News FocusAstronomy

Unwinding the Milky Way

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Science  05 Mar 2010:
Vol. 327, Issue 5970, pp. 1194-1195
DOI: 10.1126/science.327.5970.1194

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How did the Milky Way originate and evolve into what it is today? For decades, researchers have become increasingly convinced that our galaxy did not develop in isolation but rather grew at least in part by pulling in stars and even whole galaxies that formed outside its borders. But which parts of the galaxy are endemic and which exotic? Where did such alien interlopers come from, and when did they arrive? For answers, researchers have devoted themselves to finding and studying stars in the Milky Way that contain vanishingly low quantities of elements heavier than hydrogen and helium. Most of the universe's hydrogen and helium came into being within a few hundred thousand years after the big bang. But heavier elements—sweepingly referred to as "metals"—form by nuclear reactions inside stars and have been steadily building up in the cosmos for billions of years. As a result, astronomers infer that low-metallicity, or "metal-poor," stars formed in the distant past and thus can serve as a fossil record of the events that shaped our galaxy billions of years ago.