Finding the Milky Way in the Galaxy

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No visible light is emitted by the central region of the galaxy.

Interstellar dust blocks our view.

Too many stars are in the way.

Gravity curves the light away from the earth and Sun.

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So what causes spiral arms?

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Density waves

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Spiral arms are patterns

According to the density-wave theory, spiral arms are created by density waves that sweep around the Galaxy

The gravitational field of this spiral pattern causes stars and gas to slow down near the arm

This compresses the interstellar clouds, triggering the formation of stars

The entire arm pattern rotates around the Milky Way once every 500 million years

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Density waves

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Density waves

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M74

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Which of the following objects are not found primarily in the spiral arms of the Galaxy?

white dwarf stars

HII regions

supernovas

O and B stars

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Review Questions

How can we measure the mass of the Milky Way?

Why do stars behind dust clouds appear red?

Why is the sky blue?

Why are wavelengths of light outside the visible useful in studying the Milky Way?

How is the 21 cm line of Hydrogen produced?

Describe the spiral arms of the Milky Way and what causes them.

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Definitely in the form of brown dwarfs

Probably cold, dark hydrogen molecules

Likely super-massive black holes

Definitely cold gas, unknown composition

Not known

The nature of the dark matter in the Milky Way is:

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A concentration of dark matter inside 2 kpc

A spherical distribution of dark matter

Dark matter only outside 2 kpc

Uniformly distributed dark matter in the disk

No dark matter is needed to explain this plot

From the rotation curve of a hypothetical galaxy as shown in the figure, one could infer