the entire mass of the universe began in a state of fantastic compression, some 10 billion to 20 billion years ago.

How did the universe come into being in the first place? What determined its properties? Will it keep expanding forever or instead collapse on itself? A century ago, such questions were considered to lie outside the domain of science. Today, they are central to the field of astronomy.

Progress in astronomy is driven by advances in technology. In the 1930s, new communication devices led to the reception of radio waves from space. For thousands of years before, visible light had been our only way of seeing the universe. Since the 1950s, rockets and satellites have recorded infrared radiation, ultraviolet radiation, and x-rays emitted from space. Such radiation, invisible to the human eye, has revealed completely new features of many astronomical objects and has announced some objects not before known. New electronic detectors have replaced photographic plates, resulting in a 100-fold increase in sensitivity and a broader range of available wavelengths. Highspeed computers have revolutionized theoretical astronomy by permitting the simulation of millions of interacting stars or galaxies. Electronically linked to combine the data from different antennas, large arrays of radio telescopes can work together as if they were one giant eye.

As discussed in Chapter 1 and Chapter 4, astronomical exploration in the 1990s will take advantage of novel technologies to make new instruments of startling power. In equipping ourselves for the future, diversity must accompany precision. We theorize and forecast as well as we can, but if the past is a guide, some of the next decade's discoveries will catch us off guard. In astronomy, the frontiers surround us.


The Formation and Evolution of Our Solar System

In the middle of the 18th century, the German philosopher Immanuel Kant proposed that our system of planets and sun condensed out of a great rotating cloud of gas and dust. This proposal, called the nebular hypothesis, has gained enormous observational and theoretical support and is favored today. Kant suggested that a primitive gaseous cloud slowly contracted under the inward pull of its own gravity. The central, densest regions formed the sun. The outer regions collapsed along the axis of rotation, because of gravitational forces, but did not fall directly toward the nascent sun, because of centrifugal forces pushing outward. Caught between these two forces, the material formed a flattened shape, called a protostellar or protoplanetary disk, in orbit about the sun.

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