(Background) The varying interrelationships among the many components of matter in our Milky Way Galaxy comprise a sort of "galactic ecosystem." Its evolutionary balance may be as complex as that of life in a tidal pool or a tropical rainforest. Here, stars abound throughout the Lagoon Nebula, a rich stellar nursery about 1200 pc from Earth.
(Inset A) An emission nebula, the North American Nebula, glowing amidst a field of stars, its red color produced by the emission of light from vast clouds of hydrogen atoms.
(Inset B) An open cluster, the Jewel Box, containing many young, blue stars.
(Inset C) A typical globular cluster, 47 Tucanae. This true-color image reveals its dominant member stars to be elderly red giants.
(Inset D) A true-color Hubble image of part of the Cygnus Loop—a supernova remnant, the remains of a colossal stellar explosion that occurred about 15,000 years ago.
Studying this chapter will enable you to:
Describe the overall structure of the Milky Way Galaxy and specify how the various regions differ from one another.
Explain the importance of variable stars in determining the size and shape of our Galaxy.
Describe the orbital paths of stars in different regions of the Galaxy and explain how these motions are accounted for by our understanding of how the Galaxy formed.
Discuss some possible explanations for the existence of the spiral arms observed in our own and many other galaxies.
Explain what studies of galactic rotation reveal about the size and mass of our Galaxy and discuss the possible nature of dark matter.
Describe some of the phenomena observed at the center of our Galaxy.
Looking up on a dark, clear night, we are struck by two aspects of the night sky. The first is a fuzzy band of light—the Milky Way—that stretches across the heavens. From the Northern Hemisphere this band is most easily visible in the summertime, arcing high above the horizon. Its full extent forms a great circle that encompasses the entire celestial sphere. Away from that glowing band, however, our second impression is that the nighttime sky seems more or less the same in all directions. Bunches of stars cluster here and there, but overall, apart from the band of the Milky Way, the evening sky looks pretty uniform. Yet this is only a local impression. Ours is a rather provincial view. When we consider much larger volumes of space, on scales far, far greater than the distances between neighboring stars, a new level of organization becomes apparent as the large-scale structure of the Milky Way Galaxy is revealed.
