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Galaxies

Introduction

The stars of the universe are gathered together in numerous giant assemblages known as [Index] galaxies. Many such assemblages are so enormous that they contain hundreds of billions of stars. Nature has provided an immensely varied array of galaxies, ranging from faint, diffuse dwarf objects to brilliant, spiral-shaped giants. Virtually all galaxies appear to have been formed soon after the universe began, and they pervade space, even into the depths of the farthest reaches penetrated by powerful modern telescopes. Galaxies usually exist in clusters, some of which in turn are grouped into larger clusters measuring hundreds of millions of light-years across. (A light-year is the distance traversed by light in one year, traveling at a velocity of 300,000 kilometres per second, or 650,000,000 miles per hour.) These so-called superclusters are separated by nearly empty voids, causing the gross structure of the universe to look somewhat like a network of sheets and chains of galaxies.

Galaxies differ from one another in shape, with variations resulting from the way in which the systems were formed. Depending on the initial conditions in the pregalactic gas some 15,000,000,000 years ago, galaxies formed either as slowly turning, smoothly structured, round systems of stars and gas or as rapidly rotating pinwheels of such entities. Other differences between galaxies have been observed and are thought to reflect evolutionary changes. Some galaxies are rife with activity: they are the sites of star formation with its attendant glowing gas and clouds of dust and molecular complexes. Others, by contrast, are quiescent, having long ago ceased to form new stars. Perhaps the most conspicuous evolutionary changes in galaxies occur in their nuclei, where evidence suggests that in many cases supermassive objects--probably black holes--formed when the galaxies were young. Such phenomena occurred several billion years ago and are now observed as brilliant objects called quasars.

The existence of galaxies was not recognized until the early 20th century. Since then, however, galaxies have become one of the focal points of astronomical investigation. The notable developments and achievements in the study of galaxies are surveyed here. Much attention is also devoted to the Milky Way Galaxy--the local galaxy to which the Sun and Earth belong--and the galaxies and related objects that lie outside it. Included in the discussion are the properties, structures, and major components of the Milky Way system and the external galaxies; the distribution of the latter in clusters and superclusters; and the evolution of galaxies and quasars. For specifics about the components of galaxies, see STARS AND STAR CLUSTERS, NEBULA, and COSMOS.