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Conceptual Questions
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1

Why must the star responsible for an HII region be a hot star rather than a cool star?
2

Why are the HII regions around O stars generally larger than those around B stars?
3

Describe the evolution of an HII region.
4

Why wouldn't you expect to detect strong 21 cm emission from an HII region?
5

What heats the hot phase of the interstellar gas?
6

What would most bright nebulae look like if there were no interstellar dust?
7

What is the relationship between the color of a reflection nebula and the color of the star that illuminates it?
8

What fraction of the light from a star is able to penetrate 3000 pc through a typical region of interstellar dust?
9

What property of interstellar dust has made it easier for astronomers to study distant parts of the galaxy at infrared wavelengths rather than in the visible part of the spectrum?
10

What prediction would you make about how the color of the diffuse light in the galaxy compares with the average color of the bright stars in the galaxy?
11

What is the reason that the rocky material in an interstellar dust particle forms a core and the icy material forms an outer layer rather than vice versa?
12

What was the principal reason that the measurements of Herschel and Kapteyn didn't reveal the correct shape and size of the galaxy?
13

How did Herschel explain the appearance of the Milky Way in terms of the distribution of stars in the galaxy?
14

Suppose an observer on a hypothetical planet maps the distribution of globular clusters in the sky. The observer finds that globular clusters are evenly distributed throughout all parts of the sky. Where is the observer's planet?
15

What is the evidence that most of the matter in the galaxy exists in an as-yet undetected form?
16

What kinds of objects are found preferentially in the spiral arms of a galaxy?
17

Why can't observations in visible light reveal the entire spiral pattern of the galaxy?
18

How do we know that the arms of the galaxy can't be "material arms''?
19

Why is rotation speed smaller in a spiral arm than in the region between spiral arms?
20

About how often do the stars in the Sun's part of the galaxy catch up with and pass through a spiral arm?
21

According to the density wave theory, why are O and B stars found almost exclusively in spiral arms?
22

Describe the evidence that there is a massive black hole in the center of the galaxy.
23

Why do astronomers suspect that there have been violent explosions at the center of the galaxy?
24

Why are the young objects in the galaxy confined to a thin disk, but the oldest objects occupy a nearly spherical volume of space?
25

How have the shape and chemical composition of the galaxy changed as the galaxy evolved?





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