By Ron Cowen
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.
Last summer, she had her first inkling that something was wrong with this picture. In June, the prime month for viewing the galactic center with the Keck Telescope’s ultrasharp optics on Hawaii’s Mauna Kea, her team took the highest-quality spectrum ever of any of these close-in stars.