Central question

How did astronomers discover the layout of the Milky Way Galaxy, and what objects and material does it contain?

How are the stars arranged in space? The appearance of the Milky Way (Fig.23.1) prompted the search for the extent and distribution of stars. On a moonless night in a dark location you can trace out the soft, luminous band of the Milky Way wrapped like a fine scarf around the celestial sphere.

Ever since Galileo discovered that the Milky Way consists of a multitude of faint stars, astronomers have tried to determine the layout of these stars.

Figure 23.1 A wide-angle view of the southern Milky Way, taken by Art Hoag at Cerro Tololo Interamerican Observatory. Note the Milky Way, the Magellanic Clouds, and the zodiacal light.

This chapter first deals with the quest for a detailed map of the nearby regions of space. We look at this question historically to see how astronomers have attacked this puzzle and to show the key importance of distance measurements and the properties of stars in solving it. The search culminates with the discovery of the Galaxy: a vast, flat pinwheel of stars, gas, and dust. Only one out of at least 100 billion stars, the sun sits about halfway out from the Galaxy's center and swings around the center of the Galaxy once every 240 million years.

The chapter then turns to the contents of the Galaxy. It shows you that different stellar populations mark three distinct regions: the flat disk, the bulging nucleus, and the encompassing halo. Each part presents us with a different aspect of our complex Galaxy.