Imagine the Milky Way’s 100 billion stars as a puddle of calm water Now imagine someone dropped a stone the size of 400 million suns into that water The silence is broken. This wave of energy ripples across the galaxy’s surface, jostling and tossing its stars in a chaotic dance that takes forever to settle down.

Through a process that researchers liken to “galactic seismic”, the team modeled waves that could explain the strange rippling effect that is messing up stars in the Milky Way. They concluded that the ripples may have been released hundreds of millions of years ago, when the Sagittarius dwarf galaxy last passed our galaxy – “like when a stone fell into a pond,” McMillan said It looks like a second, even earlier collision between the two galaxies has occurred, the researchers add. Previous studies suggested that an ancient collision with Sagittarius could cause ripples in the center of the Milky Way, but this new study is the first to show that these ripples extend to the edge of the Milky Way. disk of the galaxy, shuffling the stars every step of the way This new study will help piece together the long and violent history of our galaxy and its smaller neighbour, the researchers write. Today, the Sagittarius dwarf galaxy is estimated to have about 400 times the mass of Earth’s Sun – just a shrimp compared to the Milky Way’s estimated 1.5 trillion Suns mass. Scientists suspect that Sagittarius was once much larger, but has lost up to 20% of its mass into our galaxy after repeated collisions over the past billion years These collisions may have also changed the shape and size of our galaxy. A 2011 study suggested that the Milky Way’s spiral arms were the result of two collisions with the dwarf galaxy Sagittarius. Another study of Gaia data published in 2020 suggested that cosmic collisions between our galaxy and Sagittarius trigger the explosion of new stars in the Milky Way whenever two galaxies meet.