andromeda is going to collide with the milky way in about 3.75 billion years.
when is andromeda going to collide with the milky way?
about 3.75 billion years
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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 a precision to draw reasonable conclusions: a lateral speed of only 7.7 km/s would mean that the andromeda galaxy is moving toward a point 177,800 light-years to the side of the milky way ((7.7 km/s) / (110 km/s) × (2,540,000 ly)), and such a speed over an eight-year timeframe amounts to only 1/3,000th of a hubble space telescope pixel (hubble's resolution≈0.05 arcsec: (7.7 km/s)/(300,000 km/s)×(8 y)/(2,540,000 ly)×180°/π×3600 = 0.000017 arcsec). until 2012, it was not known whether the possible collision was definitely going to happen or not. in 2012, researchers concluded that the collision is sure using hubble to track the motion of stars in andromeda between 2002 and 2010 with sub-pixel accuracy. andromeda's tangential or sideways velocity with respect to the milky way was found to be much smaller than the speed of approach and therefore it is expected that it will directly collide with the milky way in around four and a half billion years.
andromeda–milky way collision