"It's very exciting," Ghez says. Dark-eyed, and armed with a quick laugh, the UCLA astronomer is mercilessly pursuing this black hole, hoping to drive it into a small corral between the stars at the center of the galaxy. Then she'll be able to judge its size, shape and texture.

No one has ever seen a black hole, and they never will. This bizarre cosmic phenomenon is a place where gravity reigns supreme, where any object, and even light, becomes trapped by the gravitational forces of a massive star that has collapsed or by galaxies that have collided.

It wasn't even a sure bet that the Milky Way had a black hole, until Ghez built her first star-corral in 1998. She found a way to focus Hawaii's powerful Keck telescope on stars in the center of the galaxy, stars normally too dim and too distant to detect. Then she measured the hysterically fast motion of those faint stars, which betrayed the weight of the object they orbit. Its weight is more than 2.5 million times that of our sun. This monster is not exactly what astronomers expected to find.

Black holes were suspected of dwelling in active galaxies, super-bright masses of stars that shed a blizzard of radiation into the universe. "But our Milky Way is not an active galaxy," Ghez says. "A supermassive black hole wasn't even predicted to be there. So now, the question is, 'Why?'"

The birth of a supermassive black hole remains mysterious. Stellar black holes are one thing. Even a moderately massive star, 30 times the sun's size, can collapse into a miniature black hole after it explodes at the end of its life. Those probably litter the galaxy. But what about the origin of big black holes, now suspected of lurking in every sizable galaxy?

The jury is still way out on this question. Perhaps our black hole resulted from an implosive collision between the Milky Way and another galaxy, back in the crowded, early days of the universe.