Dubbed Shakti and Shiva, the newly-identified structures are between 12 and billion years old — so ancient they likely formed before even the oldest parts of the present-day Milky Way’s spiral arms and disk.
“What’s truly amazing is that we can detect these ancient structures at all,” said Dr. Khyati Malhan, an astronomer at the Max Planck Institute for Astronomy.
“The Milky Way has changed so significantly since these stars were born that we wouldn’t expect to recognize them so clearly as a group — but the unprecedented data we’re getting from ESA’s Gaia satellite made it possible.”
Using Gaia observations, Dr. Malhan and Max Planck Institute for Astronomy’s Dr. Hans-Walter Rix were able to determine the orbits of individual stars in the Milky Way, along with their content and composition.
“When we visualised the orbits of all these stars, two new structures stood out from the rest among stars of a certain chemical composition. We named them Shakti and Shiva,” Khyati said.
Each stream contains the mass of about 10 million Suns, with stars of 12 to 13 billion years in age all moving in very similar orbits with similar compositions.
The way they’re distributed suggests that they may have formed as distinct fragments that merged with the Milky Way early in its life.
Both Shakti and Shiva streams lie towards the Milky Way’s heart.
Gaia explored this part of the Milky Way in 2022 using a kind of galactic archaeology. This showed the region to be filled with the oldest stars in the entire Galaxy, all born before the disk of the Milky Way had even properly formed.
“The stars there are so ancient that they lack many of the heavier metal elements created later in the Universe’s lifetime,” Dr. Rix said.
“The stars in our Galaxy’s heart are metal-poor, so we dubbed this region the Milky Way’s ‘poor old heart’.”
“Until now, we had only recognized these very early fragments that came together to form the Milky Way’s…
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