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Mystery in the Middle

A stellar riddle turns up at the Milky Way's core

For nearly a decade, Andrea M. Ghez has tracked the motion of stars at the Milky Way's core. The great speed with which these centrally located stars whirl around provides the best evidence to date for the existence of an extremely dense and massive object–a supermassive black hole–right in the bull's-eye of the galaxy. Like most astronomers, Ghez, who is based at the University of California, Los Angeles, had assumed these closely orbiting stars were relatively old and lightweight.

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