Messenger - Vol. 3, No. 3, Page 6 Spring 1994 On Research Cosmic particles shed new light on ancient mysteries When a star dies, it collapses catastrophically, hurling debris into space and generating a shock wave similar to a massive sonic boom. As the blast gains momentum, it accelerates electrons and interstellar nuclei in its path, pushing them forward in a frenzied state of hyperactivity. If they happen to enter our atmosphere, these "souped-up" projectiles collide with nitrogen and oxygen, showering the Earth with "secondary" cosmic particles. The exact origins of these cosmic particles must be determined before scientists can fully explain the chemical evolution of galaxies. "All the matter we see around us is cooked inside stars and distributed during supernova explosions," says Thomas K. Gaisser, a physics professor in the Bartol Research Institute at the University. "Previous generations of stars collapsed and exploded, forming the sun and our solar system. We need to know what happens when stars 'die' to explain how our world was formed. Cosmic particles provide us with some of the clues we need to solve this mystery." Bartol's South Pole array is among the detectors used to study such high-energy particles, he says. Gaisser is particularly interested in rare particles demonstrating energies as high as 1020 electronvolts-roughly the energy of a tennis ball traveling 50 miles per hour, but carried by a single atomic nucleus. How can researchers learn more about these supercharged particles? Measurements support computer simulations that show how ionized nuclei from space interact and cascade to Earth as secondary cosmic particles. Using such techniques, Gaisser and others recently determined that very-high-energy particles may be generated outside our galaxy. Since our galaxy is believed to be the source of virtually all other cosmic particles, the apparent discovery of rare extra- galactic particles generated excitement among scientists. "We found that lower-energy particles consisted mainly of heavy nuclei, such as iron or oxygen," Gaisser says. "But higher-energy particles were mostly protons, which may have been propelled into space on a very strong extra-galactic shock." Gaisser continued his investigation of cosmic mysteries during March as a resident fellow at the Bellagio Conference and Study Center in Italy. The center hosts scholars and authors pursuing research projects in a variety of fields. -Ginger Pinholster