The Milky Way is set to collide with its closest neighbor, the Andromeda galaxy, astronomers working with the Hubble Space Telescope said Thursday. Galactic residents need not brace for impact just yet, however: The predicted collision would take place in 4 billion years.
Andromeda, officially known as Messier 31, or M31, is located about 2.5 million light-years away from the Milky Way — which would make it our closest fellow spiral galaxy. Spiral galaxies have flat, rotating, disc-shaped bodies with spiral arms anchored by a supermassive black hole at the center.
“Because Andromeda is getting closer to us, astronomers have speculated for a long time whether it might collide with our Milky Way and whether the galaxies might merge together,” said Roeland van der Marel, an astronomer with the Space Telescope Science Institute in Baltimore. “However, 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. Because 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 really hard to measure because, on the short human time scale, Andromeda almost doesn’t seem to budge. Over a few billion years, though, such minimal movement would make a big difference.