The Milky Way Almost Collided With Another Galaxy Millions of Years Ago and it Made the Stars Go Crazy

Stunning new research from the European Space Agency (ESA) shows the lasting impact of a near collision between the Milky Way and another galaxy hundreds of millions of years ago, which sparked a "ripples in a pond" effect among the stars.

Using the ESA's star-mapping mission Gaia, which measures the positions and velocities of over a billion stars, researchers could piece together the paths of a few million stars in the Milky Way. What the data revealed left them in awe.

One shape stood out—a snail shell of stars swirling around over time. The event is thought to have happened between 300 and 900 million years ago.

"At the beginning the features were very weird to us," said Teresa Antoja from Spain's Universitat de Barcelona, who led on the research, in an ESA release. "I was a bit shocked and I thought there could be a problem with the data because the shapes are so clear."

However, after rigorous validation by other teams of researchers, their findings were upheld. What they found through modelling does, in fact, exist in our galaxy. And that opened up a new question: Why did this happen?

The answer was the Milky Way came close to colliding with another galaxy. This set off millions of stars in motion and is the cause of the characteristic swirl shape in the center of the Milky Way. "It is a bit like throwing a stone in a pond, which displaces the water as ripples and waves," Antoja said.

The Milky Way is still experiencing the effects of this vast, ancient stellar near miss, a story remembered in the current velocities and paths of the stars impacted.

"Although the encounter took place around 500 million years ago, we still can see the consequences on the current orbits of stars near the Sun," Antoja told Newsweek over email.

"The present velocities of stars that we have analysed show unexpected patterns of movement indicating that they are still recovering from the past gravitational impact. In fact, it could take more than 1000 millions years for the stars to reach equilibrium (the ripples disappearing)."

Researchers believe the system responsible could be the Sagittarius dwarf galaxy, which has just a few tens of millions of stars currently being eaten up by the Milky Way.

Amina Helmi of the University of Groningen, The Netherlands, collaborated on the project and was the second author on the research paper.

"We already knew about this small galaxy, Sagittarius, but we assumed its effect on the Milky Way would be small, possibly negligible," Helmi told Newsweek. "So it was a big surprise to find that it has had an effect on the Milky Way, and near the Sun, 500 million years ago."

Helmi said such events are rarer now than they were in the past. "Encounters were more frequent in the past (because the universe was smaller), and in fact we believe the Milky Way has cannibalized already several small galaxies.

"But our thought was that it was evolving relatively quiescently and in isolation (without perturbations) for the last, say eight billion years. It will in the future merge with another galaxy (the Magellanic Clouds), but it is going to take another few billion years before we feel much of the associated action."

The new research shows smaller galaxies have a bigger impact on the Milky Way than previously thought—meaning scientists must rethink what they understand about the star system in which we live.

"This discovery implies that the disc of the galaxy is dynamically young, meaning that it is very responsive to external perturbations and that it changes strongly with time," Antoja told Newsweek.

"We now need to revise our models of galaxy dynamics. So far we would always assume that the disc is in equilibrium and does not suffer from external perturbations but we know now that this is incorrect.

"We will need to investigate if other past impacts also sculpted the fate of our galaxy disc and foresee if future encounters will also have similar consequences."

There are around 200 billion stars in the Milky Way, which is 100,000 light years in diameter.

The ESA's Gaia satellite, launched on December 19, 2013 from Kourou, French Guiana, has the task until at least 2020 of mapping in 3D our home galaxy.

The map so far charts the color, motion and distance of nearly 1.3 billion stars, and the position and brightness of nearly 1.7 billion.

This article was updated to include more comments by Antoja.