The Milky Way is 'rippling' like a pond, and scientists may finally know why - Livescience.com

Imagine the Milky Way's 100 billion stars as a flat, tranquil pool of water.

15 in the Monthly Notices of the Royal Astronomical Society, researchers explain how a nearby mini-galaxy — the Sagittarius dwarf galaxy — appears to have crashed through the Milky Way on at least two separate occasions, causing stars all around the galaxy to mysteriously oscillate at different speeds.

Through a process that the researchers equated to "galactic seismology," the team modeled a wave pattern that could explain the strange ripple effect setting the Milky Way's stars off-kilter.

They concluded that the ripples were likely released hundreds of millions of years ago, when the Sagittarius dwarf galaxy last passed through our galaxy — "a little bit like when a stone is dropped into a pond," McMillan said.

Prior studies (opens in new tab) have suggested that an ancient collision with Sagittarius may have set off ripples at the Milky Way's center, but this new research is the first to show that those ripples extended all the way to the edge of the galaxy's disk, perturbing stars every step of the way.

Today, the Sagittarius dwarf galaxy is estimated to be about 400 times the mass of Earth's sun — a mere shrimp compared with the Milky Way's estimated mass of 1.5 trillion suns.

These collisions likely changed the shape and size of our galaxy too; a 2011 study suggested that the Milky Way's spiral arm is the result of two collisions with the Sagittarius dwarf galaxy.

Another study of Gaia data released in 2020 suggested that cosmic crashes between our galaxy and Sagittarius triggered baby booms of new stars in the Milky Way every time the two galaxies met.