Andromeda galaxy will smash directly into ours

By Alexandra Witze

Web edition: May 31, 2012
Print edition: July 14, 2012; Vol.182 #1 (p. 10)

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Four billion years from now, a collision between the Milky Way (left) and Andromeda (right) galaxies will have ripped out streams of stars, warped the galactic shapes and turned Earth’s night sky into a dramatic swirl of starlight.

Credit: NASA, ESA, Z. Levay and R. van der Marel/STScI, A. Mellinger

The monstrous Andromeda galaxy and the Milky Way are destined to hit head-on, not in a glancing blow, new observations from the Hubble Space Telescope show.

By precisely locating the same stars in Andromeda in 2002 and then again in 2010, astronomers at the Space Telescope Science Institute in Baltimore have calculated how the galaxy has moved against the background of deep space — confirming that the galaxy’s sideways motion is but a fraction of the speed at which it’s hurtling toward the Milky Way.

Andromeda is 2.5 million light-years away and closing in on the Milky Way at 250,000 miles per hour. The cosmic collision will transform the heavens into a hallucinogenic swirl 4 billion years from now. Calculations suggest that the sun will be tossed out during this galactic mash-up, to drift erratically in the eventual single, large galaxy that will coalesce from the two.

The work will appear in an upcoming Astrophysical Journal.