Physics Colloquium - Probing the Interstellar Medium with X-ray Spectroscopy

The gas in our galaxy, the interstellar medium (ISM), is the source and sink of stellar material and so is important to the understanding of the process of stellar evolution.  This gas can be studied by a variety of techniques, each with its merits.    X-ray spectroscopy is a technique which is only beginning to be exploited for the study of the ISM. It uses a distant compact X-ray source as a lamp and allows study of the ISM via spectral features imprinted from K shell or L shell photoabsorption and excitation.  These features can be calculated or measured in the laboratory and thus the column densities of the various interstellar constituents can be derived.  This method has the virtue that it allows study of relatively distant sources, and it can detect and distinguish cold from hot gas, and solids.  It also provides a laboratory for atomic physics with surprisingly high precision, and the astrophysical results challenge the terrestrial laboratory measurements.  I will review recent work in this area and present results of modeling and analysis of astrophysical observations.