Chapter 22

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

Why are the HII regions around O stars generally larger than those around \(B\) stars?

Why wouldn't you expect to detect strong \(21 \mathrm{cm}\) emission from an HII region?

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

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

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?

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?

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

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?

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

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

According to the density wave theory, why are \(\mathrm{O}\) and \(\mathrm{B}\) stars found almost exclusively in spiral arms?

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

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