Apply this unit's pressure-balance framework to different real phase pairs and a hypothetical case.
Using the real published (n,T) values for the warm ionized medium (WIM: n=0.15 cm^-3, T=8e+03 K) and the hot ionized medium (HIM: n=0.0035 cm^-3, T=5e+05 K), compute both phases' P/k and state whether they are closer to or farther from pressure equilibrium than the CNM/WNM pair discussed in the lecture.
Using the H II region phase's published parameters (n=1e+02 cm^-3, T=1e+04 K), compute its P/k and compare it quantitatively to the WNM's P/k. Given that H II regions are transient, short-lived structures (Unit 6 derives their formation), discuss whether the large pressure mismatch you find is a problem for the pressure-equilibrium framework or expected given their transience.
Suppose observers discovered a seventh, previously unknown ISM phase in exact pressure equilibrium with the CNM (same P/k as the CNM's published value) but with a density of exactly 3.0 cm^-3. Compute this hypothetical phase's temperature, and state which of the six real phases in this unit's data it would most resemble in temperature (though not necessarily density).
Sum the published volume filling factors of all six phases in this unit's data. Comment on whether the sum is close to 1 (as it should be if the six phases genuinely partition the ISM's volume) and discuss one physical reason small deviations from exactly 1 might appear in real filling-factor estimates (e.g., overlapping definitions, unaccounted transitional gas).
Submit a complete derivation with equations, units, labeled quantities, and a short narrative interpretation for each problem. Show every step; a correct final number without a visible derivation receives partial credit at most, and quoting a lecture number without adapting it to this problem's specific inputs receives no credit for that step.
OpenStax Astronomy 2e (local extracted reference copy: references/openstax-astronomy-2e-extracted.txt) covers the interstellar medium in Chapter 20 ("Between the Stars: Gas and Dust in Space"): 20.1 The Interstellar Medium, 20.2 Interstellar Gas (including the 21 cm line and H II regions), 20.3 Cosmic Dust (extinction and reddening), 20.4 Cosmic Rays, 20.5 The Life Cycle of Cosmic Material, and 20.6 Interstellar Matter around the Sun. Chapter 20’s section numbering was cross-checked directly against its own embedded figure numbering this session (Figures 20.1 through 20.20 all appear under their matching section headers with no offset), so this citation is verified against the local extracted text rather than extrapolated by analogy. OpenStax’s treatment throughout Chapter 20 is descriptive and qualitative; nearly every quantitative derivation in this course (the 21 cm hyperfine radiative-transfer solution and HI column-density formula, the Jeans mass/length instability criterion, the free-fall collapse time, the Stromgren-sphere ionization-balance derivation, the Rankine-Hugoniot shock-jump conditions, and the cosmic-ray power-law spectrum) goes well beyond OpenStax Astronomy 2e’s introductory level; each lecture states explicitly where this course extends beyond the assigned reading.
materials/ASTR360/data/ where referenced above.