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Interstellar Medium


Admission requirements

Essential prior knowledge for this course is contained in the Astronomy bachelor's 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. In addition, required knowledge is basic statistical physics and quantum mechanics.


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 constituents of the ISM (ionized, atomic, and molecular gas; dust; electromagnetic radiation; magnetic fields; cosmic rays), their relation and interaction, and the different environments in which these are encountered (the multi-phase models of the ISM), as well as the corresponding observational diagnostics. 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).

The following themes are covered:

  • 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.

  • Interaction of radiation with interstellar matter: 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. 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.

  • The HI 21cm line and the 2-phase ISM: 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.

  • Ionization and recombination: photoelectric absorption and radiative recombination. The hydrogen spectrum. Recombination lines. Case A and case B recombination spectra. Measuring star formation using recombination lines.

  • HII regions: Strömgren spheres. Ionization of hydrogen, helium, and heavier elements. The role of dust. Structure and evolution of HII regions.

  • 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.

  • Molecules and their excitation: Born-Oppenheimer approximation, electronic, vibrational and rotational transitions; spectra of diatomic molecules; ortho- and para-H2. Molecular excitation and radiative trapping.

  • Molecular lines and molecular clouds: 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.

  • Interstellar dust: Extinction curves and reddening. Definitions of absorption, scattering and extinction cross sections. Constraints on dust models. Constituents of interstellar dust. PAHs. Radiative heating and cooling of dust grains. Infrared emission.

  • 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.

  • Shocks: J-type and C-type shocks in molecular clouds. Supernova shocks.

  • The 3-phase model of the ISM.

  • PDRs, XDRs, and the extragalactic ISM: Formation and destruction of H2. Self-shielding. PDRs and XDRs. The ISM at low metallicity and in the early Universe.

Course objectives

Principal course objective: upon completion of this course you will be able to explain the basic physical conditions in the Interstellar Medium (ISM) and its various constituents, and explain the relation between these constituents. You will also be able to carry out basic calculations of physical conditions in the ISM under simple assumptions.

Specifically, upon completion of this course you will be able to:

  • Explain the fundamental physical conditions in the ISM, including concepts such as kinetic temperature and excitation temperature

  • Explain and apply the principle of detailed balance in the ISM

  • Solve simple radiative transfer problems as applied to the ISM

  • Explain the 2-phase model of the ISM, including the observational evidence for it

  • Explain photoionization-recombination balance and the resulting recombination spectra

  • Calculate simple properties of spherical HII regions

  • Explain collisional excitation including the concept of critical density and the limiting cases at high and low densities

  • Calculate level populations of atoms and molecules in optically thin conditions

  • Explain radiative trapping, its effect on the line spectrum and level populations, and radiative transfer and excitation under optically thick conditions

  • Explain the spectra of simple molecules and the use of molecular lines as probes of physical conditions

  • Explain the properties, role and observable effects of dust in the ISM

  • Explain the thermal balance in the various constituents of the ISM

  • Calculate ISM temperatures under simplified conditions

  • Explain the nature and effects of shocks in the ISM

  • Explain the 3-phase model of the ISM

  • Explain the nature and physics of PDRs, XDRs, and of the ISM at low metallicity and in the early Universe

Soft skills

At the end of this course, you will have been trained in the behaviour-oriented skill of correctly explaining and analyzing complex concepts.


See Astronomy master schedules

Mode of instruction

  • Lectures

  • Exercise classes

Assessment method

  • Written exam


Students should enroll on Blackboard before the first lecture. Blackboard will be used to communicate with students and to share lecture slides, homework assignments, and any extra materials. To have access, you need a student ULCN account.

Reading list

Physics of the Interstellar and Intergalactic Medium, Draine, ISBN 9780691122144 (paperback) or ISBN 9780691122137 (hardcover) (required)


Via uSis. More information about signing up for your classes can be found here. Exchange and Study Abroad students, please see the Prospective students website for information on how to apply.

Contact information

Lecturer: Prof.dr. P.P. (Paul) van der Werf
Assistant: Kirsty Butler