mountain profile Institute for Astronomy University of Hawaii



Ast 622: The Interstellar Medium
Spring 2014 TuTh 3:00-4:30pm
Jonathan Williams, Institute for Astronomy


The Interstellar Medium (ISM) is the gas and dust between the stars. Stars form from it, their winds and supernova enrich and replenish it. Temperatures in the ISM range from the very hot, >106K, to the very cold, <10K. Observed densities span an even wider dynamic range, from <10-3 to >106 particles per cubic centimeter. Even the highest densities, however, are more rarefied than the best vacuums currently attainable on Earth and thus the ISM allows us to explore physical processes in unique environments. This course will cover observations and theories of a wide range of ISM environments from pervasive diffuse, ionized gas to dense, star forming cores in molecular clouds.

Although this is a major area of research in general, there is relatively little direct work within the IfA. However, many of the physical processes that I will lecture on have applications from planetary atmospheres to galaxy formation. The structure of the class is designed to emphasize utility by alternating between lectures and student-led problem solving on the whiteboard. This second part will be largely free-form order-of-magnitude estimates and should provide a useful experience for the oral qualifying exams. Participation and performance in the problem solving classes accounts for 50% of the course grade.

I will give handouts for each lecture and pointers to additional reading. There is no official course book but I frequently refer to a recent book by Bruce Draine for more detailed discussions and derivations.

The second main part of the course will be an independent research project chosen from the list below. These require analzying (reduced) datasets and/or carrying out your own calculations. Most are based on published work but the goal is for the student to go through the analysis steps themselves and to see how the concepts introduced in the lectures are applied in research. The student should produce a 4-page paper in the style of an ApJ Letter 2 weeks before the end of the semester and present their work in the style of a short conference talk to the rest of the class. The paper and presentation will together account for the remaining 50% of the course grade.


  1. Some fundamentals
    • Statistical mechanics
    • The non-equilibrium ISM
    • Radiative transfer
    • Fluid dynamics

  2. Dust
    • Reddening and extinction
    • Mie theory
    • Temperature
    • Absorption and emission

  3. Ionized regions
    • Stromgren sphere
    • Bremsstrahlung & synchrotron radiation
    • Ultra-compact HII regions, super star clusters
    • The 2-level system
    • The 3-level system

  4. Atomic regions
    • The 21cm line
    • Equivalent width

  5. Heating and cooling
    • Heating and cooling mechanisms
    • The 2-phase ISM
    • The 3-phase ISM

  6. Molecules
    • Vibrational and rotational states
    • Molecular hydrogen
    • Carbon monoxide
    • Molecular clouds
    • Astrochemistry
    • Photon dominated regions

  7. Dynamics
    • The Jeans' mass
    • Shocks
    • Expanding supernova remnants

  8. The circumstellar medium
    • Make planets not stars