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Our thin disk of young stars, gas, and dust is embedded in a thicker but more diffuse disk of older stars; this thicker disk extends about 3000 light-years above and below the midplane of the thin disk and contains only about 5% as much mass as the thin disk.
Close in to the galactic center (within about 10,000 light-years), the stars are no longer confined to the disk but form a central bulge (or nuclear bulge ). When we observe with visible light, we can glimpse the stars in the bulge only in those rare directions where there happens to be relatively little interstellar dust. The first picture that actually succeeded in showing the bulge as a whole was taken at infrared wavelengths ( [link] ).
The fact that much of the bulge is obscured by dust makes its shape difficult to determine. For a long time, astronomers assumed it was spherical. However, infrared images and other data indicate that the bulge is about two times longer than it is wide, and shaped rather like a peanut. The relationship between this elongated inner bulge and the larger bar of stars remains uncertain. At the very center of the nuclear bulge is a tremendous concentration of matter, which we will discuss later in this chapter.
In our Galaxy, the thin and thick disks and the nuclear bulge are embedded in a spherical halo of very old, faint stars that extends to a distance of at least 150,000 light-years from the galactic center. Most of the globular clusters are also found in this halo.
The mass in the Milky Way extends even farther out, well beyond the boundary of the luminous stars to a distance of at least 200,000 light-years from the center of the Galaxy. This invisible mass has been give the name dark matter because it emits no light and cannot be seen with any telescope. Its composition is unknown, and it can be detected only because of its gravitational effects on the motions of luminous matter that we can see. We know that this extensive dark matter halo exists because of its effects on the orbits of distant star clusters and other dwarf galaxies that are associated with the Galaxy. This mysterious halo will be a subject of the section on The Mass of the Galaxy , and the properties of dark matter will be discussed more in the chapter on The Big Bang .
Some vital statistics of the thin and thick disk s and the stellar halo are given in [link] , with an illustration in [link] . Note particularly how the ages of stars correlate with where they are found. As we shall see, this information holds important clues to how the Milky Way Galaxy formed.
| Characteristics of the Milky Way Galaxy | |||
|---|---|---|---|
| Property | Thin Disk | Thick Disk | Stellar Halo (Excludes Dark Matter) |
| Stellar mass | 4 × 10 10 M Sun | A few percent of the thin disk mass | 10 10 M Sun |
| Luminosity | 3 × 10 10 L Sun | A few percent of the thin disk luminosity | 8 × 10 8 L Sun |
| Typical age of stars | 1 million to 10 billion years | 11 billion years | 13 billion years |
| Heavier-element abundance | High | Intermediate | Very low |
| Rotation | High | Intermediate | Very low |
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