Researchers have obtained the largest set of detailed observations yet of the oldest stars in the centre of the Milky Way.

The Pristine Inner Galaxy Survey (Pigs) team found that this group of stars is slowly spinning around the centre of our galaxy, despite being thought to have formed in a chaotic fashion.

They also seem to spend most of their long lives near the centre of the galaxy, according to the findings presented at the National Astronomy Meeting 2023 at the University of Cardiff by Pigs team member Dr Anke Arentsen from the University of Cambridge.

Some of the stars born in the first billion years after the Big Bang are still around today, and can be used to study what galaxies were like as they were starting to form.

Their pristine chemical composition, mostly hydrogen and helium, with a much lower abundance of heavier elements than younger stars like the sun, makes them easy to recognise.

Astronomers typically search for these ancient stars away from the Milky Way disc plane, in the low-density halo around our galaxy, where they are easier to find.

According to galaxy models, the oldest stars are expected to be in the dense inner parts of the Milky Way.

Finding them in this region is challenging as large amounts of interstellar dust make them difficult to see, and ancient stars are extremely rare compared with the overwhelming majority of their younger peers.

Dr Arentsen said: “It is exciting to think that we are seeing stars that formed in the earliest phases of the Milky Way, previously largely out of reach.

“These stars likely formed less than a billion years after the Big Bang, so are relics from the early universe.

“The available data for these ancient objects is growing rapidly. I’m excited to see what we will learn about these first stars to populate our galaxy in the next few years.”

In the Pigs project, Dr Arentsen and her team employed a special imaging filter on the Canada-France-Hawaii Telescope (CFHT) to efficiently pre-select candidate stars.

They were confirmed with the Anglo-Australian Telescope (AAT), resulting in the largest set yet of detailed observations for pristine inner galaxy stars.

The observations were then combined with data from the Gaia space mission to study how these ancient stars move through the Milky Way.

The researchers suggest the older the stars are, the more chaotic their motions.

However, even the oldest stars found still show some average rotation around the centre of the galaxy.

They also show that many of these stars spend almost all of their lives in the inner galaxy, inside a sphere that reaches only halfway between the galactic centre and the sun.