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June 1, 2012

Milky Way, Andromeda galaxies set to crash, but not for 4 billion years

Los Angeles Times (MCT)

LOS ANGELES — The Milky Way is set for a head-on collision with its closest neighbor, the Andromeda galaxy, astronomers predicted this week. Galactic residents need not brace for impact just yet, however: The mash-up won’t take place for another 4 billion years.

Andromeda, officially known as Messier 31, or M31, is located about 2.5 million light-years away from our Milky Way. But that gap is slowly narrowing, a fact that has prompted scientists to wonder whether the two spiral galaxies are destined to merge.

“To know if this will in fact happen, it’s necessary to know not only how Andromeda is moving in our direction, but also what its sideways motion is,” Roeland van der Marel, an astronomer at the Space Telescope Science Institute in Baltimore, said Thursday. “That will determine whether Andromeda will miss us at a distance - or whether it might be heading straight for us.”

The galaxy’s gradual sideways movement is extremely hard to measure because, on the short human time scale, Andromeda doesn’t seem to budge. Over a few billion years, though, such minimal movement would make a big difference.

So scientists used the Hubble Space Telescope to painstakingly measure the tiny sideways shifts in the galaxy’s stars over a 5- to 7-year period. They used those movements to plot Andromeda’s most-likely path - and it brought M31 straight into the Milky Way’s spiral arms.

Over about 2 billion years, the two galaxies would merge, forming a more globular, non-rotating agglomeration known as an elliptical galaxy. The sun would probably be flung farther from the Milky Way’s center, but Earth would almost certainly remain undisturbed: The scientists said it’s extremely unlikely that any star would come close enough to our solar system for its gravity to disrupt our planet’s position around the sun.

Andromeda is now headed toward us at a relative speed of 250,000 mph - a pace that would allow it to reach the moon from the Earth in an hour. As gravity pulls the two galaxies inexorably closer, it will accelerate up to five times the current speed, van der Marel said.

Andromeda’s smaller companion galaxy, M33, could join the collision, making it an intergalactic group hug.

 

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