mountain profile Institute for Astronomy University of Hawaii


Ast 622: The Interstellar Medium
Fall 2023 Wednesdays, Fridays 1:30-2:45pm
Jonathan Williams, C-209, jw[at]


Almost every astronomical observation must account for the passage of light through the interstellar medium (ISM); the gas and dust between the stars. Furthermore, understanding the ISM is of fundamental importance for understanding the origin of stars and the evolution of galaxies. The environments are extreme: temperatures 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 the ISM from pervasive diffuse ionized gas to collapsing cores in molecular clouds, with a diversion into planet-forming disks (the circumstellar medium). We will discuss applications in current (ALMA and JWST) research based on studies in the Milky Way, nearby galaxies, and the high-redshift Universe.


I have studied various aspects of the ISM for ~35 years now, from studies of extragalactic HI as an REU student at Arecibo, to thesis work on molecular clouds at UC Berkeley, then star-forming cores as a postdoc at CfA and NRAO, and I now focus mainly on planet-forming disks. I recently wrote a book on the ISM based on my research experience and many years of teaching. This book is a required text for the class but, to make up for forcing you buy this, I'll use my (far from lucrative) royalties to buy treats during class.


Available at Cambridge University Press and amazon.
Figures and code on github.

The course will follow the book closely. Indeed my intent was that you can follow much of the book material largely by yourselves and I will assign reading ahead of the lectures so that we can use the in-class time to expand on various topics in more detail and show the application of the material to current research.

One of the best ways to learn is by doing. There will be a problem set every two weeks, many of which will involve analysis of real data. You can use any programming language for this but I recommend python as you will almost certainly need to know it as you progress toward a PhD. If you are new to scientific computing and/or python, there are many online resources that I will point out and we will have collaborative hackathons as necessary so I and your classmates can share advice on how to tackle common issues in the problem sets.

Your grade will be a mix of 25% for participation (discussion of reading assignments, assisting in hackathons), 10% for each problem set (5 total), and 25% on the final exam. As a three credit core course, your out-of-class workload should average about 6 hours per week.


Any student who feels s/he may need an accommodation based on the impact of a disability is invited to contact me privately. I would be happy to work with you, and the KOKUA Program (Office for Students with Disabilities) to ensure reasonable accommodations in my course. KOKUA can be reached at (808) 956-7511 or (808) 956-7612 (voice/text) in room 013 of the Queen Liliuokalani Center for Student Services.