An article published in the journal “Nature” reports the discovery of a primordial stellar stream at the edge of the Milky Way cataloged as C-19. A team of researchers used the GRACES instrument on the Gemini North telescope in Hawaii to study this stellar stream discovered in data collected by the Gaia space probe. The stars that are part of C-19 were identified in the Pristine survey as well, which aims to search for stars with the lowest metallicity in the Milky Way with the CFHT (Canada-France-Hawaii Telescope), also at Hawaii. Further data was collected using the Gran Telescopio Canarias in the Canaries. The study’s conclusion is that C-19’s stars were torn from an ancient star cluster when the Milky Way was very young. This new information could help better understand the formation of the first stars.
Stellar streams are associations of stars that orbit a galaxy. These are the remnants of globular clusters or dwarf galaxies that were victims of gravitational forces, in the case of C-19 by the Milky Way. Over the years, several stellar streams have been discovered around the Milky Way and now C-19 is added to them.
The oldest stars are those with the lowest metallicity, which means that they have the lowest amount of heavy elements. That’s because the oldest stars formed from clouds containing almost only hydrogen and helium while subsequent generations also contain heavier elements ejected by previous supernovae.
Studying very ancient stars means better understanding the first phase of the history of the universe with the formation of galaxies and the first stars. For this reason, the Pristine survey looked for the Milky Way stars with the lowest metallicity.
Data collected by ESA’s Gaia space probe, a special space telescope that aims to map stars and other objects, was examined using an algorithm created by the authors of the C-19 discovery to detect stellar streams. The image (International Gemini Observatory/NOIRLab/NSF/AURA/J. da Silva/Spaceengine. Acknowledgment: M. Zamani (NSF’s NOIRLab)) shows an illustration of the Milky Way and the Magellanic Clouds with C-19 on the left side.
The result of the search for stellar streams was positive but follow-up observations were needed to understand the origin of C-19. For this purpose, the researchers used the GRACES (Gemini Remote Access to CFHT ESPaDOnS Spectrograph) instrument installed on the Gemini North telescope. The results indicated that C-19’s stars have a low amount of elements heavier than hydrogen and helium with characteristics typical of stars that are part of ancient globular clusters.
The most likely reconstruction is that the stars that formed C-19 were torn from a primordial globular cluster when the Milky Way was very young. It’s possible that that cluster was destroyed by the Milky Way’s gravity and those stars are all that remain. That’s a normal event in galaxies history but it’s not easy to find traces after billions of years.
C-19 and other stellar streams can provide useful information on ancient catastrophic events and therefore on the formation and evolution of the Milky Way. They’re like fossils of ancient structures that help to better understand the formation of the first stars.
