Beyond Hubble

PHOTO COMPOSITE BY THE GEMINI OBSERVATORY
ALMOST HEAVEN: The dome of Gemini North sits perched under a brilliantly clear sky 14,000 ft. above sea level on Hawaii's Mauna Kea

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UCLA astronomer Andrea Ghez, meanwhile, has focused her attention on the center of our home galaxy, the Milky Way, far closer than Djorgovski's gamma-ray bursts but hundreds of times farther away than Marcy's planets. Shrouded in thick clouds of dust, the galactic core is invisible to ordinary light detectors. But among the Keck's suite of specialized instruments is an electronic camera sensitive to infrared light--the same kind of invisible light that your remote control uses to communicate with your TV. Infrared light of some wavelengths can penetrate dust as though it weren't there, giving Ghez a perfect view of the Milky Way's core.

Armed with the combination of the Keck's power and the detector's sensitivity, Ghez has been able to measure the motions of stars that lie 100 times as close to the core as the nearest star, Proxima Centauri, lies to the sun, and he finds that they're whipping around the galactic center at 1,600 miles per second, nearly 100 times as fast as Earth orbits the sun. It only takes high school physics to calculate that the object they're orbiting is as massive as 3 million suns yet packed into an area no bigger than the orbit of Mars.

The only thing that reasonably fits this description is a black hole, an object whose gravity is so strong even light can't escape from it. "We have evidence of these supermassive black holes in several other galaxies," says Ghez, "but this is the most convincing case we know of."

With their six-year head start, the Kecks have done more science than the newer telescopes, but the newcomers haven't wasted any time catching up. The European Southern Observatory's VLT, for example, built and operated by a consortium of eight countries, got the first of its four 8.2-m telescopes up and running in 1998 and achieved "first light" with the fourth in September.

But it's already doing first-rate science. Earlier this year, for example, astronomers from Sweden, Italy, Denmark and Germany used one of the scopes to help solve astronomy's so-called age paradox. In the mid-1990s, astronomers used the Hubble to measure the age of the universe at between 8 billion and 12 billion years. But other experts insisted they knew of stars that were at least 14 billion years old--obviously a problem, since stars can't be older than the cosmos. Using the VLT, though, observers have measured minute traces of radioactive uranium and thorium in the oldest stars--a technique akin to radiocarbon dating--and proved that they're more like 12 billion years old (the age of the universe, meanwhile, is now estimated at 14 billion years).

In fact, whereas the Europeans started later than their American competitors, they could pull ahead before too long. Not only do they have four giant telescopes on one site, but they've also budgeted more money than anyone else for state-of-the-art light detectors.

Still, U.S.-based telescopes remain ahead on several fronts, including the detwinkling of starlight. The technology that does this is called adaptive optics, and it was originally developed in secrecy by the Department of Defense to help military snoops take sharp pictures of Soviet spy satellites. Largely declassified in the 1980s, it's now being adapted for major telescopes everywhere. The idea is straightforward: stars and galaxies twinkle and shimmer because turbulent pockets of air act as weak, light-distorting lenses (heat rising from a car's hood or an asphalt parking lot causes a similar effect). With adaptive optics, though, a computer can measure the shimmer and cancel it out (see diagram).

Adaptive-optics systems do have limitations. To start with, they work well only with infrared radiation. That's not a huge problem, given that infrared is ideal for spotting new planets and for studying the early universe, the core of the Milky Way and the formation of stars. A bigger drawback is that adaptive optics can currently correct only for a small patch of atmosphere at the center of the telescope's field of view. But pockets of atmospheric turbulence are small enough that a slight change in viewing angle means a whole different pattern of distortions, which in turn requires a different pattern of corrections.

Even with these limitations, astronomers at both the Keck and Gemini have taken pictures that are every bit as clear as the Hubble's. Clearer, in fact, because a large telescope's images are inherently sharper than a small one's. Indeed, Ghez's latest and sharpest Keck images of the galactic center have been made with the adaptive optics.

Will adaptive optics make space telescopes obsolete? Not entirely. Space is still the best place to take supersharp pictures in ordinary light. And some radiation--ultraviolet, for example, and some wavelengths of infrared--can't penetrate the atmosphere at all. Moreover, telescopes radiate infrared light of their own, which contaminates celestial images. That's why NASA's plan to launch a Next Generation Space Telescope by 2009 still makes sense. With an 8-m mirror of its own, NGST will be able to see distant galaxies, for example, that no earthly telescope could ever see through the glare of its own heat.