Discovery of the least 'metallic' stellar structure in the Milky Way

98.5% of the Sun is made up of two light chemical elements, hydrogen and helium, while the remaining 1.5% consists of other heavier elements such as carbon, oxygen, and iron. The abundance of these heavier elements in a star is called its 'metallicity', and varies from star to star. It now turns out that our Galaxy is home to a stellar structure uniquely made of stars with extremely low metallicity, with a heavy element content 2,500 times lower than that of the Sun. This is well below that of any other known stellar structure in the Universe. This discovery1 , 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/Université Côte d'Azur), is published on January 5, 2022 in the journal Nature. This group of stars all belong to a stellar structure in the Milky Way called C-19. 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 unique and direct window onto the very earliest ages of star formation and the development of stellar structures in the very distant past. Since heavy elements were produced by successive generations of massive stars, the very low metallicity of the C-19 stars shows that they were formed only a short time after the birth of the Universe.

• 1This work was carried out as part of the Pristine survey, with ESA's Gaia space observatory, the Canada-France-Hawaii Telescope (Hawaii), the Gemini North telescope (Hawaii) and the GTC telescope (Canary Islands).