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Concept
Andromeda–Milky Way collision
Summary
The Andromeda–Milky Way collision is a galactic collision predicted to occur in about 4.5 billion years between the two largest galaxies in the Local Group—the Milky Way (which contains the Solar System and Earth) and the Andromeda Galaxy. The stars involved are sufficiently far apart that it is improbable that any of them will individually collide, though some stars will be ejected. The resulting galaxy is often nicknamed Milkomeda or Milkdromeda, a portmanteau of the respective galaxy names.
The Andromeda Galaxy is approaching the Milky Way at about as indicated by blueshift. However, the lateral speed (measured as proper motion) is very difficult to measure with sufficient precision to draw reasonable conclusions. Until 2012, it was not known whether the possible collision was definitely going to happen or not. Researchers then used the Hubble Space Telescope to measure the positions of stars in Andromeda in 2002 and 2010, relative to hundreds of distant background galaxies. By averaging over thousands of stars, they were able to obtain the average proper motion with sub-pixel accuracy. The conclusion was that Andromeda is moving southeast in the sky at less than 0.1 milliarc-seconds per year, corresponding to a speed relative to the Sun of less than 200 km/s towards the south and towards the east. Taking also into account the Sun's motion, Andromeda's tangential or sideways velocity with respect to the Milky Way was found to be much smaller than the speed of approach (consistent with zero given the uncertainty) and therefore it will eventually merge with the Milky Way in around 5 billion years.
Such collisions are relatively common, considering galaxies' long lifespans. Andromeda, for example, is believed to have collided with at least one other galaxy in the past, and several dwarf galaxies such as Sgr dSph are currently colliding with the Milky Way and being merged into it.
The studies also suggest that M33, the Triangulum Galaxy—the third-largest and third-brightest galaxy of the Local Group—will participate in the collision event, too.
Official source
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