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Based on 20 years of solar investigations, the extreme-ultraviolet (EUV) spectrum in the range 50-500 Â has proved to be extraordinarily rich in providing information on the density, temperature, and element abundances of the plasma in the Sun's chromosphere and corona, especially when observed with high spectral resolution. With the advent of new instruments using multilayer optics and CCD detectors, similar measurements with a spectral resolution of about 10,000 become possible on large numbers of nearby stars, in contrast with lower resolution instruments now scheduled for space flight. In this paper we provide a description of the lines from the most abundant ions in this spectral region that have special diagnostic significance. These lines are from plasmas in the temperature range from 10 5 to 10 7 K. We demonstrate their utility for determining temperature, density, and abundances. We also describe an objective grating spectrometer, based on the Naval Research Laboratory Skylab slitless spectrograph, for making celestial observations in the EUV spectral region with high spectral resolution.

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