INTRODUCTION: 



ing a myriad of stars revolving about a particular 

 "spot," the galactic center. 



Our galaxy is lens-shaped, consisting of stars con- 

 centrated in a central nucleus and in "arms" that 

 spiral outward from the nucleus (Figure 1.1). These 

 arms are composed of millions of individual stars, of 

 star systems, and of gas and dust clouds. The parts 

 of our galaxy's arms visible from the earth (about 

 two-thirds of the way out from the center of the 

 galaxy) are called the Milky Way. Individual parts 

 of the arms and nucleus revolve rapidly about the 

 center; our sun takes about 200 million years for a 

 single revolution about the galactic center, which 

 means that it travels about 600,000 miles per hour. 

 In addition to the galactic nucleus and the "arms," 

 our galaxy also contains globular clusters. These 

 groups of stars occur just beyond the margin of the 

 "lens" and orbit in random paths about the galactic 

 center. 



Figure 1.1 Our galaxy, an exomple of the spiral nebula type, and 

 some surrounding star clusters. 



The next larger unit of astronomical organiza- 

 tion — the largest unit known — is the galactic system 

 or local group. In it, individual galaxies are grouped 

 into a unit by being relatively close together and dis- 

 tinct from other such units. Our galactic system is 

 composed of twenty-two, or perhaps more, galaxies. 



DISTANCES 



If one attempts to visualize the vast distances in- 

 volved in all these units of universe organization, it 

 becomes obvious that the universe is mostly space. 

 Measurement in miles is rarely used, except within 

 our solar system. The common measurements are 

 light years and pa r.'.ecs. A light year is the distance light 



will travel during one earth year. Light travels at a 

 speed of 186,326 miles per second, so a light year is 

 approximately 6 trillion miles. The parsec, 3.26 light 

 years, is used by astronomers more frequently than 

 the light year because it is based upon astronomical 

 relationships. 



These measurements can be used to show how 

 "empty" space really is. The earth is approximately 

 93 million miles from the sun. The next nearest star 

 (actually three very close stars). Alpha Centauri, is 

 4.4 light years from the sun. The sun is 30,000 light 

 years from our galactic center. Our galaxy, larger 

 than most, has a diameter of about 100,000 light 

 years; it is near the center of our galactic system, 

 whose diameter is about 4 million light years. One 

 end of our galactic system is marked by our sister 

 galaxy, the very similar Andromeda. Beyond the 

 limits of our galactic system other galaxies, the so- 

 called island universes, are also organized into galactic 

 systems. The distance between adjacent galaxies in 

 other galactic systems may exceed 4 million light 

 years, but those in our galactic system do not reach 

 2 million light years apart. However, only 50,000 

 light years from our galaxy are two galaxies; the 

 greater and lesser Magellanic Clouds. 



GALAXY ORGANIZATION 



Not all galaxies are organized in the same way. 

 The basic galactic structures are of three types: 

 spiral, elliptical, and irregular. These variations in 

 structure are generally explained on the basis of their 

 different speeds of revolution. Spiral galaxies, whose 

 components have the fastest revolution velocities 

 about their galactic center, are most common; they 

 comprise about 80 per cent of all galaxies, including 

 our own. The elliptical galaxies, spheres to saucer- 

 like discs, have slower revolution speeds than those 

 of the spiral galaxies; they encompass about 17 per 

 cent of all galaxies. The irregular galaxies are form- 

 less clouds of stars without either concentration into 

 a galactic nucleus or systematic orbiting of individual 

 stars. This group, the slowest moving, constitutes 

 the remaining 3 per cent of the galaxies. 



There is no general agreement among astronomers 

 as to why galaxies have different revolution speeds 

 and, hence, spiral, elliptical, and irregular structures. 

 Some believe the three types represent the evolution- 

 ary sequence along the path from galaxy birth to 

 death; others believe that the types are no more than 



