Homing In On Black Holes- page 2 | Science | Smithsonian
(Laurie Hatch)

Homing In On Black Holes

To gain insight into the most mysterious objects in the universe, astronomers shine a light at the chaotic core of our Milky Way

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(Continued from page 1)

Even so, there's tension in the room. This team of astronomers, led by Andrea Ghez of UCLA, is in an ongoing competition with astronomers at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. Since the early 1990s, Garching astrophysicist Reinhard Genzel and his colleagues have studied the black hole at the center of the Milky Way using the New Technology Telescope and the Very Large Telescope array in Chile. Ghez, 45, pushes her students to get the most out of each observation session at Keck. Six years ago she was elected to the National Academy of Sciences—quite an honor for someone still in her 30s. "It's easy to be at the forefront of astronomy if you have access to the best telescopes in the world," she says.

Nearly a decade ago the American and the German teams independently deduced that only a giant black hole could explain the behaviors of stars at the Milky Way's core. Stars circling a hefty mass—whether a black hole or some large star—travel through space much faster than those circling a smaller mass. In visual terms, the larger mass creates a deeper funnel in the fabric of space around which the stars revolve; like leaves circling in a whirlpool, the deeper the whirlpool, the faster the leaves spin. Other astronomers had seen fast-moving stars and clouds of gas near the center of the Milky Way, so both Ghez and Genzel suspected that a dense cluster of matter was hidden from view.

By painstakingly compiling infrared photographs taken months and years apart, the two teams tracked the innermost stars, those within one light-month of the galaxy's center. Combined, the images are like time-lapse movies of the stars' motions. "Early on, it was clear there were a few stars that were just hauling," Ghez recalls. "Clearly, they were extremely close to the center." Something was trapping them in a deep whirlpool. A black hole made the most sense.

The clincher came in 2002, when both teams sharpened their images using adaptive optics, technology that compensates for the atmosphere's blur. The scientists followed stars that orbit perilously close to the galaxy's center and found that the fastest star's top speed was 3 percent of the speed of light—about 20 million miles per hour. That's a startling speed for a globe of gas far bigger than our sun, and it convinced even the skeptics that a supermassive black hole was responsible for it.

The blur of Earth's atmosphere has plagued telescope users since Galileo's first studies of Jupiter and Saturn 400 years ago. Looking at a star through air is like looking at a penny on the bottom of a swimming pool. Air currents make the starlight jitter back and forth.

In the 1990s, engineers learned to erase the distortions with a technology called adaptive optics; computers analyze the jittering pattern of incoming starlight on a millisecond by millisecond basis and use those calculations to drive a set of pistons on the back of a thin and pliable mirror. The pistons flex the mirror hundreds of times each second, adjusting the surface to counteract the distortions and form a sharp central point.

The technology had one major limitation. The computers needed a clear guiding light as a kind of reference point. The system worked only if the telescope was aimed close to a bright star or planet, limiting astronomers to just 1 percent of the sky.

By creating an artificial guide star wherever it is needed, the Keck Observatory's laser removes that limitation. The laser beam is tuned to a frequency that lights up sodium atoms, which are left by disintegrating meteorites in a layer of the atmosphere. Keck's computers analyze the distortion in the column of air between the telescope mirror and the laser-created star.

Inside the telescope's 101-foot-tall dome, the laser system sits within a bus-size enclosure. The laser starts out with a jolting 50,000 watts of power, amplifying the light beam within a dye solution made from 190-proof ethanol. But by the time the light is adjusted to its correct color and its energy is channeled along a single path, its power dwindles to about 15 watts—still bright enough that the Federal Aviation Administration requires the observatory to shut down the laser if an airplane is expected to fly near its path. From several hundred feet away the laser looks like a dim amber pencil beam. From a bit farther it isn't visible at all. As far as the rest of the island is concerned, there is no laser show at Mauna Kea.

Identifying a black hole is one thing; describing it is another. "It's difficult to paint a picture that relates to the world as we understand it, without using mathematical complexity," Ghez says one afternoon at the Keck control center. The next day, she asks her 6-year-old son if he knows what a black hole is. His quick response: "I don't know, Mommy. Shouldn't you?"


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