An identified structure in the Milky Way has a lower proportion of heavy elements than any other star system known in our galaxy. The discovery, described in a scientific article published this Wednesday (5) in the journal Nature, was made through the Gemini Observatory, a program of the US National Optical-Infrared Astronomy Research Laboratory (NOIRLab).
According to observations, the stars of this stream were ripped from an ancient star cluster and are relics of the early Milky Way, which could provide data on the formation of the first stars.
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An international team of researchers, including members of the European Union, Canada and Russia, is responsible for the discovery of C-19, as the stellar stream is called, which lies south of the Milky Way. Its orbit extends about 20,000 light-years from the galactic center at its closest position and about 90,000 light-years at its furthest point.
Star system occupies an area equivalent to 30 full moons in the Milky Way
According to scientists, the star system spans an impressive area of the night sky — about 30 times the width of the The key — although not visible to the naked eye.
Using the Gemini North telescope, located in Hawaii, and the Gemini Remote Access to CFHT ESPaDOnS Spectrograph (GRACES) instrument, both at the Gemini Observatory, the team realized that C-19 is a remnant of a globular cluster.
It was previously thought that globular clusters had no less than 0.2% metallics, but C-19 has an unprecedented level: less than 0.05% metallicity.
The discovery that a low-metal flux originated from a globular cluster has implications for star formation, clusters stars and galaxies in the early universe.
In fact, the very existence of this flow indicates that globular clusters and the Milky Way’s first building blocks must have been able to form in low-metal environments, before successive generations of stars provided the universe with heavier elements.
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“It was not known if there were globular clusters with so few heavy elements. Some theories even suggested that they could not form,” commented Nicolas Martin of the Astronomical Observatory of Strasbourg, lead author of the study. “Other theories suggest that they all disappeared long ago, making this a key discovery for our understanding of how stars formed in the early universe.”
Team members originally detected C-19 in data from the Gaia satellite using an algorithm they specifically designed to detect stellar outflows. The stars in C-19 were also identified by the Pristine survey — a search for the lowest metallicity stars in and around the Milky Way using the Canada-France-Hawaii Telescope — as interesting enough to merit follow-up observations.
To identify the origin of C-19’s constituent stars, astronomers needed detailed GRACES spectra. The team also gathered data using a spectrograph mounted on the Gran Telescopio Canarias in La Palma in the Canary Islands.
“GRACES provided the critical clues that C-19 is a disrupted globular cluster and not the most common disrupted dwarf galaxy,” explained Kim Venn of the University of Victoria, the principal investigator of the GRACES observations. “We already knew that this was a very metal-poor stream, but identifying it as a globular cluster required the precision of metallicity and the detailed chemical abundances available only with high-resolution spectra.”
The new study also suggests that C-19 must have formed from very early generations of stars, making the cluster a remarkable relic from the time when the first groups of stars were forming.
Consequently, this discovery improves our understanding of the formation of stars and star clusters that emerged shortly after the Big Bang, and provides a natural laboratory for studying the oldest structures in galaxies.
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