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Galactic train wrecks show our future

Five billion years from now our galaxy, the Milky Way, will collide with the Andromeda galaxy, triggering the birth of stars from smashed together clouds of cosmic gas and dust. This is old news, but exactly what the galactic wreckage will look like is unknown.

Part of the problem is that these mergers take place over millions to billions of years, which is much too long for anyone to witness the whole process. As a work around, astronomers study a variety colliding galaxies at various stages of merging to piece together the picture of what will happen to us.

They've now gotten enough data to assemble an atlas of the galactic train wrecks from start to finish.

"This atlas is the first step in reading the story of how galaxies form, grow, and evolve," Lauranne Lanz of the Harvard Smithsonian Center for Astrophysics, said in a news release announcing the accomplishment.

She and colleagues combined recent data from NASA's Galaxy Evolution Explorer (GALEX) spacecraft and Spitzer Space Telescope and presented the images Wednesday at a meeting of the American Astronomical Society in Boston.

GALEX observes in ultraviolet light, which captures emission from hot young stars. Spitzer sees the infrared emission from warm dust heated by those stars, as well as from stellar surfaces. The combined data highlight areas where stars are forming most rapidly, and together permit a more complete census of the new stars.

In general, galaxy collisions trigger star formation, though some mergers trigger few stars than others. Lanz and her colleagues want to figure out what differences in physical processes cause these varying outcomes, which will help guide computer simulations of these smashups.

"We're working with the theorists to give our understanding a reality check," she said in the news release. "Our understanding will really be tested in five billion years when the Milky Way experiences its own collision."

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John Roach is a contributing writer for Connect with the Cosmic Log community by hitting the "like" button on the Cosmic Log Facebook page or following's science editor, Alan Boyle, on Twitter (@b0yle).