Andromeda on Collision Course with the Milky Way

Milkywaycollision-630It's headed straight for us: Ron Cowen in Nature:

It’s a definite hit. The Andromeda galaxy will collide with the Milky Way about 4 billion years from now, astronomers announced today. Although the Sun and other stars will remain intact, the titanic tumult is likely to shove the Solar System to the outskirts of the merged galaxies.

Researchers came to that conclusion after using the Hubble Space Telescope between 2002 and 2010 to painstakingly track the motion of Andromeda as it inched along the sky. Andromeda, roughly 770,000 parsecs (2.5 million light years) away, is the nearest large spiral galaxy to the Milky Way.

Roeland van der Marel and Sangmo Tony Sohn, astronomers at the Space Telescope Science Institute in Baltimore, Maryland, announced the findings today at a NASA press briefing in Washington DC. The results will be reported in an upcoming issue of Astrophysical Journal1–3.

“We’ve been able to extract dynamical information about Andromeda that has been hidden from astronomers for a century,” says van der Marel.

For decades, scientists have known that Andromeda is falling towards our home Galaxy at a rate of 110 kilometres per second and that the two might eventually collide as a result of their mutual gravity. But because astronomers could easily measure Andromeda’s velocity only along the line of sight to Earth, no one could be sure whether the future encounter would constitute a major merger, a near-miss or a glancing blow.