Meet the Milky Way’s Neighbor: The Andromeda Galaxy

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      By some estimates, the Andromeda Galaxy contains roughly one trillion stars. And it stretches more than 200,000 light-years in diameter. That’s significantly bigger than the Milky Way, which more recent estimates suggest is 150,000 light-years across (though the exact boundary of where either of these galaxies “end” is a bit nebulous). Astronomers are still struggling to get an accurate count, but our galaxy also appears to have roughly a quarter to a half as many stars as Andromeda.

      In the decades ahead, other astronomers started noticing supernovae exploding in Andromeda, too. One astronomer in particular, Heber Curtis, used the known brightness of these explosions to calculate the distance to Andromeda. He estimated that this “spiral nebulae” was an unprecedented 500,000 light-years away, which would put it well outside the confines of our Milky Way.

      And a few years before, Vesto Slipher, an astronomer at Lowell Observatory in Flagstaff, had turned the 24-inch Clark Telescope on M31 and measured it moving toward us at an astonishing rate. Meanwhile, Slipher’s measurements of more than a dozen other spiral nebulae showed that all but three were moving away from Earth. His evidence supported the idea that Andromeda wasn’t within our Milky Way. It lived outside it. This served as the first strong evidence for an expanding universe.

      As Slipher’s observations first showed, over the next five billion years or so, the Milky Way and Andromeda will draw uncomfortably close together. The pair will interact in glancing blows, ripping stars from one another into long, drawn out tails. As seen from Earth, the Andromeda Galaxy will loom large in our night sky during these encounters. But as the two eventually tangle up entirely, they’ll merge into one enormous group of stars. But instead of a spiral galaxy, the final object will be an elliptical. Astronomers call this future galaxy Milkomeda, and these kinds of mergers happen all the time.

      Jesus: Hey, Dad? God: Yes, Son? Jesus: Western civilization followed me home. Can I keep it? God: Certainly not! And put it down this minute--you don't know where it's been! Tom Robbins in Another Roadside Attraction

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