University Communications

Winter 1998
Rising Star
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"I just love that," Ghez reflects, smiling broadly.

How did she do it?

It's no small feat to find a black hole, those infinitely dense phantoms created, scientists postulate, from the remnants of collapsed stars (so-called "stellar black holes") or from an aggregate of stellar holes ( "supermassive black holes," such as Ghez revealed). Whatever its origin, the mass of a black hole is so dense, that nothing -- not even light -- can escape its grasp. Yet, when one is in your neighborhood, there are signs. Outside the ominously titled "Event Horizon" -- about 10 million miles wide -- gravity from a black hole rises to Looney Toon effect, grabbing the near side of a passing star and stretching it away from the backside like the neck of a rubber chicken. In our eeriest sci-fi imaginings -- not an approach to which Ghez nor most other astronomers would ascribe, but which serves nicely as illustration -- we might envision a black hole, as described by New Zealand astronomer Roy Kerr, as a "magic ring" of infinitely forceful spin, like the eye of a hurricane, through which an astronaut could, POOF!, pass through to an alternate universe.

In the farther reaches of the Milky Way, there is about one star flickering within each three cubic light years of space. Near the middle, where the black hole is supposed to be, the population is a million times that. In a slow pan of the galaxy, the black hole's glowing "accretion disk" is so inconspicuous it had been compared by one astronomer to "finding a Coke machine in the desert."

To find that Coke machine, Ghez in 1995, using the Keck I telescope, started to track the movement of 100 stars near the galactic core. Over the course of three years of observation from the mountain top, she found that these stars showed the telling signs of influence by extreme gravitational forces.

Within that grouping, Ghez found that a nucleus of 20 stars were spiraling around the black hole at speeds up to 3 million miles per hour, about 10 times the speed at which stars typically move. In order to account for their speed, Ghez determined that an object 2.6 million times more massive than our sun must be concentrated into a single black hole.

"In our first year, we could only collect the data, and we didn't really know that we could see the faint sources because we hadn't worked out all the analytical codes," Ghez says. "But the second year, when we knew we'd gotten the measurements, there was actual cheering. We were ecstatic."

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