Components of the Galaxy

We may divide the galaxy into several components, some of which are visible and some of which are not.
Visible Components

A spiral galaxy like the Milky Way has three basic components to its visible matter: the disk (containing the spiral arms), the halo, and the nucleus or central bulge. These components are indicated schematically in the following figure. The halo and the nucleus are also referred to collectively as the spherical component of the galaxy since they have an approximately spherical distribution with respect to the center of the galaxy.

Nonvisible Components

In addition to these visible components, the galaxy also contains at least three other components that are "invisible": the galactic magnetic field, charged particles trapped in the galactic magnetic field, and a halo of "dark matter" that is of unknown composition but that makes itself felt by its gravitational influence on the visible matter.
Star Populations

The present discussion of Pop I and Pop II is oversimplified. There is not such a clear distinction for real stars, and astronomers often divide these categories up into subcategories such as extreme Pop I and intermediate Pop I (the latter is the category of the Sun) according to their detailed spectral characteristics. For our discussion the simple division into two populations will be sufficient.

Two Star Populations
As we already suggested in our discussion of star clusters in Chapter 19, it is useful to divide the stars of the galaxy into two populations that can be distinguised by their spectral characteristics. The first generation of stars that formed were poor in elements heavier than hydrogen and helium ("metal-poor"). They are distributed rather spherically, in the halo of globular clusters and in the central bulge. This set of old, metal-poor stars is called Population II, or "Pop II", for short. On the other hand, the young stars in the galaxy have formed in the galactic disk, because that is where the star-forming material is concentrated today. They contain more metals because the galaxy has been enriched in heavy elements by earlier generations of stars. These stars, which are metal-rich and typically form in stellar associations and open clusters, constitute Population I, or "Pop I" for short. Some principle characteristics of these two populations are given in the following table.

Characteristics of Population I and Population II
Characteristic Population I Population II
Metal Concentration 2-3% < 1%
Main Spectral Classes O, B, A K, M
Most Luminous Stars Blue Supergiants Red Giants
Location Galactic Disk Nucleus and Halo
Age < 1.5 x 109 years > 1.5 x 109 years

To summarize, Population I is the mix of stars similar to that found in open clusters or in spiral arms. Thus, the stars near the Sun are Pop I. In contrast, Population II is the mixture of stars typical for globular clusters. As the preceding table indicates, these populations have substantially different characteristics.

Formation of the Galaxy
The two populations of stars are a result of the way that our galaxy formed. Initially the galaxy was a great ball of gas and dust. This ball may have formed as a large cloud, or it may have formed by the accretion of smaller clouds that possibly had already made a generation of stars. The first stars that formed were spherically distributed, and because the initial cloud was spinning slowly the stars formed in rather random orbits within the initially spherical galaxy. This early generation of spherically distributed stars become the present Pop II; they were low in metals because little star formation had preceded them.

As the galaxy flattened and began to spin faster the star-forming material concentrated in the disk, so most newer star formation has occurred there. These newer disk stars, which constitute Pop I, are richer in metals because of the stellar evolution associated with earlier generations of stars. They also were likely to be formed in more circular orbits confined to the disk because of the more rapid rotation of the galaxy as it contracted.