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About 20 years ago one of the authors of this article took his father's binoculars and tiptoed out of the house at night. The budding astronomer decided that he would look for playmates on other planets going around stars in the sky. To his chagrin, the binoculars made no difference whatsoever. The stars appeared as twinkling points of light to his naked eye, and they were pointlike through binoculars as well. Although the largest stars could engulf our entire solar system within their luminous diameters, every star (aside from the sun) is simply too distant to be resolved with binoculars.
Two decades later the same kid can see not just a point of light but a circular disk--at least for some of the brightest stars. This stellar resolution takes advantage of a technique that was suggested more than 130 years ago: interferometry. Instead of looking through binoculars or even a conventional telescope, he must use a computer display connected to a device called an optical interferometer. For more than half a century, interferometry at radio wavelengths has succeeded brilliantly, mapping the structures of distant galaxies and quasars by their radio emissions. Only in the past 15 years, however, has technology allowed interferometry at infrared and visual wavelengths to take off--and the results have been well worth the wait. The Hubble Space Telescope reigns supreme for taking crisp photographs of faint objects, but ground-based optical interferometers can see, for the brightest stars, details 100 times finer than Hubble can.