Chapter summary

In this chapter we saw how stars and interstellar material are arranged in the Milky Way.

We saw how the rotation curve tells us about the mass distribution in the galaxy, and how that rotation curve is determined. Different techniques are needed for material inside and outside the Sun's orbit about the galactic center. We saw how the rotation provides evidence for dark matter. Once the rotation curve is known, velocities of objects can be used to estimate distances. This works better outside the Sun's orbit than inside, because of the distance ambiguity.

In looking at the average gas distribution, we found that the HI is extended beyond the Sun's orbit, while the number of molecular clouds falls off more sharply with distance from the center of the galaxy. The molecular clouds are also more tightly concentrated toward the galactic plane than the HI clouds. This concentration toward the plane suggests that the molecular clouds are younger.

We discussed evidence for spiral structure in the Milky Way and the difficulties in tracing out spiral arms in our galaxy. We saw that tracers for spiral structure include HII regions, OB associations and molecular clouds.

In looking at the galactic center, we found that it contains a small, active region. The center has a mass concentration which may be a few million M0 black hole.

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