Strange, Ancient Stars of the Milky Way -Challenge Current Understanding


Globular Cluster M 80


“The discovery of this exceptional globular cluster remnant will help refine models of the formation of the first stellar structures of the Universe,” astronomer Nicolas Martin told The Daily Galaxy about the discovery of a stellar structure known as C-19 uniquely comprising stars with extremely low metallicity. Recent studies suggest that planetary systems made up of terrestrial-type planets in compact, tightly-spaced orbits are most likely to form around lower-metallicity stars. Small planetary systems orbiting low-metallicity stars have been around a lot longer, and may be the earliest type of planetary system, making them an ideal place to search for life. 

The C-19 Remnant

“We believe,” Martin noted, “that most stars form in clusters but current models have issues reproducing the formation of a cluster as metal-poor as the C-19 progenitor. We are clearly missing some ingredients in those models. Hopefully, the discovery of C-19 and maybe the future discovery of more exceptional systems like this one will help refine the models.”

C-19s content of heavy elements such as carbon, oxygen, and iron is 2,500 times lower than that of our Sun, well below that of any other known stellar structure in the universe. Even dense globular clusters scattered throughout the halo of our Galaxy have slightly higher metallicities than the stars in the C-19 stream. The low metallicity of the C-19 stars shows that they were formed only a short time after the birth of the universe, prior to heavy elements that were produced by successive generations of massive stars.

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Globular Clusters

A map of the Milky Way globular clusters overlaid on top of the Milky Way shown below was obtained by the Gaia satellite. Each cluster is a grouping of thousands to millions of stars as shown in the insert. The color of the symbols represent their “metallicity,” which corresponds to their fraction in heavy elements with respect to the Sun. The stars of the C-19 stream are shown with star symbols and have a much lower metallicity than any cluster, 2500 times fewer heavy elements when compared to the Sun. (N. Martin & Observatoire Astronomique de Strasbourg; Canada-France-Hawaii Telescope / Coelum; ESA/Gaia/DPAC).

C-19 Stream

“This observation opens a unique window directly onto the first epoch of star formation in the universe” says Nicolas Martin, a researcher at Observatoire Astronomique de Strasbourg in a separate IAC paper about the stars which have very low proportions of heavy metals: “We have found a relic of the epoch when the first stellar structures were formed. Until now, he adds, “nobody knew that there were globular clusters with so little heavy element content, so that this work is a key discovery for understanding how stars formed in the primitive universe.”

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Alien Galaxies

“Most of the stars in our neighborhood, such as the Sun, were formed in our galaxy. However, a tiny fraction of the stars and star clusters in the Milky Way, which can be found in its surroundings, were brought here in smaller galaxies,” explains Jonay González, a researcher at the Instituto de Astrofísica de Canarias and a co-author of the IAC  article. “The cluster we have discovered was probably introduced into the galaxy this way, but has been losing its stars in its orbit around the Galaxy as a result of tidal attractions, leaving a “celestial footprint” of stars“, he adds.

This discovery, made by an international team led by a CNRS researcher at the Strasbourg Astronomical Observatory (CNRS / University of Strasbourg), and involving scientists from the Galaxies, Stars, Physics and Instrumentation Laboratory (Paris Observatory—PSL / CNRS) and at the J-L Lagrange Laboratory (CNRS / Côte d’Azur Observatory), is published on January 5, 2022 in the journal Nature.

C-19 Stream

Artist’s picture of the remnant of globular cluster C-19 in the Milky Way. Credit: Gabriel Pérez Díaz (SMM, IAC).

Not only does this discovery challenge our current understanding and models of the formation of these stellar groupings, which exclude the existence of structures composed only of such stars, it also opens a direct window to the very earliest ages of star formation and the development of stellar structures in the very distant past.



At face value, nothing distinguishes C-19 stars from other stars of the Milky Way. Only the tiny but very precise movement of the star measured by the Gaia satellite and the observation of its metallicity in the metallicity map from the Pristine survey made it possible to isolate this star (one of the C-19 stars observed with the Gran Telescopio Canarias) as part of the C-19 remnant of an exceptionally low metallicity stream of stars. (c) Aladin (Observatoire Astronomique de Strasbourg) & SDSS.

Formed Only a Short Time after the Birth of the Universe

Since heavy elements were produced by successive generations of massive stars, the low metallicity of the C-19 stars shows that they were formed only a short time after the birth of the universe. The oldest and most metal-poor stars formed in the halo of our Milky Way Galaxy before the gas flattened into a spiral disk where current generations of stars are made today. Most metal-poor halo stars are isolated or reside in dense globular clusters that have survived the 13 billion year dynamical evolution of our Milky Way. The stars in C-19 are not only extremely metal poor, but also extend across a sizable fraction of the galactic halo within a sparse stellar stream. The progenitor of C-19 was possibly a loose globular cluster or extremely metal-poor satellite galaxy that got stretched apart from galactic tidal forces. 

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GAIA and the Pristine Survey

This finding of the primitive stellar structure reports the IAC was possible thanks to data taken by the GAIA satellite of the European Space Agency (ESA), and the identification of these primitive stars in the Pristine Survey, which is being carried out with the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea, Hawaii. The research team explored the map recorded by the Gaia satellite, using a new algorithm which helps to isolate these rare stellar groupings including a new stellar stream which the team labelled “C-19”. At the same time the Pristine study, from Hawaii, had been mapping the sky to make systematic measurements of the abundances of heavy elements in millions of stars. The combination of the two studies showed that C-19 contains stars whose heavy element abundances are extremely low.

Source: Nicolas Martin, A stellar stream remnant of a globular cluster below the metallicity floor, Nature (2022). DOI: 10.1038/s41586-021-04162-2Image at the top of the page M80 (NGC 6093) is one of the densest globular star clusters in the Milky Way Galaxy. Courtesy The Hubble Heritage Team (AURA/STSCI/NASA)

Image at the top of the page M80 (NGC 6093) is one of the densest globular star clusters in the Milky Way Galaxy. Courtesy The Hubble Heritage Team (AURA/STSCI/NASA)

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Nicolas Martin, IAC and CNSR

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