Supernova explosions are thought to be the underlying cause for the acceleration of protons and heavier atoms to extremely high energies. These particles, called cosmic rays, move at nearly the speed of light, gyrating in the magnetic field of the galaxy, to bring us news about distant parts of the galaxy, news that we can currently decipher only imprecisely. Some cosmic rays enter the earth's atmosphere and can be detected by telescopes that look for the light given off when cosmic rays collide with stationary molecules in the atmosphere. Yet an innovative telescope called the Fly's Eye (Plate 2.9), which consists of hundreds of photosensitive tubes that scan the skies over Utah, has found tracks of cosmic rays of such high energy that even supernova explosions may have been inadequate to accelerate them. If these cosmic rays come from outside the galaxy itself, nobody can explain why they are there. An enhanced Fly's Eye telescope planned for the 1990s may solve this question.

The causes of stellar explosions, and the enrichment by supernovae of interstellar gas with life-giving heavy elements, will be the focus of extensive study with many of the telescopes to be built in the 1990s. Measurements of cosmic rays can determine abundances of the elements in the galaxy directly. A satellite called the Advanced Composition Explorer will measure the abundances in cosmic rays of all the elements up to zirconium, an element 90 times heavier than hydrogen. Scrutiny of the x-rays and gamma rays emitted by supernova remnants will help identify the various kinds and proportions of atoms dispersed in supernovae (Plate 2.10). Such a task will be on the dockets of the Gamma Ray Observatory; an Explorer satellite equipped with a gamma-ray spectrometer; AXAF; and two ultraviolet instruments currently under construction, the Far Ultraviolet Spectroscopy Explorer (FUSE) and the Extreme Ultraviolet Explorer. Chemical elements manufactured and spewed out into space by supernovae can also be identified by their infrared emissions. The proposed SOFIA telescope, quickly deployable because it is aboard an aircraft, will have the flexibility to study supernova debris on short notice. The high sensitivity of the proposed SIRTF mission will permit infrared measurements of supernovae out to 30 million light-years from the earth.

THE LIFE HISTORY OF GALAXIES

The Milky Way as a Galaxy

The Milky Way, the faint opalescent band of light that sweeps across the night sky, is our galaxy of 100 billion stars, all orbiting in a flattened disk. The central portions of the Milky Way, obscured at visual wavelengths by intervening interstellar dust, are revealed by infrared observations from ground-based telescopes and by orbiting satellites. Clear views of the stars and gas clouds that form the disk of the Milky Way have been obtained by the IRAS and the Cosmic Background Explorer (COBE) satellite and are shown on the



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