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This is an image of the sky in the light of the hydrogen Alpha line. It shows light emitted by hydrogen atoms which are recombining from the ionized state, and thus demonstrates that much of the interstellar medium is a plasma. Most of this light comes from a phase of the interstellar medium called the "Diffuse Ionized Gas", which has a density of about 0.1 atoms/cc, and a temperature of order 8000K. Spectroscopic measurements of the H Alpha line, and other lines from the same medium, such as singly ionized sulfur, have permitted diagnostic measurements of plasma density, electron temperature, ion temperature, and ionization fraction.
Brighter, compact regions of emission are HII regions, denser regions of photoionized plasma surrounding hot, luminous stars.
This image was made by the Wisconsin H-Alpha Mapper (WHAM), an instrument dedicated to studying the Diffuse Ionized Gas of the interstellar medium. This instrument was developed in the Department of Astronomy, University of Wisconsin at Madison. The effort is led by L. Matthew Haffner and Ronald J. Reynolds.