National Geographic : 2007 Mar
the silicon in rocks and computer chips, the iron in our blood and our machines-just about every atom heavier than hydrogen and helium- was forged inside ancient stars and strewn across the universe when they exploded billions of years ago. Eager to understand our origins and, in some cases, simply wild about things that go bang, astronomers have been struggling for decades to understand why stars that shine peacefully for millions of years suddenly blow up. Lately they've had two big breaks. One is a revelation about potent blasts of high-energy gamma rays that come from distant points in the heavens. For decades astronomers have puz zled over their origins, but space probes recently clinched the answer, which Woosley proposed more than a decade ago: Many gamma-ray bursts are the early warning signals from supernovas, emitted minutes before the explosion. The link offers a glimpse of events leading up to the actual explosion-another mystery. There, too, researchers have made headway. Looking not at the heavens but at computer models of supernovas, some think they have figured out what may trigger the final cataclysm. The miss ing element may be unimaginably powerful reverberations-the sound of a star singing its own swan song. or astronomers, there's usually no rush to study something before it vanishes. "The universe usually evolves as slowly as watching paint dry," says one. But these days, hundreds of astronomers keep cell phones and beepers close by so they can rush to work like doctors on call. They're waiting for word from a spacecraft called Swift. Swift, launched in 2004, scans the skies for gamma rays. When it detects a burst, it swivels its telescopes toward the source to get a good fix and detect the afterglow-the lingering point of light that marks the spot where a burst originated. It also sends an alert to earthbound astronomers, who can take a closer look with bigger telescopes. Early on February 18, 2006, Swift recorded an outpouring of gamma rays from somewhere toward the constellation Aries. Within three 84 NATIONAL GEOGRAPHIC * MARCH 2007 minutes, the satellite had determined the posi tion of the burst and broadcast an alert. Two days later, astronomers at a telescope in Arizona reported that the burst came from a small, near by galaxy, only a fraction as far away as usual. Astronomers had already traced a connection between bursts and supernovas. But this burst was so close, and Swift had spotted it so quickly, that scientists hoped it would help confirm what they suspected: A gamma-ray burst is an exploding star's opening act. After an unusually long flood of gamma rays and x-rays, lasting more than half an hour rather than the typical few seconds, the Febru ary 18 burst gave way to visible and infrared light. Within three days this afterglow was fading away-and then the supernova grabbed the spotlight. Astronomers at the Very Large Telescope in northern Chile were watching the afterglow dwindle when they noticed a brightening. The star had exploded just a minute or so after the burst, but most of its energy was invisible ultraviolet and x-ray radiation. Its visible light had brightened more slowly, and now it was finally outshining the afterglow. For the first time, astronomers had seen a gamma-ray burst evolve into a supernova from the very beginning. Eighteen days after the supernova flared into view, astronomers were still watching. Atop Palomar Mountain in southern California, the observatory dome's twin shutters slid open under patchy clouds, letting a sliver of night sky fall onto the caged mirror of the 200-inch Hale Telescope. Caltech astronomer Avishay Gal-Yam had two hours before the supernova would dip too low in the sky for the telescope to see it. Still more luminous than a billion suns, the supernova outshone the combined light from all the stars in its home galaxy, glowing white-hot from the radioactive decay of unstable nickel atoms forged in the explosion. Gal-Yam pointed to a computer screen showing a squiggly line -the glow broken down into its component colors, or wavelengths. Each dip in the line represented a wavelength of light absorbed by a different element-silicon, cobalt, calcium, iron-in the debris of the star.