Bottomless Pits

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Andrea Ghez is getting very close to the black hole in the center of our Milky Way galaxy. Despite the violent reputation of black holes, she appears undaunted.

"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.

Or, perhaps a modest black hole once roamed the universe alone, eventually attracting the gas that later formed our galaxy and stars.

It does seem probable that the Milky Way's black hole went through a growth phase. It may have acquired more black holes, neutron stars and other "dark matter" during additional galactic smash-ups. Or, it may have slowly attracted the stellar black holes manufactured right in its own galaxy. Even in this day and age, Ghez says she expects to find small black holes headed like invisible rocks toward the supermassive black hole.

After a growth spurt, perhaps middle age sets in. Although our black hole is huge, it is categorized as "quiescent." Unlike an "active galactic nucleus," which blazes with radiation as it pulls in a thick disk of gas and shredded stars, our black hole sits in a wispy disk of fuel, and emits only weak radiation. In astronomical terms, it is not sucking loudly like a flushing toilet. It doesn't stand a chance of swallowing the Earth, Ghez says. It probably will never snare even her close-orbiting stars, although one has mysteriously disappeared. "This black hole is supermassive," she says, "But it's still only a small fraction of the mass of the galaxy."

Theoretically, our black hole, and all others, will eventually fade and die, although Ghez confesses no clue as to how such an event would unfold.

Quiescent though it is, you would not want to go skating around the black hole. That is strictly for professionals.

"When you get very close to a black hole, the pull of gravity on your feet is much stronger than on your head," Ghez warns. "Your experience would be that you'd get torn to shreds — well, we call it 'tidally disrupted.'"

Feet first, you'd be yanked over the "event horizon," the boundary around the black hole. Your loved ones, Ghez says, would never see you disappear. They'd just note that you were moving slower and slower.

"One of the oddities of relativity," she says. Your tidally disrupted matter, however, would whirl toward the center, approaching the speed of light, light whirling in with you.

Your destination would be the "singularity" at the center of this mind-boggling density. For the sake of comparison, consider a neutron star, that compressed-iron core left behind after a massive star explodes most of its gas into space. The density of a neutron star is, according to the cocktail-napkin calculations of supernova scholar Stanford Woosley, similar to 800 million elephants jammed into a cubic inch. The density of the singularity at the center of our black hole is greater than that, to a degree that is even more unfathomable.

Ghez remains unfazed, however. With her new-and-improved way to drive stakes into the galactic ground, she continues in pursuit of our black hole.