Galaxies in the Universe An Introduction

Our position in the Milky Way's disk gives us a detailed and close-up view of a fairly typical large spiral galaxy. We begin this chapter by looking at the Sun's immediate vicinity. Examining the closest stars gives us a sample of the disk stuff, and we can ask how many stars of each luminosity, mass, composition, and age are present. Combining this information with theories of stellar evolution, we investigate the star-forming past of the solar neighborhood.

In Section 2.2 we venture further afield. Measuring stellar distances allows us to map out the Milky Way's structure: the thin and thick disks, the metal poor halo, and the central bulge. Star clusters, where all the members were born together, with the same initial composition, are especially useful; comparing their color–magnitude diagrams with the predictions of stellar models yields the age and composition jointly with the cluster's distance. We find that the youngest stars belong to the disk, and are relatively rich in elements heavier than helium, whereas the stars of the metal-poor halo are extremely old.

Most of the Milky Way's stars and almost all of its gas lie in the disk, orbiting the center like the planets around the Sun. Section 2.3 deals with the Galaxy's rotation: how we measure it, and how we use it to find the distribution of gas within the disk.