ANCHORAGE, ALASKA—Two large neighbors of our own Milky Way galaxy—Andromeda (upper right) and Triangulum (lower left)—experienced a close encounter in the distant past. When they brushed past each other billions of years ago, cold hydrogen gas was pulled out of the galaxies by their mutual gravity. The faint radio emission of the resulting hydrogen "bridge," the red material stretching from Andromeda in the direction of Triangulum, was first hypothesized based on observations from a Dutch radio telescope in 2004 and has now been mapped in detail by the Green Bank Telescope in West Virginia. The bridge, described here yesterday at the 220th meeting of the American Astronomical Society, extends over hundreds of thousands of light-years, and the hydrogen gas appears to be clumped into clouds a few thousand light-years across. Next time, the two galaxies may not be so lucky. Recent Hubble Space Telescope observations have revealed that Andromeda will collide and merge with our Milky Way in a few billion years. By then, the smaller Triangulum galaxy may have already collided with either of its two larger neighbors. Ultimately, the trio will meld into one giant elliptical galaxy, flinging stars and planets in all possible directions.
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