Someday our little corner of the universe will have a ringside seat for one of the biggest events in the cosmos. Two billion years from now, the Milky Way and Andromeda, our closest neighboring galaxy, will begin to fuse into one giant football-shaped galaxy. The gigantic merger will relocate our solar system and thereby change forever the appearance of the constellations, although it's unlikely that any Earthly life will be greatly disturbed.
Astronomers have known about the prospective merger for decades, as the two galaxies--currently separated by about 2.5 million light years--inexorably churn toward each other. But no one had developed detailed models to predict what would happen to our solar system as Andromeda's stars and dark matter begin roaring through the neighborhood at speeds of several hundred kilometers per second.
Researchers at the Harvard Center for Astrophysics in Cambridge, Massachusetts, used computer simulations to project the paths and interactions of Andromeda and the Milky Way over the next 5 billion years. In a paper submitted for publication in the Monthly Notices of the Royal Astronomical Society, they report that the first contact will occur about 2 billion years from now, so far into the future that the sun--burning considerably hotter and brighter than it does now--will have boiled off Earth's oceans. Then, over hundreds of millions of years, the tug of Andromeda's mass will gently but steadily transport the sun and its planets to a location about 100,000 light-years from the new galactic center.
Although the titanic event will transform the familiar swath of white stars that gave the Milky Way its name into a bigger, more diffuse stellar cloud, it's unlikely to endanger Earth directly. For one thing, there is almost no chance of a head-on collision between two stars, says theoretical astrophysicist and co-author Avi Loeb. That's because stars within a galaxy are spread out so thinly it's very difficult for them to collide. Also, the realignment of stars will take place so gradually it is unlikely to cause many tremors on the ground. More likely, Loeb says, passing stars will disturb comets lurking within the Oort cloud located beyond Pluto's orbit, thereby generating comet showers and perhaps more than one cataclysmic impact.
The findings provide further confirmation that "galaxies fall together and basically stick," says astronomer Joshua Barnes at the University of Hawaii in Honolulu. But it's not at all certain where our solar system will end up. The movement of the stars and planets is like the "froth on the wave," he says. The lingering unknown dynamic, not yet included in the simulations, involves how the galactic dark matter is distributed and how its currents will be disturbed when the Milky Way and Andromeda begin to merge.