Milky Way’s Ancient Star Rivers Predate Spiral Arms

Hidden in the cosmic swirl, the Milky Way’s origins have been traced through newfound relics.

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Tucked near our galaxy’s bustling center, two stellar streams as ancient as the cosmos itself have been spotted orbiting the Milky Way’s core. Thanks to the European Space Agency’s Gaia telescope, it’s now believed these starry rivers, Shiva and Shakti, flowed before our galaxy fashioned its iconic spirals.

Astrophysicist Khyati Malhan and astronomer Hans Walter-Rix from Germany’s Max Planck Institute for Astronomy stumbled upon these streams. “It’s kinda mind-blowing that we can spot such old structures,” admits Malhan. “Given how much the Milky Way’s morphed over billions of years, spotting these stars as a distinct bunch is unexpected. But with Gaia’s next-level data, we’ve done just that.”

The Milky Way started to take shape about 13 billion years back, in the Universe’s youth, rapidly churning out stars and galaxies. Deciphering this epic past falls to galactic archaeologists, who hunt for star clusters with ancient characteristics.

That’s where Gaia’s mission is key. This space observatory is sketching the Milky Way in unprecedented detail, capturing stars’ 3D positions, velocities, and motions. It even measures star metallicity — the presence of metallic elements, which hint at a star’s age. With fewer metals, a star is often much older.

Armed with this data, astronomers can spot star clusters that otherwise might go unnoticed. Stars that share space, speed, and metal content likely hail from the same original cluster.

These celestial formations also help unfurl the Milky Way’s story. Some star streams are all that’s left of star clusters the Milky Way has pulled apart. Others are remnants of entire galaxies our galaxy’s gravity has shredded.

This is how Shiva and Shakti caught the eye of Malhan and Walter-Rix. They were analyzing Gaia’s data on the Milky Way’s inner regions in 2022 when they noticed two streams populated by incredibly old, metal-scarce stars.

“Plotting the star paths, these two entities popped among stars with certain chemical traits,” Malhan mentions. “We dubbed ’em Shakti and Shiva.”

Weighing in at about 10 million solar masses each, Shiva and Shakti whirl in the Milky Way’s rotational direction. Shiva spins closer to the center, with more oval orbits, while Shakti, a bit further out, traces rounder routes.

Their composition suggests these streams are remnants of original structures that merged with the Milky Way, fueling its growth. The researchers think our galaxy began as a starry blob, expanding as cosmic filaments and tendrils fell in and wrapped around it. Shiva and Shakti seem to be two such filaments.

“Gaia’s mission is shedding light on our galaxy’s baby pictures, and it’s nailing it,” notes Gaia project scientist Timo Prusti.

“To piece together the Milky Way’s birth and evolution, we need super precise star data. Now, with Gaia, we’ve got it. Discovering unexpected galaxy parts like Shiva and Shakti means we’re slowly completing our cosmic puzzle, understanding our home and its earliest days.”

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