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Getting Ready for Next Supernova Explosion

Once every 30 to 50 years, a giant star somewhere in the Milky Way is thought to exhaust its fuel and explode with a brightness that outshines the entire galaxy. Its a great show, and astronomers dont want to miss the next one.

A small group of prominent researchers is filling in the outlines for a network that could give colleagues worldwide at least an hours notice that a star in our galaxy has erupted as a supernova. Armed with such lead time, researchers would be able to train an array of high-tech detectors on the event to catch it from its inception. Thousands of amateur astronomers also would be alerted to watch for the explosion to help track its visual beginnings and evolution.

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The wealth of data that would result, supporters say, could unlock some tightly bound secrets of these

cosmic fireballs. The value of a galactic supernova and the value of early warning is very, very high, says Robert Kirshner, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Supernovae act as the foundries for most of the chemical elements heavier than helium. The carbon in firewood, the sulfur in a match head, the calcium in marble, the sodium in table salt, the iron in a bridges beams, and the oxygen that turns them red with rust were first forged and dispersed by ancient stellar explosions. Under the right conditions, supernovae are thought to spawn black holes objects with gravity so intense that not even light can escape their pull.

Yet most of the lessons astronomers and astrophysicists have learned about supernovae have come from watching them in other galaxies or from studying the remains of a handful of supernovae relics in our own galaxy. These have yielded significant clues, but not enough to reconcile conflicting descriptions of the mechanism driving the supernovae blasts or fill gaps in the descriptions themselves.

Much of supernova theory is still up in the air, acknowledged Adam Burrows, an astrophysicist at the University of Arizona in Tucson, during the first international workshop on an early alert network, held earlier this month at Boston University.

To bring the theories back down to Earth, astronomers and astrophysicists say their ideal is to study a galactic supernova, enough to observe in unprecedented detail. A supernova that exploded in the Large Magellanic Cloud in February 1987 whetted their appetite. It was the closest, brightest supernova humans have observed since before the invention of the telescope.

Supernova 1987A helped answer some of the questions researchers had, but it also raised more, particularly about the earliest phases of the explosion. Hence the desire for a heads-up on the next one.

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The forms of radiation most useful for early warning also turn out to be some of the most exotic physicists have discovered or predicted. The first signals to arrive would be recorded by detectors designed to measure slight disturbances in Earths gravity field, notes Rainer Weiss, a physics professor at the Massachusetts Institute of Technology in Cambridge and a leading collaborator on LIGO, one of three gravity-wave projects being built worldwide to study such disturbances.

A massive star goes supernova when it exhausts its fuel, and the outward push of its radiation, which keeps it from shrinking, weakens. Beyond a certain point, the star can no longer resist collapsing under its own gravity. In an instant, the core collapses to the size of Manhattan, its material packed as densely as particles in an atoms nucleus. Then the core rebounds, setting up a shock wave that blasts away the stars outer layers. What remains is the center now a neutron star. Or, if the collapse continues, a black hole.

This collapse and rebound are expected to trigger gravity waves that spread like ripples from a stone dropped in a pond and travel unhindered at the speed of light. By pooling data from gravity-wave detectors, Dr. Weiss says, researchers should be able to hand astronomers a precise location in the sky for the event, even before the light arrives.

Within a hundred microseconds after gravity waves arrive, come phalanxes of neutrinos tiny subatomic particles that interact only rarely with other particles, but which carry the vast majority of the energy from the explosion. They, too, travel at the speed of light. But during the earliest stages of the rebound, the material at the supernovas core is thought to be too dense even for neutrinos to escape.

Once they do, they will hit underground detectors around the world, providing more-detailed information about what happens during the supernovas first few seconds. At least two detectors one in Japan and one in Canada will give an independent estimate of where the supernova will appear in the sky once its light escapes the expanding shell of debris.

Indeed, neutrino detectors may be the main tools available for studying some galactic supernovae, according to Dr. Kirshner. Under certain conditions, a supernova might stall and fizzle out before it unleashed its full array of fireworks, he adds. There may be events where the neutrino signal is the only signal.

All the detectors that could be linked into an early warning network either are built, being upgraded, or are under construction. So far, two facilities have been tied in a prototype network: the MARCO neutrino detector in Italy and Super Kamiokande in Japan.

Some workshop participants expressed concern that rivalries among labs, or a desire to hold data close to the vest until no doubt remains that they indicate a supernova and so avoid professional embarrassment, could slow the networks response time. Still, the network holds the promise of uncovering a rich array of physics germane to the frontier of supernovae, neutron stars, and black holes, Dr. Burrows says.

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