Number 51 (Story #3), October 10, 1991 by Phillip F. Schewe and Ben Stein THE EXTREME ULTRAVIOLET (EUV) is a wavelength range (5-25 nm) impossible to see through the atmosphere with telescopes on the ground. The EUV detector on the Rosat satellite has therefore opened up a whole window on astronomy. British scientists in charge of Rosat's EUV survey have reported more than 700 sources, whereas only a dozen were previously known. One consequence is a downward revision of the estimated density of hydrogen in the region of the solar system; it was thought that such hydrogen would limit EUV radiation, much of it coming from hot stars, from reaching the earth. (New Scientist, 28 Sept. 1991.)