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Andromeda is coming

The Andromeda galaxy is moving toward the Milky Way at about 110 kilometres per second. A collision in roughly four to five billion years follows from that velocity.

Strong2 min read

Slipher measured Andromeda’s spectrum in 1912 and found it shifted toward the blue. Modern Hubble data give a three-dimensional velocity: not a glancing false alarm, a trajectory that meets us. Van der Marel and colleagues put the first close passage a bit over four billion years out.

Stars are far apart. ‘Collision’ is a mixing of two galaxies, not a hail of suns hitting suns. The night sky, on that schedule, grows a second great band and then a single elliptical galaxy. The Sun will still be a star, barely. The prediction is orbital mechanics, and the input is a measured velocity.

Caveats

  • The date is model-dependent at the level of hundreds of millions of years. The approach itself is not.
  • Dark-matter halos do most of the interacting. That part is inferred.

Sources

  1. 1van der Marel R. P. et al.. The M31 Velocity Vector. III. Future Milky Way–M31–M33 Orbital Evolution. The Astrophysical Journal 753, 2012.

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