Physics Colloquium - X-ray Emission from Beyond the Solar System: uncertainties and opportunities

X-ray astronomy is a relatively new field which began by putting detectors above the atmosphere on balloons and sounding rockets more than fifty years ago. These early glimpses revealed X-ray emission from space in all directions. Today the field is mature with three operating space telescopes (Chandra, XMM-Newton, and Suzaku), but technological challenges still limit wavelength resolution which in turn hampers scientific understanding. X-ray emitting environments typically are classified as being either in collisional ionization equilibrium (CIE) or photoionization equilibrium (PIE) with the ionization being driven by electron collisions or some high-energy photon source, respectively. Within the last decade, X-ray astronomers began to consider a third mechanism, charge exchange (CX) due to the collision of highly charged ions with neutrals. It is highly likely, the emission from most sources involve at least two, if not all three, mechanisms and disentangling the various contributions is fraught with huge uncertainties. In this talk, I'll review the status of X-ray emission modeling focusing on the astrophysics and atomic physics of CX. I'll end with an atomic physicist's perceptive of how X-ray spectroscopy fits within the full range of astronomy (radio to gamma-ray) and prospects for the future.