Milky Way set for head-on crash with Andromeda galaxy

Posted by Emma Woollacott

Look out: we're headed for a crash. In, ooh, just four billion years or so our Milky Way galaxy is set to collide with neighboring Andromeda.

The Milky Way, says NASA dryly, is set to get 'a major makeover' in the process. But while our sun will be flung into a new region of the galaxy, it and the rest of the solar system are in no danger of being destroyed. This is because the stars are far enough apart to make collisions highly unlikely.

"Our findings are statistically consistent with a head-on collision between the Andromeda galaxy and our Milky Way galaxy," said Roeland van der Marel of the Space Telescope Science Institute (STScI) in Baltimore.

Andromeda, also known as M31, is now 2.5 million light-years away, but is inexorably falling toward the Milky Way under the pull of gravity between the two galaxies and the invisible dark matter that surrounds them both.

Another two million years later, the two galaxies will merge and reshape into a single ellipse.

"After nearly a century of speculation about the future destiny of Andromeda and our Milky Way, we at last have a clear picture of how events will unfold over the coming billions of years," says Sangmo Tony Sohn of STScI.

The stars in both galaxies will be thrown into different orbits around the new galactic center, and simulations show that our solar system will probably end up much farther from the galactic core than it is today.

To make matters more complicated, M31's small companion, the Triangulum galaxy, M33, will join in the collision and perhaps later merge with the M31/Milky Way pair. Indeed, there's a small chance that M33 will hit the Milky Way first.

While the encounter between Andromeda and the Milky Way has been predicted for a long time, it wasn't known whether there'd be a narrowmiss, a glancing blow or a head-on smash, as until now it's proved impossible to measure M31’s tangential motion.

"This was accomplished by repeatedly observing select regions of the galaxy over a five- to seven-year period," says Jay Anderson of STScI.

"In the worst-case-scenario simulation, M31 slams into the Milky Way head-on and the stars are all scattered into different orbits," says Gurtina Besla of Columbia University in New York.

"The stellar populations of both galaxies are jostled, and the Milky Way loses its flattened pancake shape with most of the stars on nearly circular orbits. The galaxies' cores merge, and the stars settle into randomized orbits to create an elliptical-shaped galaxy."