13 billion-year-old 'streams of stars' discovered near Milky Way's center may be earliest building blocks of our galaxy

Astronomers peering into the heart of the Milky Way have discovered two gargantuan, never-before-seen structures. These vast "streams" of stars each contain the mass of 10 million suns and are up to 13 billion years old. They span wide swathes of the galaxy and may be some of the earliest building blocks of our Milky Way, scientists with the Max Planck Institute for Astronomy (MPIA)  said.

The two structures — described in a new study published March 21 in The Astrophysical Journal — have been named Shiva and Shakti, after the divine Hindu couple whose union is said to have brought harmony to the universe. The newfound stellar streams appear to have merged with the early Milky Way between 12 billion and 13 billion years ago, fueling our galaxy's growth.

"What's truly amazing is that we can detect these ancient structures at all," lead study author Khyati Malhan, an astrophysicist at MPIA said in a statement. "The Milky Way has changed so significantly since these stars were born that we wouldn't expect to recognise them so clearly as a group."

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The researchers spotted the cosmic structures using the European Space Agency's Gaia space telescope — a floating observatory that's been mapping the shape and structure of the Milky Way since 2014. By charting the speed, position and motion of more than 1.5 billion stars in our galaxy, Gaia's observations enable astronomers to draw connections between groups of stars that share similar origins, helping to piece together our galaxy's history.

It's thought that the Milky Way has collided with neighboring galaxies at least a dozen times over the last 12 billion years, with each merger funneling fresh stars into our evolving galaxy. The Gaia telescope has helped reveal several of these collisions — including a previously unknown merger with the so-called Gaia sausage dwarf galaxy, which our insatiable Milky Way swallowed 10 billion years ago, giving our galaxy its bulging center.

The millions of stars that make up Shiva and Shakti also seem to have contributed to our galaxy's overall structure but are located a bit further from the galactic center than previously identified fragments like the Gaia sausage.