The Galaxy Collision Next Door

Giant galaxies such as the Milky Way and its neighbor Andromeda originated long ago after smaller galaxies crashed together and grew larger. Observing this process in action, however, is difficult because it requires detecting collisions between dwarf galaxies near the edge of the observable universe, where we see galaxies as they appeared more than 10 billion years ago. Now astronomers have uncovered evidence of a similar collision much closer to home—a mere 2.6 million light-years from Earth—in a small galaxy named IC 10, allowing them to watch a dwarf–dwarf smashup in detail.

David Nidever, an astronomer at the University of Michigan, calls IC 10 one of the most intriguing galaxies in the heavens. "It's the only starburst galaxy in our Local Group of galaxies," he says. A starburst is a galaxy spawning stars at a rapid rate. IC 10 emits only a few percent as much light as the Milky Way. Unfortunately, astronomers have taken the better part of a century to unravel the nature of this unusual neighbor, because our galaxy tries to block the view. IC 10 lies behind the Milky Way's dust in the W-shaped northern constellation Cassiopeia.

In his 1936 book, The Realm of the Nebulae, astronomer Edwin Hubble suggested that IC 10 might belong to the Local Group, a gathering of nearby galaxies that now encompasses more than six dozen known members. But for decades astronomers failed to pin down the galaxy's distance; some estimates were four times greater than others.

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