Interstellar Medium (NB Old version; will be updated for 2019 asap)

General Book Organisation Schedule

General information

Credits and level - MSc. Astronmy, level 500, 6EC

Course description - The space between the stars is filled with matter, magnetic fields, and radiation. This course describes this Interstellar Medium (ISM) as an integral part of galactic ecosystems. It provides an overview of the known constituents of the ISM (ionized, atomic, and molecular gas; dust; magnetic fields; cosmic rays; EM radiation), and the different environments in which these are encountered (the 2- and 3-phase models of the ISM) along with the observational diagnostics (atomic and molecular spectroscopy; spectral energy distributions). It discusses the physical processes that govern the interactions within the ISM and with stars (radiation, shocks), and it highlights the relationships between the ISM and stars and their host galaxies (birth and death of stars; supernovae; nuclei of active galaxies).

Prerequisites - Essential prior knowledge for this course is contained in the BSc course Radiative Processes. Crucial in particular is knowledge of emission and absorption processes, the Einstein coefficients, spectral lines, Kirchhoff's Law, and the radiative transfer equation, including the concept of optical depth. Although these concepts are recapulated in the first two lectures, this will be done quite quickly, and all students should review this material before the start of the course (taken from the BSc course Radiative Processes). In addition, required knowledge is basic statistical physics and quantum mechanics.

Lectures - The course will have 12 lectures of 2 hours each. The corresponding textbook chapters are given in the schedule. Prepare each lecture by reading those chapters. The textbook approaches the material in a bottom-up way, starting the basic physics. The lectures will adopt this the same approach, but add a lot of practical examples, which will be included in the handouts. After the lectures, re-read the indicated chapters and study the handouts.

Problem sets - the course will also comprise several problem classes, in which a number of problems will be assigned. Problems assigned during problem classes must be handed in (by email to the assistant) by the deadline indicated, and will be graded. The grades for the problem sets will be averaged and this average, rounded to 1 decimal after the comma, will count for 25% of the final grade for the course. If you expect that you will be late in handing in a particular problem set, you can request permission for handing it in later, explaining the reasons for your request. You must do this by email to the teacher, before the deadline. Not handing in a problem set, or handing it in after the deadline without explicit permission, results in a "1" on that particular problem set. The problems will be fairly challenging, and discussing them amongst yourselves will be part of the learning experience. Therefore you must solve the problems in groups of 2 persons (if necessary there can be one group of 3 persons). Per group only 1 set of solutions needs to be handed in. Do not hesiate to seek help from the assistant. The clarity of your solutions will factor significantly into the grades. It is not sufficient to write a few equations. You must define your variables, draw well-labeled figures where appropriate, and explain what you are doing. It is critical that you start working on the problem set shortly after it is assigned. Allow yourself plenty of time to solve them, and to seek help (after you've put in serious effort), by discussing the problems with your fellow students and the assistant.

Exam and grade - The course will be closed off with an oral exam, which must be taken in December 2014 or January 2015. The student should make an appointment with the teacher for taking the exam. The oral exam will not involve complex mathematical derivations, and example questions will be given during the lectures. Course book and handouts may be brought to the exam (but it is unliekly that you will need them). The final grade for the course will be calculated from the grade for the oral exam (75%) and the average grade (rounded to to 1 decimal after the comma) of the problem sets (25%).

Material not covered - Given the limited amount of time available, choices had to be made as to what to present and what to leave out. As a result, a number of topics are covered only superficially. The choice of these was in some (but not all) cases dictated by the fact that they are covered elsewhere in our curriculum. This concerns in the first place Astrochemistry and Star formation, on which the present course touches only superficially, but which are covered in depth in other courses in our MSc programme. Other topics that receive scant attention are:

  1. collisionally ionized plasmas
  2. ultraviolet absorption lines and the intergalactic medium
  3. astrophysical gas dynamics
  4. radio continuum processes
  5. high-energy processes


During the this course we will closely follow parts of an excellent and very recent textbook:

All students should have a copy of the above book, which will be the primary reference for this course. Note that Bruce Draine maintains a list of errata and a problem collection, which can be downloaded from his website.

Another good text (not compulsary for this course) is: Finally, the following two classical texts (also not compulsory for this course) should be mentioned; they contain a wealth of excellent material, and have educated a complete generation of researchers: In addition to these books, handouts will be provided and can be downloaded from this website (see schedule).


Teacher Paul van der Werf, J.H. Oort Gebouw 565, tel. 071-5275883
Assistant TBD, J.H. Oortgebouw TDB, tel. 071-TDB


  1. Lecture 1: Introduction, overview and fundamental physical processes - September 5, 2014, 11:15-13:00, HL414
    Topics: Class organization and introduction. History of ISM research. The Galactic ecosystem: HII regions, reflection nebulae, SNRs, dark clouds. Distribution in the Milky Way. ISM mass budget. Objects vs. phases. Properties of ISM phases and cycle of material between phases. Energy sources and energy densities in the ISM. Fundamental physical conditions: Maxwell velocity distribution and kinetic temperature. Lack of LTE. Excitation temperature. Statistical equilibrium.
    Literature: Draine, Ch. 1
    [ Lecture 1 slides (pdf) ]
  2. Lecture 2: Interaction of radiation with interstellar matter - September 12, 2014, 11:15-13:00, HL414
    Topics: Description of the radiation field: radiation intensity, specific energy density. Definition of the Einstein coefficients for absorption, spontaneous emission and stimulated emission. Relation between the Einstein coefficients. Relation to cross section. Line profiles. Natural broadening and Doppler broadening. Equation of transfer for radiation. Relation of emissivity and absorption coefficient to Einstein coefficients. Optical depth and source function. Kirchhoff's law. Population inversion and masers.
    Literature: Draine, Ch. 6 except Sections 6.6 & 6.7; Draine, Ch. 7
  3. Problem class 1: September 12, 2014, 13:45-15:30, HL414
  4. Lecture 3: The HI 21cm line and the 2-phase ISM - September 19, 2014, 11:15-13:00, HL414
    Topics: The HI 21cm line. Equation of transfer in terms of Rayleigh-Jeans brightness temperature. Spin temperature. Deriving HI column density and mass from optically thin HI emission. HI absorption. Emission-absorption observations and the evidence for the 2-phase ISM.
    Literature: Draine, Ch. 8 and 9 except Sections 9.8 and 9.11
    [ Lecture 3 slides (pdf) ]
  5. Lecture 4: Photoionization, and recombination lines - September 26, 2014, 11:15-13:00, HL414
    Topics: Ionization processes: photoelectric absorption. Recombination processes: radiative recombination. The hydrogen spectrum. Recombination lines. Case A and case B recombination spectra. Measuring star formation using recombination lines.
    Literature: Draine, Ch. 13 except Sections 13.2 and further; Draine, Ch. 14 except Sections 14.2.4 and further
    [ Lecture 4 slides (pdf) ]
  6. Lecture 5: HII regions - October 10, 2014, 11:15-13:00, HL414
    Topics: HII regions. Strömgren spheres. Ionization of hydrogen, helium, and heavier elements. The role of dust. Structure and evolution of HII regions.
    Literature: Draine, Ch. 15 except Sections 15.4, 15.7, and 15.8
    [ Lecture 5 slides (pdf) ]
  7. Problem class 2: October 10, 2014, 13:45-15:30, HL414
  8. Lecture 6: Collisional excitation - October 24, 2014, 11:15-13:00, HL414
    Topics: Collisional excitation. Critical density for a 2-level system. Behaviour of the population ratio in limiting cases of very high and very low density. Implications for HI spin temperature. Generalization to multi-level systems. Line ratios as diagnostics for density, temperature and abundance.
    Literature: Draine, Ch. 17 except Sections 17.6 and 17.7; Draine, Ch. 18 except Sections 18.1.2, 18.4, 18.6 and 18.7, qualitatively only
    [ Lecture 6 slides (pdf) ]
  9. Lecture 7: Molecules and molecular clouds - October 31, 2014, 11:15-13:00, HL414
    Topics: Molecular energy levels: Born-Oppenheimer approximation, electronic, vibrational and rotational transitions; spectra of diatomic molecules; ortho- and para-H2. Molecular excitation and radiative trapping
    Literature: Draine, Ch. 5 (only global properties of spectra of diatomic molecules) except Sections 5.1.8 and further; Draine, Ch. 19 until beginning of Section 19.2
    [ Lecture 7 slides (pdf) ]
  10. Problem class 3: October 31, 2014, 13:45-15:30, HL414
  11. Lecture 8: Molecular lines and molecular clouds: November 7, 2014, 11:15-13:00, HL414
    Topics: Radiative trapping. Solving excitation and radiative transfer for very optically thick lines: escape probabilities. Using CO as a tracer of the mass of molecular clouds. Global properties of molecular clouds. The X-factor.
    Literature: Draine, Ch. 19; Draine Ch. 32 except Sections 32.6 - 32.11
    [ Lecture 8 slides (pdf) ]
  12. Lecture 9: Interstellar dust - November 21, 2014, 11:15-13:00, HL414
    Topics: Interstellar dust. Extinction curves and reddening; features on the extinction curve. Definitions of absorption, scattering and extinction cross sections. Constraints on dust models. Constituents of interstellar dust. PAHs. The MRN dust model. Radiative heating and cooling of dust grains. Steady state temperatures. Temperature spikes in very small dust grains. Infrared emission.
    Literature: Draine, Ch. 21, except Sections 21.3, 21.4, 21.6 and 21.8; Draine Ch. 22.1, only first 4 definitions and discussion around Eq. 22.6; Draine Ch. 23 except Sections 23.4, 23.6 - 23.9 and 23.10.1; Draine Ch. 24 except Sections 24.1.2 and 24.4
    [ Lecture 9 slides (pdf) ]
  13. Lecture 10: Thermal balance - November 28, 2014, 11:15-13:00, HL414
    Topics: Thermal balance of the ISM. Heating and cooling of HII regions. HII region temperatures. Heating and cooling of the neutral ISM and the 2-phase model.
    Literature: Draine, Ch. 27 except Section 27.2; Draine Ch. 30
    [ Lecture 10 slides (pdf) ]
  14. Problem class 4: November 28, 2014, 13:45-15:30, HL214
  15. Lecture 11: Shocks, and the 3-phase model of the ISM - December 5, 2014, 11:15-13:00, HL414
    Topics: Shock waves. Supernova remnants. Hot gas and the 3-phase model of the ISM.
    Literature: Draine, Ch. 36, excepts Sections 36.3 and further, without magnetic fields; Draine, Ch. 39, qualitative only; skip Section 39.3
    [ Lecture 11 slides (pdf) ]
  16. Lecture 12: PDRs, XDRs, and extragalactic ISM - December 12, 2014, 11:15-13:00, HL414
    Topics: Formation and destruction of H2. Self-shielding. PDRs. J-type and C-type shocks in molecular clouds. XDRs. Hot topics and open questions
    Literature: Draine, Ch. 31 except Sections 31.6-31.8; Draine, Ch. 36 only Section 36.6 (qualitative only)
    [ Lecture 12 slides (pdf) ]
  17. Problem class 5: December 12, 2012, 13:45-15:30, HL414

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Last modified: Thu Jun 6 17:53:11 2019
Paul van der Werf