National Geographic : 2009 Jul
• degrees, the full moon spanning about one half of a degree. Without AO, a powerful telescope on a ne night can perceive objects separated from each other by as little as one 3,600th of a degree, or one arc second. anks to Keck s AO system, UCLA astronomer Andrea Ghez was able to make a motion picture of seven bright stars whirling around the invisible black hole at the center of our galaxy over a period of 14 years: The entire movie takes place in- side a box measuring only one arc second on a side. Based on the frenzy of the stars in the grip of the black hole, Ghez calculated that it has a mass of four million suns, generating enough gravitational force to slingshot some stars that pass too close right out of our gal- axy. Several such hypervelocity stars have been located, speeding o toward the depths of inter- galactic space like party crashers ejected from an exclusive nightclub. What s next? Even bigger telescopes, of course, with the capability to shoot cosmic pictures faster, wider, and in even greater de- tail. Among the behemoths due to come on line within a decade are the Giant Magellan Telescope, the Thirty Meter Telescope, and the 42-meter European Extremely Large Tele- scope---a scaled-down version of the 100-meter Overwhelmingly Large Telescope, which was tabled at the planning stage when its projected budget turned out to be overwhelming too. Particularly innovative is the Large Synoptic Survey Telescope, or LSST, whose 8.4-meter pri- mary mirror was cast last August in a spinning furnace under the stands of the University of Arizona Wildcats football stadium in Tucson. (The rotation technique produces a mirror blank that is already concave, reducing the amount of glass that must be ground away to bring the mirror to a proper gure.) Conven- tional telescopes have narrow elds of view, typ- ically spanning no more than half a degree on a side---much too narrow to take in the enormous patterns that grew out of the big bang. e LSST will have a eld of view covering ten square de- grees, the area of 50 full moons. From its site in the Chilean Andes, it will be able to image galaxies far across the universe in exposures of just 15 seconds each, capturing eeting events to distances of over ten billion light-years, 70 percent of the way across the observable uni- verse. "Since we ll have a big eld of view, we can take a whole lot of short exposures and--- bang, bang, bang, bang---cover the entire visible sky every several nights, and then repeat," says LSST Director Tony Tyson. "If you keep doing that for ten years, you have a movie---the rst movie of the universe." e LSST s fast, wide-angle imaging could help answer two of the biggest questions con- fronting astronomers today: the nature of dark matter and of dark energy. Dark matter makes its presence known by its gravitational attrac- tion---it explains the rotation speed of galax- ies---but it emits no light, and its constitution is unknown. Dark energy is the name given to the mysterious phenomenon that, for the past ve billion years, has been accelerating the rate at which the universe expands. "It s a little bit scary," says Tyson, "as if you were ying an air- plane and suddenly something unknown took over the controls." The LSST could help solve these immense riddles thanks in part, oddly enough, to the science of acoustics. e big bang was noisy. Although sound cannot propagate through the vacuum of today s space---as pedants are fond of reminding the directors of science-fiction ONE DAY, OBSERVATORIES SITTING IN CRATERS ON THE FAR SIDE OF THE MOON MAY PROBE THE UNIVERSE FROM IDEAL SURROUNDINGS.