Galactic merger to "evict" Sun and Earth.

23:26 14 May 2007
NewScientist.com news service
Hazel Muir

It's hard to know where you'll be in five years' time, never mind 5 billion. But astronomers have it all figured out. In 5 billion years, they say, the Sun and Earth – along with our atomic remains and any living Earthlings – will inhabit "Milkomeda", the wreckage of a violent collision between the Milky Way and the giant Andromeda galaxy.

Now a new study has simulated the crash and asked: where will the Sun end up in this mammoth galactic union? "We're living in the suburbs of the Milky Way right now, but we're likely to move much farther out after the coming cosmic smash-up," concludes T J Cox from the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, US.

The Milky Way belongs to a group of about 40 galaxies that mill around in our cosmic neighbourhood, bound together by their mutual gravitational pull. The two largest by far are the Milky Way and Andromeda, a swirling spiral of a trillion stars that lies about 2.5 million light years away.

These two galaxies are currently rushing towards each other at about 120 kilometres per second and will likely collide in future. Now Cox and his CfA colleague Avi Loeb have run computer simulations of this collision, using 2.6 million particles to model the matter in both galaxies and in intergalactic space. They accounted for the unidentified invisible "dark matter" known to exist in space as well as the ordinary matter in stars and clouds of gas.

The scientists watched how gravity choreographed the motions of the two galaxies up to 10 billion years into the future. The results suggest they will pass close to each other in less than 2 billion years, well within the Sun's lifetime. At this point, their mutual gravity would start to mess up their structures and tug out long tails of stars and gas.

The two galaxies would then overshoot and come together again for a second close passage before finally merging about 5 billion years from now. The merged galaxy, which Loeb dubs Milkomeda, will be a blobby elliptical galaxy, rather than a neat spiral like Andromeda or the Milky Way today.

The Sun will almost certainly hang onto its clutch of planets throughout the mayhem, even if the Earth is no longer habitable. To find out its fate, Loeb and Cox tagged all the stars in the simulation that currently share the Sun's orbit around the Milky Way, which lies some 26,000 light years from the galaxy's centre. Then they noted the future positions of these stars in Milkomeda.

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