their lives, the stars are said to be more enriched or metallic. Therefore, a very old primitive star will have almost no heavy elements.
“We were very surprised to discover that Phoenix Stream is clearly different from all other globular clusters in the Milky Way,” explained lead author Zhen Wan of the University of Sydney. “Although the cluster was destroyed billions of years ago, we can still say that it formed in the early universe.”
Because other known globular clusters are enriched by the presence of heavy elements forged by previous stellar generations, it was theorized that a minimal abundance of heavier elements was required for a globular cluster to form.
But the parent Phoenix Stream falls well short of this predicted minimal metallicity, posing a major problem for earlier ideas about how globular clusters are born. “One possible explanation is that Phoenix Stream represents the last of its kind, the remnant of a globular cluster population that was born in radically different environments than we see today,” Li said.
The researchers proposed that these globular clusters that are no longer with us were constantly depleted by the gravitational forces of the Milky Way, which tore them apart. The remains of other ancient globular clusters can also live as weak currents that can still be discovered before they dissipate over time.
“There is a lot of theoretical work to be done, and now we have many new questions to explore about how galaxies and globular clusters form,” said co-author Geraint Lewis, also of the University of Sydney.
###
Reference: July 29, 2020, Nature.
DOI: 10.1038 / s41586-020-2483-6
This study was part of the Southern Star Currents Spectroscopic Survey, or simply S5 for short, an international collaboration using the 2dF / AAOmega instrument at the Anglo-Australian Telescope in Coonabarabran, NSW, to examine star currents discovered during the Survey Dark Energy (DES)
The Carnegie Institution for Science (carnegiescience.edu) is a private, non-profit organization based in Washington, DC, with six research departments across the US Since its founding in 1902, the Carnegie Institution has been a pioneering force in basic scientific research. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science.
