Apparatus and Setup
This is a computational lab (no telescope required) that builds this unit's six-phase pressure-balance table and a schematic thermal-equilibrium (heating=cooling) curve from Lectures 01-02's real, standard published ISM phase parameters (McKee & Ostriker 1977).
Materials and Data
| Phase | n (cm^-3) | T (K) | P/k (cm^-3 K) | Filling factor |
|---|---|---|---|---|
| Molecular clouds (H2) | 200 | 15 | 3,000 | 0.001 |
| Cold neutral medium (CNM) | 30 | 80 | 2,400 | 0.010 |
| Warm neutral medium (WNM) | 0.3 | 8e+03 | 2,400 | 0.350 |
| Warm ionized medium (WIM) | 0.15 | 8e+03 | 1,200 | 0.200 |
| H II regions | 100 | 1e+04 | 1,000,000 | 0.001 |
| Hot ionized medium (HIM) | 0.0035 | 5e+05 | 1,750 | 0.430 |
All values are standard published McKee & Ostriker (1977)/Draine (2011) canonical ISM phase parameters, level 2 (not independently re-verified this session; see ../reference-log.md).
Procedure
- Compute P/k=nT for all six phases (reproduce the table above from the raw n, T values) and rank the phases by pressure.
- Compute the ratio of the WNM's pressure to the CNM's pressure and state whether the two phases are consistent with approximate pressure equilibrium (define "approximate" quantitatively, e.g., within a factor of 2).
- Compute the ratio of the molecular-cloud phase's pressure to the CNM's pressure, and explain physically why this large ratio is not evidence against the pressure-balance framework (Lecture 01's self-gravity point).
- Build the schematic thermal-equilibrium curve of Lecture 02 (log P/k vs log T) by combining a CNM-like linear branch (cooling ∝ T) and a WNM-like branch (cooling ∝ T^-0.35) with a logistic-function blend, calibrated so the curve passes through both real (T,P/k) points; identify the local slope sign on the connecting branch and confirm it is negative (thermally unstable) between the two stable points.
Analysis and Uncertainty
State explicitly which of this lab's numbers are real, standard published values (level 2) and which are this lab's own illustrative curve-fitting choice (level 3, the specific logistic blending function and its width parameter, chosen only to reproduce the qualitative S-shape, not itself a measured quantity). Discuss what physical pressure terms (magnetic, cosmic-ray, turbulent) are omitted from the simple P=nkT model and how their omission could account for any residual CNM/WNM pressure mismatch.
Deliverables
- The six-phase pressure table with your own computed P/k values.
- A plot (or SVG, following the lecture figure style) of your thermal-equilibrium curve with the unstable branch's slope sign explicitly stated.
- A one-paragraph discussion of the CNM/WNM pressure-ratio result and what physics could explain any residual mismatch from exact equality.
Assessment Criteria
- Correct P/k arithmetic for all six phases with correct units (cm^-3 K).
- Correctly identified pressure ratio and a reasoned (not merely asserted) equilibrium judgment.
- Correctly identified negative-slope region on the constructed thermal-equilibrium curve.
References and Provenance
- 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.
- ISM phase parameters: McKee & Ostriker (1977, ApJ 218, 148); Draine (2011) textbook Table 1.1 -- standard published, level 2; see
materials/ASTR360/reference-log.md. - Generated by
materials/ASTR360/src/generate_astr360_content.py.