observation (luminous stars and heated dust and gas) fall far short of the critical mass required for closure. Many recent studies, both theoretical and observational, indirectly suggest that more mass might exist, perhaps in extremely faint halos around galaxies. Attempts to observe faint galactic halos are suggestive, but limited by the sky background. 5) Active Galaxies and Quasars. Events leading to and the effects of the tremendously large energy sources in the centers of exploding galaxies and quasars have captivated the interests of both astronomers and the public for nearly a generation. Although the nuclei of these objects are often bright, the material ejected into space and the processes that supply energy to the nuclei are very faint and difficult to observe. 6) Faint Regions of Nearby Galaxies. Nearby galaxies afford favorable observing opportunities because of their proximity, and intense efforts to understand their structure and evolution have characterized modern astronomy. These faint regions not only may contain the mass necessary to close the universe but also reflect the subtle tidal effects that galaxies exert on each other. The Milky Way. Our galaxy is one of the most interesting, yet enigmatic, of astronomical objects. We look to our solar neighborhood to understand how stars are born, live their lives, and die, and to understand how our galaxy is structured. The nuclei of many galaxies harbor unusual and, as yet, not understood energetic processes. What about our own? Do other stars have planets? To complicate observations, large clouds of dust hide 90% of the galaxy from view to observers using optical and ultraviolet techniques. The effective size of the visible galaxy would be severely diminished by even small increases in the night sky levels. 1) Diffuse Galactic Light. "Hot tunnels" carved through the galaxy by supernova blast waves appear to permeate the galaxy. Many studies are underway to understand the physical conditions (e.g., temperatures of a
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