Coverage of Topics and Study Notes:

In 2007, we are only covering the high points of Ch 18, but with a little more coverage than last year.  My viewpoint of the organization is different than the authors', so I've described it below:

Study of the interstellar medium is an important field occupying many astronomers, including me in my own research. The topics are diverse, and it's harder to find central themes than as it was for the sun, or for the stars. However, remember that only half of the matter in our galaxy is found in stars--the other half is in the interstellar medium.

It may be helpful to take an alternative organizational overview from the text:

The interstellar medium consists of several types of interstellar clouds situated in an intercloud medium. The intercloud medium is such low density that it makes up only a small fraction of the mass of the overall mass in the interstellar medium. We'll accept the description given in Discovery 18-1, Ultraviolet Astronomy and the "Local Bubble" of very low density and temperature of 500,000K "probably carved out by supernova explosions in the past."  [For those of you who are deeply interested, here's a better description from a current reference book, Astrophysical Quantities.  The intercloud medium contains three components of gas in three different conditions. One component of the intercloud medium is an extensive, very low density, very hot gas with temperature greater than 300,000K. The "local bubble" described in Discovery 18-1 is a prime example. The second and third components (not mentioned in the text) are low density, but not as low as the hot intercloud medium, and both much cooler with temperatures of 8000K. One of these 8000K components is neutral atomic hydrogen (HI) and one is ionized hydrogen (HII).]

About half of the mass in the interstellar medium is located in "diffuse clouds" of neutral hydrogen, or "HI regions." They are only specifically mentioned in the text, at the end of the section on Observations of Emission Nebulae on p 475, but they are some of the "Dark Dust Clouds" in section 18.3.  HI regions are relatively small and low density, and they are one source of the 21-cm radiation discussed in section 18.4. (The other source is the atomic intercloud medium.) The HI regions have temperature of about 80K . The text treats the HI component as a general clumpy medium between molecular clouds, but I see the HI as organized into the diffuse clouds = HI regions and intercloud HI medium.

The other half of the mass is in the "molecular" clouds, mostly discussed in section 18.5. These clouds come in various sizes. The smallest are included in our text in the discussion of "dark dust clouds" discussed in section 18.3. High density small molecular clouds are often called "Bok globules," after the great Dutch-American astronomer who called attention to them. Bok globules have higher density than HI clouds, and are colder (10-20K rather than 80K). The Rho Ophiuchi cloud in Figure 18.14 is a small molecular cloud, not an HI cloud. Molecular clouds have various temperatures. It's probably reasonable to say that they have a range from 10-50K. Note that "molecular cloud complexes" were first believed to be single giant clouds, and so they are also called Giant Molecular Clouds.

Emission Nebulae, commonly called HII regions, are the most beautiful interstellar clouds, but also the least common! Only a tiny fraction of the interstellar medium is in them. They are warm (8000K) clouds of ionized created by O and B stars. These stars are very important because they dominate the luminosity of a galaxy like our own, even though they are rare. They contain some of the most intersting astrophysics.  My research is partly in infrared observations of HII regions.

Review and Discussion:

Numbers 1 and 12, "Give a brief description" of the interstellar medium and of a dark dust cloud are too vague and open ended to be used on an quiz.   In fact, I will always specify an HI region or a molecular cloud in a question about dark clouds.

Skip numbers 3, 4, 8, 12, 16, 19, and 20.

Conceptual Self-Test,

Skip numbers 2, 12, 13,15, 20.

Number 14 choice (a) should read "exciting" instead of "exiting".  I would have dropped the word "enough" because the atoms are either excited or not.  Maybe the author meant "excite enough atoms to emit light we can detect."

Problems: Skip them all. They're great, but not for AS151. Maybe we'll use them in AS231.