An international team of researchers has obtained the most comprehensive set of detailed observations to date of the oldest stars in the Milky Way, at the center of our Galaxy. The Pristine Inner Galaxy Survey (PIGS) team found that this star cluster slowly rotates around the center of the Milky Way, despite its irregular formation. They also seem to spend most of their long lives near the galactic center. PIGS team member Dr Anke Arentsen from the University of Cambridge presents the new study this week at the National Astronomy Meeting 2023 at Cardiff University.

Some stars born in the first billion years after the Big Bang are still around and can be used to study what galaxies were like when they were just starting to form. They can be recognized by their primitive chemical composition, mostly hydrogen and helium, with much less heavy elements than younger stars like the Sun. Astronomers often look for these ancient stars in the low-density halo around our Galaxy, far from the plane of the Milky Way disk, where they are easier to find.

Galaxy formation models suggest that the oldest stars are expected to be found in the dense interior of the Milky Way. They are difficult to find in this region because our line of sight to the center of the Galaxy is blocked by a large amount of interstellar dust, and old stars are extremely rare compared to the vast majority of their younger peers.

In the PIGS project, Arensen and his team used a custom image filter on the Canada-France-Hawaii Telescope (CFHT) to efficiently preselect candidate stars. These were confirmed by spectroscopic observations on the Anglo-Australian Telescope (AAT), resulting in the largest set of detailed observations of pristine stars in the inner Galaxy to date.

The PIGS observations were then combined with data from the Gaia space mission to study how these ancient stars moved in the Milky Way. It seems that the older the stars, the more chaotic their motion, but even the oldest stars found still show some average rotation around the Galactic centre. They also show that many of these stars spend most of their lives in the interior of the Galaxy, in a sphere that only reaches the middle of the galactic center and the Sun.

Arentsen comments: “It’s very exciting to think that we’re seeing stars forming in the early stages of the Milky Way that were previously largely inaccessible. These stars, like remnants from the early universe, probably formed less than a billion years after the Big Bang. The available data on these ancient objects is growing rapidly. I’m excited to see what we’ll learn about these first stars to populate our Galaxy in the next few years!” Source