Overview

Throughout this book we have discussed the components of our galaxy: stars, clusters of stars, interstellar gas and dust. We now look at how these components are arranged in the galaxy. The study of the large scale structure of our galaxy is difficult from our particular viewing point. We are in the plane of the galaxy, so all we see is a band of light (Fig. 16.1). The interstellar dust prevents us from seeing very far into the galaxy. We see a distorted view.

The first evidence on our true position in the galaxy came from the work of Harlow Shapley, who studied the distribution of globular clusters (Fig. 16.2). He found the distances to the clusters from observations of Cepheids and RR Lyrae stars. Shapley found that the globular clusters form a spherical distribution. The center of this distribution is some 10 kpc from the Sun. Presumably, the center of the globular cluster distribution is the center of the galaxy. This means that we are about 10 kpc from the galactic center.

In Chapter 13, when we studied HR diagrams for clusters, we introduced the concept of stellar populations I and II. The distribution of these populations in the galaxy can help us understand how the galaxy has evolved. Population I material is loosely thought of as being the young material in the galaxy. Population I stars are found in galactic clusters, and are characterized by high metalicity. Some are also associated with interstellar gas and dust, suggesting that they are young enough to have some of their parent cloud around them. Population I stars are confined to the galactic plane.

Population II stars are thought of as being the "old" component of the galaxy. They are found in globular clusters and are characterized by low metalicity. They have no gas and dust around them. Their galactic distribution is very different from that of population I stars. The population II stars form a spherical distribution, as opposed to a disk. This spherical distribution is sometimes called the halo. When we talk about a spherical distribution, we do not mean just a spherical shell around the galaxy. Instead, we mean the spherically symmetric distribution whose density falls off with increasing distance from the galactic center. Population II objects also seem to have a larger velocity spread in their motions than do population I objects. Table 16.1 shows the characteristic thicknesses and velocity dispersions for some components of the galaxy.

The schematic arrangement of these components is shown in Fig. 16.3. First we see the disk and the halo. Note that the halo has a spherical distribution with a density of material that falls off radially. We then look at the disk. First there is an overhead view, showing the location of the Sun. The best estimates, which we will discuss later in this chapter, place the Sun 8.5 kpc from the galactic center, about halfway out to the edge of the disk. We then look at a side view of the disk. The inner part of the disk (closer to the center than the Sun) is relatively thin and flat. The outer part of the disk is warped and also gets thicker, that is, it flares. We will also discuss the evidence for this in this chapter. Surrounding the center region is a bulge.

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