National Geographic : 1974 May
shift more than twice that of quasar 3C 273. A red shift, as astronomers have known for more than a hundred years, means that the object is moving away from the observer. It is the same phenomenon-known as the Doppler effect-that makes the sound of a train whistle suddenly fall off in pitch as the train passes and rushes away into the dis tance. The sound waves are, in effect, length ened and the frequency decreased as the source of the sound recedes. Similarly with a celestial object: The waves of light are lengthened (that is, become more red) as the source of light moves away from the observer. The faster it moves, the more the waves are lengthened. Fortunately for astronomers, the red shift is an admirable yardstick. The amount of red shift tells how fast an object is receding, and by Hubble's law that in turn tells how far away the object is. So the relatively large red shift for 3C 273 put it some two billion light years away (it is still the closest, as well as the brightest, of the known quasars). But the much larger red shift for 3C 48 put it at four billion light-years distant. That was near the fringe of the known universe when Dr. Schmidt made his mo mentous discovery. MORE QUASARS have been discovered in the years since then-some 350 have been confirmed-although surveys in dicate that they are short-lived, cosmological ly speaking, and that there were many more of them in the early history of the universe than there are today. The most remote known object, and there fore the oldest known object, as this is writ ten, is quasar OQ 172, discovered in 1973. Its furious rush into space at 91 percent of the speed of light puts it at an estimated ten bil lion light-years away. Thus quasars have doubled the horizons of the observable universe in just a decade. As Professor John Wheeler, the distin guished theoretical physicist at Princeton, puts it, "Quasars are the beacons that light up the far away and long ago." Inevitably this new class of heavenly bod ies has aroused vigorous controversy as well as intriguing mystery. Quasars fluctuate rapidly in brightness, often within weeks or months, sometimes in only a few days. It is a principle well understood by astronomers that a celestial body does not change significantly Fiery string of stellar beads depicts the life cycle of the medium-size star that is our sun (left). Five billion years ago gravity collapses a nebula, center, far too large to show in true scale. A star forms, its nuclear furnace ignites, and it quickly matures to today's yellow star. Five billion years from now, its inner fuel consumed, it expands into a red giant a hundred times its pres ent size. Then it becomes a white dwarf and finally a cinder. Giants and dwarfs: stars in their death throes. A red giant (top panel) looms balloonlike over our sun, a typi cal medium-size star. The sun over whelms a white dwarf (second). A white dwarf appears huge beside a neutron star (third), the corpse of a star originally more massive than our sun. And the neutron star dwarfs a black hole (bottom), a collapsed star only a few miles across-so dense that even light cannot escape its gravity. The Incredible Universe 607 a I'~'~""-""~'~-';:"