Review Questions
Identify several dark nebulae in photographs in this chapter. Give the figure numbers of the photographs, and specify where the dark nebulae are to be found on them.
Why do nebulae near hot stars look red? Why do dust clouds near stars usually look blue?
Describe the characteristics of the various kinds of interstellar gas (HII regions, neutral hydrogen clouds, ultra-hot gas clouds, and molecular clouds).
Prepare a table listing the different ways in which dust and gas can be detected in interstellar space.
Describe how the 21-cm line of hydrogen is formed. Why is this line such an important tool for understanding the interstellar medium?
Describe the properties of the dust grains found in the space between stars.
Why is it difficult to determine where cosmic rays come from?
What causes reddening of starlight? Explain how the reddish color of the Sun’s disk at sunset is caused by the same process.
Why do molecules, including H2 and more complex organic molecules, only form inside dark clouds? Why don’t they fill all interstellar space?
Why can’t we use visible light telescopes to study molecular clouds where stars and planets form? Why do infrared or radio telescopes work better?
The mass of the interstellar medium is determined by a balance between sources (which add mass) and sinks (which remove it). Make a table listing the major sources and sinks, and briefly explain each one.
Where does interstellar dust come from? How does it form?